Linux Quick Reference Guide (6th Ed.)

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Linux
Quick Reference Guide
6th edition August 2018
Foreword
This guide stems from the notes I have been taking both while working as a Linux sysadmin and while preparing the
certification exams LPIC-1 (Linux Professional Institute Certification level 1), LPIC-2, RHCSA (Red Hat Certified System
Administrator), and RHCE (Red Hat Certified Engineer). It contains a good amount of topics for these certification exams,
with some subjects handled in more details than others, plus other useful information about standards and tools for Linux
system administration. Unless otherwise specified, shell commands and operations refer to Bash.
This is an independent publication and is not affiliated with, authorized by, sponsored by, or otherwise approved by LPI or
Red Hat. You can freely use and share this whole guide or the single pages, both in electronic or in printed form, provided
that you distribute them unmodified and not for profit.
Happy Linux hacking,
Daniele Raffo
Version history
1st edition
2nd edition
3rd edition
4th edition
5th edition
6th edition
May 2013
September 2014
July 2015
June 2016
September 2017
August 2018
Bibliography and suggested readings
●Evi Nemeth et al., UNIX and Linux System Administration Handbook, O'Reilly
●Rebecca Thomas et al., Advanced Programmer's Guide to Unix System V, McGraw-Hill
●Mendel Cooper, Advanced Bash-Scripting Guide, http://tldp.org/LDP/abs/html
●Adam Haeder et al., LPI Linux Certification in a Nutshell, O'Reilly
●Heinrich W. Klöpping et al., The LPIC-2 Exam Prep, http://lpic2.unix.nl
●Michael Jang, RHCSA/RHCE Red Hat Linux Certification Study Guide, McGraw-Hill
●Asghar Ghori, RHCSA & RHCE RHEL 7: Training and Exam Preparation Guide, Lightning Source Inc.
●Colin Barschel, Unix Toolbox, http://cb.vu/unixtoolbox.xhtml
●Ellen Siever et al., Linux in a Nutshell, O'Reilly, http://archive.oreilly.com/linux/cmd
●Christoph Braun, Unix System Security Essentials, Addison-Wesley
●Bruce Barnett, The Grymoire, http://www.grymoire.com/Unix
●Brendan Gregg, Linux performance, http://www.brendangregg.com/linuxperf.html
●RHEL manuals, https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux
●A-Z index of Bash command line, http://ss64.com/bash
●GNU software manuals, http://www.gnu.org/manual
●Shell command line snippets, http://www.commandlinefu.com
●Bash command line snippets, http://www.bashoneliners.com
●RAM management in Linux, http://www.linuxatemyram.com
●Regular expressions tester, http://www.regextester.com
●Bash pitfalls, http://mywiki.wooledge.org/BashPitfalls
●Linux man pages, https://www.kernel.org/doc/man-pages
●CentOS 7 man pages, https://www.unix.com/man-page-centos-repository.php
Index
LVM................................................1
LVM commands................................2
System boot....................................3
SysV startup sequence.....................4
Login..............................................5
Runlevels........................................6
SysV vs Systemd.............................7
/etc/inittab......................................8
Filesystem Hierarchy Standard...........9
Partitions......................................10
mount..........................................11
Filesystem types............................12
Swap............................................13
/etc/fstab......................................14
Filesystem operations.....................15
Filesystem maintenance..................16
XFS, ReiserFS, CD-ROM fs...............17
AutoFS..........................................18
RAID............................................19
Bootloader....................................20
GRUB 2 configuration......................21
GRUB 2 usage................................22
GRUB Legacy.................................23
Low-level package managers...........24
High-level package managers..........25
Package management tools.............26
Backup.........................................27
Archive formats..............................28
Documentation..............................29
Shell basics...................................30
Text filters.....................................31
Advanced text filters.......................32
Regular expressions........................33
File management...........................34
Directory management...................35
I/O streams...................................36
read and echo................................37
Processes......................................38
Signals.........................................39
Resource monitoring.......................40
vmstat and free.............................41
File permissions.............................42
File attributes................................43
ACLs.............................................44
Links............................................45
Find system files............................46
Shell variables...............................47
Shell operations.............................48
Shell scripting................................49
Script execution.............................50
Tests............................................51
Flow control...................................52
Text processors..............................53
Vi commands.................................54
Vi options......................................55
SQL..............................................56
SQL SELECT..................................57
SQL JOIN......................................58
MySQL..........................................59
MySQL tools..................................60
MySQL syntax................................61
MySQL status................................62
MySQL recipes...............................63
MySQL operations..........................64
PostgreSQL...................................65
X..................................................66
X tools..........................................67
X keysim codes..............................68
/etc/passwd...................................69
User management..........................70
UID and GID..................................71
su and sudo...................................72
Terminals......................................73
Messaging.....................................74
cron.............................................75
at.................................................76
Utilities.........................................77
Localization...................................78
System time..................................79
syslog...........................................80
E-mail...........................................81
SMTP............................................82
Sendmail.......................................83
Exim.............................................84
Postfix..........................................85
Postfix configuration.......................86
Procmail........................................87
Courier POP configuration................88
Courier IMAP configuration..............89
Dovecot........................................90
Dovecot mailbox configuration.........91
Dovecot POP and IMAP configuration.92
Dovecot authentication...................93
FTP..............................................94
vsftpd...........................................95
CUPS............................................96
IP addressing.................................97
Subnetting....................................98
Network services............................99
Network configuration commands...100
Wireless networking......................101
Network tools...............................102
Network monitoring......................103
Packet sniffing..............................104
netcat.........................................105
Network settings..........................106
Network configuration...................107
nmcli..........................................108
Teaming and bridging....................109
TCP Wrapper................................110
Routing.......................................111
iptables.......................................112
iptables rules...............................113
iptables NAT routing......................114
firewalld......................................115
firewalld rules..............................116
SSH............................................117
SSH operations............................118
SSH configuration.........................119
OpenSSL.....................................120
CA.pl..........................................121
GnuPG........................................122
OpenVPN.....................................123
Key bindings - terminal.................124
Key bindings - X...........................125
udev...........................................126
Kernel.........................................127
Kernel management......................128
Kernel compile and patching..........129
Kernel modules............................130
/proc..........................................131
System recovery..........................132
DNS............................................133
DNS configuration.........................134
DNS zone file...............................135
Apache........................................136
Apache configuration....................137
Apache virtual hosts.....................138
Apache directory protection...........139
Apache SSL/TLS...........................140
Apache proxy...............................141
Tomcat........................................142
Samba server..............................143
Samba client................................144
Samba global configuration............145
Samba share configuration............146
Samba access configuration...........147
Samba setup...............................148
NFS............................................149
/etc/exports.................................150
NFS setup....................................151
iSCSI..........................................152
iSCSI setup.................................153
DHCP..........................................154
PAM............................................155
LDAP..........................................156
OpenLDAP...................................157
SELinux.......................................158
AVC............................................159
KVM............................................160
Git..............................................161
Vagrant.......................................162
HTML 4.01 components.................163
HTML 4.01 text............................164
HTML 4.01 images........................165
HTML 4.01 tables..........................166
7-bit ASCII table..........................167
1/167 LVM
LVM
Logical Volume Management (LVM) introduces an abstraction between physical and logical storage allowing a more versatile
use of filesystems. LVM uses the Linux device mapper feature (/dev/mapper).
Disks, partitions, and RAID devices are made of Physical Volumes, which are grouped into a Volume Group.
A Volume Group is divided into small fixed-size chunks called Physical Extents, which are mapped 1-to-1 to Logical Extents.
Logical Extents are grouped into Logical Volumes, on which filesystems are created.
How to create a Logical Volume
1. Add a new physical or virtual disk to the machine
2. lsblk Check that the new disk is being recognized e.g. as
/dev/sda
3. fdisk /dev/sda Create a new partition (of type 0x8E = Linux LVM) on
the new disk.
This is not necessary but recommended, because other
OSes might not recognize LVM and see the whole
unpartitioned disk as empty
4. pvcreate /dev/sda1 Initialize the Physical Volume to be used with LVM
5. vgcreate -s 8M myvg0 /dev/sda1 Create a Volume Group and define the size of Physical
Extents to 8 Mb (default value is 4 Mb)
or vgextend myvg0 /dev/sda1 or add the Physical Volume to an existing Volume Group
6. lvcreate -L 1024M -n mylv myvg0 Create a Logical Volume
7. mkfs -t ext3 /dev/myvg0/mylv Create a filesystem on the Logical Volume
8. mount /dev/myvg0/mylv /mnt/mystuff Mount the Logical Volume which is now ready to be used
How to increase the size of a Logical Volume (only if the underlying filesystem allows it)
1. Add a new physical or virtual disk to the machine; this will provide the extra disk space
2. fdisk /dev/sdc Partition the new disk
3. pvcreate /dev/sdc Initialize the Physical Volume
4. vgextend myvg0 /dev/sdc Add the Physical Volume to an existing Volume Group
5. lvextend -L 2048M /dev/myvg0/mylv Extend the Logical Volume by 2 Gb
or lvresize -L+2048M /dev/myvg0/mylv
or lvresize -l+100%FREE /dev/myvg/mylv or extend the Logical Volume taking all free space
6. resize2fs /dev/myvg0/mylv Extend the filesystem
How to reduce the size of a Logical Volume (only if the underlying filesystem allows it)
1. resize2fs /dev/myvg0/mylv 900M Shrink the filesystem to 900 Mb
2. lvreduce -L 900M /dev/myvg0/mylv Shrink the Logical Volume to 900 Mb
or lvresize -L 900M /dev/myvg0/mylv
How to snapshot and backup a Logical Volume
1. lvcreate -s -L 1024M -n snapshot0 /dev/myvg0/mylv Create the snapshot like a Logical Volume
2. tar cvzf snapshot0.tar.gz snapshot0 Backup the snapshot with your preferred backup tool
3. lvremove /dev/mvvg0/snapshot0 Delete the snapshot
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
2/167 LVM commands
LVM commands
PV commands VG commands LV commands
pvs Report information about
Physical Volumes
vgs Report information
about Volume Groups
lvs Report information
about Logical Volumes
pvscan Scan all disks for
Physical Volumes
vgscan Scan all disks for
Volume Groups
lvscan Scan all disks for
Logical Volumes
pvdisplay Display Physical Volume
attributes
vgdisplay Display Volume Group
attributes
lvdisplay Display Logical Volume
attributes
pvck Check Physical Volume
metadata
vgck Check Volume Group
metadata
pvcreate Initialize a disk or
partition for use with
LVM
vgcreate Create a Volume Group
using Physical Volumes
lvcreate Create a Logical
Volume in a Volume
Group
pvchange Change Physical Volume
attributes
vgchange Change Volume Group
attributes
lvchange Change Logical Volume
attributes
pvremove Remove a Physical
Volume
vgremove Remove a Volume
Group
lvremove Remove a Logical
Volume
vgextend Add a Physical Volume
to a Volume Group
lvextend Increase the size of a
Logical Volume
vgreduce Remove a Physical
Volume from a Volume
Group
lvreduce Shrink the size a
Logical Volume
pvresize Resize a disk or partition
in use with LVM
lvresize Modify the size of a
Logical Volume
vgmerge Merge two Volume
Groups
vgsplit Split two Volume
Groups
vgimport Import a Volume Group
into a system
vgexport Export a Volume Group
from a system
pvmove Move the Logical Extents
on a Physical Volume to
wherever there are
available Physical
Extents (within the
Volume Group) and then
put the Physical Volume
offline
LVM global commands
lvmdiskscan Scan the system for disks and partitions usable by LVM
dmsetup command Perform low-level LVM operations
/dev/mapper/vgname-lvname
/dev/vgname/lvname
Mapping of Logical Volumes in the filesystem
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
3/167 System boot
System boot
Boot sequence
POST
(Power-On Self Test) Low-level check of PC hardware.
BIOS
(Basic I/O System) Detection of disks and hardware.
Chain loader
GRUB
(GRand Unified
Bootloader)
GRUB stage 1 is loaded from the MBR and executes GRUB stage 2 from filesystem.
GRUB chooses which OS to boot on.
The chain loader hands over to the boot sector of the partition on which resides the OS.
The chain loader also mounts initrd, an initial ramdisk (typically a compressed ext2
filesystem) to be used as the initial root device during kernel boot; this make possible to
load kernel modules that recognize hard drives hardware and that are hence needed to
mount the real root filesystem. Afterwards, the system runs /linuxrc with PID 1.
(From Linux 2.6.13 onwards, the system instead loads into memory initramfs, a cpio-
compressed image, and unpacks it into an instance of tmpfs in RAM. The kernel then
executes /init from within the image.)
Linux kernel
Kernel decompression into memory.
Kernel execution.
Detection of devices.
The real root filesystem is mounted on / in place of the initial ramdisk.
init
Execution of init, the first process (PID 1).
The system tries to execute in the following order:
/sbin/init
/etc/init
/bin/init
/bin/sh
If none of these succeeds, the kernel panics.
Startup The system loads startup scripts and runlevel scripts.
Login
If in text mode, init calls the getty process, which runs the login command that asks
the user for login and password.
If in graphical mode, the X Display Manager starts the X Server.
Newer systems use UEFI (Unified Extensible Firmware Interface) instead of BIOS. UEFI does not use the MBR boot code; it
has knowledge of partition table and filesystems, and stores its application files required for launch in a EFI System
Partition, mostly formatted as FAT32.
After the POST, the system loads the UEFI firmware which initializes the hardware required for booting, then reads its Boot
Manager data to determine which UEFI application to launch. The launched UEFI application may then launch another
application, e.g. the kernel and initramfs in case of a boot loader like GRUB.
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
4/167 SysV startup sequence
SysV startup sequence
Startup sequence Debian Red Hat
At startup /sbin/init executes all
instructions on /etc/inittab . This script
at first switches to the default runlevel...
id:2:initdefault: id:5:initdefault:
... then it runs the following script (same for
all runlevels) which configures peripheral
hardware, applies kernel parameters, sets
hostname, and provides disks initialization...
/etc/init.d/rcS /etc/rc.d/rc.sysinit or
/etc/rc.sysinit
... and then, for runlevel N, it calls the script /
etc/init.d/rc N (i.e. with the runlevel
number as parameter) which launches all
services and daemons specified in the
following startup directories:
/etc/rcN.d/ /etc/rc.d/rcN.d/
The startup directories contain symlinks to the init scripts in /etc/init.d/ which are executed in numerical order.
Links starting with K are called with argument stop, links starting with S are called with argument start.
lrwxrwxrwx. 1 root root 14 Feb 11 22:32 K88sssd -> ../init.d/sssd
lrwxrwxrwx. 1 root root 15 Nov 28 14:50 K89rdisc -> ../init.d/rdisc
lrwxrwxrwx. 1 root root 17 Nov 28 15:01 S01sysstat -> ../init.d/sysstat
lrwxrwxrwx. 1 root root 18 Nov 28 14:54 S05cgconfig -> ../init.d/cgconfig
lrwxrwxrwx. 1 root root 16 Nov 28 14:52 S07iscsid -> ../init.d/iscsid
lrwxrwxrwx. 1 root root 18 Nov 28 14:42 S08iptables -> ../init.d/iptables
The last script to be run is S99local -> ../init.d/rc.local ; therefore, an easy way to run a specific program
upon boot is to call it from this script file.
/etc/init.d/boot.local
/etc/init.d/before.local (SUSE)
/etc/init.d/after.local (SUSE)
runs only at boot time, not when switching runlevel.
runs only at boot time, before the scripts in the startup directories.
runs only at boot time, after the scripts in the startup directories.
To add or remove services at boot sequence: update-rc.d service defaults
update-rc.d -f service remove
chkconfig --add service
chkconfig --del service
When adding or removing a service at boot, startup directories will be updated by creating or deleting symlinks for the
default runlevels: K symlinks for runlevels 0 1 6, and S symlinks for runlevels 2 3 4 5.
Service will be run via the xinetd super server.
Service operation parameters supported by the init scripts
start Start the service
Mandatory
stop Stop the service
restart Restart the service (stop, then start)
status Display daemon PID and execution status
force-reload Reload configuration if service supports it, otherwise restart
condrestart
try-restart Restart the service only if already running Optional
reload Reload the service configuration
Linux Standard Base (LSB)
The Linux Standard Base defines a format to specify default values on an init script /etc/init.d/foo :
### BEGIN INIT INFO
# Provides: foo
# Required-Start: bar
# Defalt-Start: 2 3 4 5
# Default-Stop: 0 1 6
# Description: Service Foo init script
### END INIT INFO
Default runlevels and S/K symlinks values can also be specified as such:
# chkconfig: 2345 85 15
# description: Foo service
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
5/167 Login
Login
/etc/init/start-ttys.conf (Red Hat) Start the specified number of terminals at bootup via getty, which
manages physical or virtual terminals (TTYs)
/etc/sysconfig/init (Red Hat) Control appearance and functioning of the system during bootup
/etc/machine-id (Red Hat) Randomly-generated machine ID
rm /etc/machine-id && \
systemd-machine-id-setup (Red Hat)
Initialize the machine ID
/etc/securetty List of TTYs from which the root user is allowed to login
/etc/issue Message that will be printed before the login prompt.
Can contain the following escape codes:
\b Baudrate of line
\d Date
\s System name and OS
\l Terminal device line
\m Architecture identifier of machine
\n Nodename aka hostname
\o Domain name
\r OS release number
\t Time
\u Number of users logged in
\U "n users" logged in
\v OS version and build date
/etc/issue.net Message that will be printed before the login prompt on a remote session
/etc/motd Message that will be printed after a successful login, before execution of
the login shell
/etc/nologin If this file exists, login and sshd deny login to the system.
Useful to prevent users to log in when doing system maintenance
To prevent a user to log in, their shell can be set either as:
- /bin/false (user will be forced to exit immediately)
- /sbin/nologin (user will be prompted a message, then forced to exit; message is "This account is currently not available"
or, if the file /etc/nologin.txt exists, the contents of that file)
cat /etc/debian_version (Debian)
cat /etc/fedora-release (Fedora)
cat /etc/redhat-release (Red Hat)
cat /etc/lsb-release
lsb_release -a
cat /etc/os-release
Show the Linux distribution name and version
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
6/167 Runlevels
Runlevels
Runlevel
(SysV)
Target
(Systemd) Debian Red Hat
0Shutdown
1Single user / maintenance mode
default
runlevels
2Multi-user mode
(default) Multi-user mode without network
3multi-user.target Multi-user mode Multi-user mode with network
4Multi-user mode Unused, for custom use
5graphical.target Multi-user mode Multi-user mode with network and X
(default)
6Reboot
SSingle user / maintenance mode
(usually accessed through runlevel 1)
Systemd's target runleveln.target emulates a SysV's runlevel n.
runlevel
who -r
Display the previous and the current runlevel
init runlevel
telinit runlevel
Change to runlevel
systemctl get-default Get the default target
systemctl set-default target Set the default target
systemctl isolate target Change to target
systemctl emergency Change to maintenance single-user mode with only /root filesystem mounted
systemctl rescue Change to maintenance single-user mode with only local filesystems mounted
init 0
telinit 0
shutdown -h now
halt
poweroff
Halt the system
init 6
telinit 6
shutdown -r now
reboot
Reboot the system
shutdown Shut down the system in a secure way: all logged-in users are notified via a
message to their terminal, and login is disabled. Can only be run by the root user
shutdown -a Non-root users that are listed in /etc/shutdown.allow can use this command to
shut down the system
shutdown -h 16:00 message Schedule a shutdown for 4 PM and send a warning message to all logged-in users
shutdown -f Skip fsck on reboot
shutdown -F Force fsck on reboot
shutdown -c Cancel a shutdown that has been already initiated
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
7/167 SysV vs Systemd
SysV vs Systemd
System V Systemd Action
/etc/init.d/service operation
service service operation (Red Hat)
rcservice operation (SUSE)
systemctl operation service Perform one of these operations on
the specified service:
start
stop
restart
status
force-reload
condrestart
try-restart
reload
update-rc.d service defaults (Debian)
chkconfig --add service (Red Hat)
Add a service at boot
update-rc.d -f service remove (Debian)
chkconfig --del service (Red Hat)
Remove a service at boot
update-rc.d -f service \
start 30 2 3 4 5 . stop 70 0 1 6 .
Add a service on the default
runlevels; create S30 symlinks for
starting the service and K70
symlinks for stopping it
chkconfig --levels 245 service on Add the service on runlevels 2 4 5
chkconfig service on systemctl enable service Add the service on default runlevels
chkconfig service off systemctl disable service Remove the service on default
runlevels
chkconfig service systemctl is-enabled service Check if the service is enabled on
the current runlevel
chkconfig service reset Reset the on/off state of the service
for all runlevels to whatever the LSB
specifies in the init script
chkconfig service resetpriorities Reset the start/stop priorities of the
service for all runlevels to whatever
the LSB specifies in the init script
chkconfig --list service Display current configuration of
service (its status and the runlevels
in which it is active)
chkconfig
chkconfig --list
systemctl list-unit-files \
--type=service
List all active services and their
current configuration
ls /etc/rcn.d (Debian) List services started on runlevel n
systemctl List loaded and active units
systemctl --all List all units, including inactive ones
systemctl -t target List targets
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
8/167 /etc/inittab
/etc/inittab
/etc/inittab
# The default runlevel.
id:2:initdefault:
# Boot-time system configuration/initialization script.
# This is run first except when booting in emergency (-b) mode.
si::sysinit:/etc/init.d/rcS
# What to do in single-user mode.
~~:S:wait:/sbin/sulogin
# /etc/init.d executes the S and K scripts upon change of runlevel.
l0:0:wait:/etc/init.d/rc 0
l1:1:wait:/etc/init.d/rc 1
l2:2:wait:/etc/init.d/rc 2
l3:3:wait:/etc/init.d/rc 3
l4:4:wait:/etc/init.d/rc 4
l5:5:wait:/etc/init.d/rc 5
l6:6:wait:/etc/init.d/rc 6
# Normally not reached, but fall through in case of emergency.
z6:6:respawn:/sbin/sulogin
# /sbin/getty invocations for the runlevels.
# Id field must be the same as the last characters of the device (after "tty").
1:2345:respawn:/sbin/getty 38400 tty1
2:23:respawn:/sbin/getty 38400 tty2
/etc/inittab describes which processes are started at bootup and during normal operation; it is read and executed by
init at bootup.
All its entries have the form id:runlevels:action:process.
id 1-4 characters, uniquely identifies an entry.
For gettys and other login processes it should be equal to the suffix of the corresponding tty
runlevels Runlevels for which the specified action must be performed.
If empty, action is performed on all runlevels
action
respawn Process will be restarted when it terminates
wait Process is started at the specified runlevel and init will wait for its termination
(i.e. execution of further lines of /etc/inittab stops until the process exits)
once Process is executed once at the specified runlevel
boot Process is executed at system boot. Runlevels field is ignored
bootwait Process is executed at system boot and init will wait for its termination.
Runlevels field is ignored
off Does nothing
ondemand Process is executed when an on-demand runlevel (A, B, C) is called
initdefault Specifies the default runlevel to boot on. Process field is ignored
sysinit Process is executed at system boot, before any boot or bootwait entries.
Runlevels field is ignored
powerfail Process is executed when power goes down and an UPS kicks in.
init will not wait for its termination
powerwait Process is executed when power goes down and an UPS kicks in.
init will wait for its termination
powerfailnow Process is executed when power is down and the UPS battery is almost empty
powerokwait Process is executed when power has been restored from UPS
ctrlaltdel Process is executed when init receives a SIGINT via
kbdrequest Process is executed when a special key combination is pressed on console
process Process to execute. If prepended by a +, utmp and wtmp accounting will not be done
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
CTRL ALT DEL
9/167 Filesystem Hierarchy Standard
Filesystem Hierarchy Standard
Filesystem Hierarchy Standard (FHS)
/bin Essential command binaries
/boot Bootloader files (e.g. OS loader, kernel image, initrd)
/dev Virtual filesystem containing device nodes to devices and partitions
/etc System configuration files and scripts
/home Home directories for users
/lib Libraries for the binaries in /bin and /sbin, kernel modules
/lost+found Storage directory for recovered files in this partition
/media Mount points for removable media
/mnt Mount points for temporary filesystems
/net Access to directory tree on different external NFS servers
/opt Optional, large add-on application software packages
/proc Virtual filesystem providing kernel and processes information
/root Home directory for the root user
/sbin Essential system binaries, system administration commands
/srv Data for services provided by the system
/sys Virtual filesystem providing information about hotplug hardware devices
/tmp Temporary files (deleted at reboot)
/usr User utilities and applications
/usr/bin Non-essential command binaries (for all users)
/usr/include C header files
/usr/lib Libraries for the binaries in /usr/bin and /usr/sbin
/usr/local Software installed locally
/usr/local/bin Local software binaries
/usr/local/games Local game binaries
/usr/local/include Local C header files
/usr/local/lib Local libraries for the binaries in /usr/local/bin and /usr/local/sbin
/usr/local/man Local man pages
/usr/local/sbin Local system binaries
/usr/local/share Local architecture-independent hierarchy
/usr/local/src Local source code
/usr/sbin Non-essential system binaries (daemons and services)
/usr/share Architecture-independent files (e.g. icons, fonts, documentation)
/usr/share/doc Package-specific documentation not included in man pages
/usr/share/man Man pages
/usr/share/info Documentation in Info format
/usr/src Source code for the actual OS
/var Variable files (e.g. logs, caches, mail spools)
/var/log Logfiles
/var/opt Variable files for the application software installed in /opt
/var/spool Queued items to be processed (e.g. mail messages, cron jobs, print jobs)
/var/tmp Temporary files that need to be stored for a longer time (preserved between reboots)
The manpage man hier contains information about filesystem hierarchy.
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
10/167 Partitions
Partitions
/dev/hda IDE hard drive
/dev/sda SCSI, PATA, or SATA hard drive
/dev/vda Virtual disk for KVM-based virtual machines
/dev/hda, /dev/hdb, /dev/hdc ... First, second, third ... hard drive
/dev/sda1, /dev/sda2, /dev/sda3 ... First, second, third ... partition of the first hard drive
The superblock contains information relative to the filesystem e.g. filesystem type, size, status, metadata structures.
The Master Boot Record (MBR) is a 512-byte program located in the first sector of the hard disk; it contains information
about hard disk partitions and has the duty of loading the OS. On recent systems, the MBR has been replaced by the GUID
Partition Table (GPT).
Most modern filesystems use journaling; in a journaling filesystem, the journal logs changes before committing them to the
filesystem, which ensures faster recovery and less corruption in case of a crash.
Partitioning limits for Linux using MBR:
Max 4 primary partitions per hard disk, or 3 primary partitions + 1 extended partition Partition numbers: 1-4
Max 11 logical partitions (inside the extended partition) per hard disk Partition numbers: 5-15
Max disk size is 2 Tb
GPT makes no difference between primary, extended, or logical partitions; and it has practically no limits concerning number
and size of partitions.
fdisk /dev/sda Disk partitioning interactive tool
fdisk -l /dev/sda List the partition table of /dev/sda
parted Disk partitioning interactive tool
sfdisk /dev/sda Disk partitioning non-interactive tool
cfdisk Disk partitioning tool with text-based UI
gparted
gnome-disks
Disk partitioning tool with GUI
partprobe This command can be run after fdisk operations to notify the OS of partition table
changes. Otherwise, the changes will take place only after reboot
mkfs -t fstype device Create a filesystem of the specified type on a partition (i.e. format the partition).
mkfs is a wrapper utility for the actual filesystem-specific maker commands:
mkfs.ext2 aka mke2fs
mkfs.ext3 aka mke3fs
mkfs.ext4
mkfs.msdos aka mkdosfs
mkfs.ntfs aka mkntfs
mkfs.reiserfs aka mkreiserfs
mkfs.jfs
mkfs.xfs
mkfs -t ext2 /dev/sda
mkfs.ext2 /dev/sda
mke2fs /dev/sda
Create an ext2 filesystem on /dev/sda
mke2fs -j /dev/sda
mkfs.ext3 /dev/sda
mke3fs /dev/sda
Create an ext3 filesystem (ext2 with journaling) on /dev/sda
mkfs -t msdos /dev/sda
mkfs.msdos /dev/sda
mkdosfs /dev/sda
Create a MS-DOS filesystem on /dev/sda
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11/167 mount
mount
mount
cat /proc/mounts
cat /etc/mtab
Display the currently mounted filesystems.
The commands mount and umount maintain in /etc/mtab a database of
currently mounted filesystems, but /proc/mounts is authoritative
mount -a Mount all devices listed in /etc/fstab, except those indicated as noauto
mount -t ext3 /dev/sda /mnt Mount a Linux-formatted disk. The mount point (directory) must exist
mount -t msdos /dev/fd0 /mnt Mount a MS-DOS filesystem floppy disk to mount point /mnt
mount /dev/fd0 Mount a floppy disk. /etc/fstab must contain an entry for /dev/fd0
mount -o remount,rw / Remount the root directory as read-write, supposing it was mounted read-only.
Useful to change flags (in this case, read-only to read-write) for a mounted
filesystem that cannot be unmounted at the moment
mount -o nolock 10.7.7.7:/export/ /mnt/nfs Mount a NFS share without running NFS daemons.
Useful during system recovery
mount -t iso9660 -o ro,loop=/dev/loop0 cd.img /mnt/cdrom Mount a CD-ROM ISO9660 image file like a CD-ROM
(via the loop device)
umount /dev/fd0
umount /mnt
Unmount a floppy disk that was mounted on /mnt (device must not be busy)
umount -l /dev/fd0 Unmount the floppy disk as soon as it is not in use anymore
eject /dev/fd0
eject /mnt
Eject a removable media device
mountpoint /mnt Tell if a directory is a mount point
The UUID (Universal Unique Identifier) of a partition is a 128-bit hash number, which is associated to the partition when the
partition is initialized.
blkid /dev/sda1 Print the UUID of the specified partition
blkid -L /boot Print the UUID of the specified partition, given its label
blkid -U 652b786e-b87f-49d2-af23-8087ced0c667 Print the name of the specified partition, given its UUID
findfs UUID=652b786e-b87f-49d2-af23-8087ced0c667 Print the name of the specified partition, given its UUID
findfs LABEL=/boot Print the name of the specified partition, given its label
e2label /dev/sda1 Print the label of the specified partition, given its name
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12/167 Filesystem types
Filesystem types
Partition types
0x00 Empty
0x01 FAT12
0x02 XENIX root
0x03 XENIX usr
0x04 FAT16 <32M
0x05 Extended
0x06 FAT16
0x07 HPFS/NTFS/exFAT
0x08 AIX
0x09 AIX bootable
0x0a OS/2 Boot Manager
0x0b W95 FAT32
0x0c W95 FAT32 (LBA)
0x0e W95 FAT16 (LBA)
0x0f W95 extended (LBA)
0x10 OPUS
0x11 Hidden FAT12
0x12 Compaq diagnostics
0x14 Hidden FAT16 <32M
0x16 Hidden FAT16
0x17 Hidden HPFS/NTFS
0x18 AST SmartSleep
0x1b Hidden W95 FAT32
0x1c Hidden W95 FAT32 (LBA)
0x1e Hidden W95 FAT16 (LBA)
0x24 NEC DOS
0x27 Hidden NTFS WinRE
0x39 Plan 9
0x3c PartitionMagic recovery
0x40 Venix 80286
0x41 PPC PReP Boot
0x42 SFS
0x4d QNX4.x
0x4e QNX4.x 2nd part
0x4f QNX4.x 3rd part
0x50 OnTrack DM
0x51 OnTrack DM6 Aux1
0x52 CP/M
0x53 OnTrack DM6 Aux3
0x54 OnTrackDM6
0x55 EZ-Drive
0x56 Golden Bow
0x5c Priam Edisk
0x61 SpeedStor
0x63 GNU HURD or SysV
0x64 Novell Netware 286
0x65 Novell Netware 386
0x70 DiskSecure Multi-Boot
0x75 PC/IX
0x80 Old Minix
0x81 Minix / old Linux
0x82 Linux swap / Solaris
0x83 Linux
0x84 OS/2 hidden C: drive
0x85 Linux extended
0x86 NTFS volume set
0x87 NTFS volume set
0x88 Linux plaintext
0x8e Linux LVM
0x93 Amoeba
0x94 Amoeba BBT
0x9f BSD/OS
0xa0 IBM Thinkpad hibernation
0xa5 FreeBSD
0xa6 OpenBSD
0xa7 NeXTSTEP
0xa8 Darwin UFS
0xa9 NetBSD
0xab Darwin boot
0xaf HFS / HFS+
0xb7 BSDI fs
0xb8 BSDI swap
0xbb Boot Wizard hidden
0xbe Solaris boot
0xbf Solaris
0xc1 DRDOS/sec (FAT-12)
0xc4 DRDOS/sec (FAT-16 < 32M)
0xc6 DRDOS/sec (FAT-16)
0xc7 Syrinx
0xda Non-FS data
0xdb CP/M / CTOS / ...
0xde Dell Utility
0xdf BootIt
0xe1 DOS access
0xe3 DOS R/O
0xe4 SpeedStor
0xeb BeOS fs
0xee GPT
0xef EFI (FAT-12/16/32)
0xf0 Linux/PA-RISC boot
0xf1 SpeedStor
0xf4 SpeedStor
0xf2 DOS secondary
0xfb VMware VMFS
0xfc VMware VMKCORE
0xfd Linux raid autodetect
0xfe LANstep
0xff BBT
The command sfdisk -T provides the above list of partition IDs and names.
Most used Linux-supported filesystems
ext2 Linux default filesystem, offering the best performances
ext3 ext2 with journaling
ext4 Linux journaling filesystem, an upgrade from ext3
Reiserfs Journaling filesystem
XFS Journaling filesystem, developed by SGI
JFS Journaling filesystem, developed by IBM
Btrfs B-tree filesystem, developed by Oracle
msdos DOS filesystem, supporting only 8-char filenames
umsdos Extended DOS filesystem used by Linux, compatible with DOS
fat32 MS-Windows FAT filesystem
vfat Extended DOS filesystem, with support for long filenames
ntfs Replacement for fat32 and vfat filesystems
minix Native filesystem of the MINIX OS
iso9660 CD-ROM filesystem
cramfs Compressed RAM disk
nfs Network filesystem, used to access files on remote machines
SMB Server Message Block, used to mount Windows network shares
proc Pseudo filesystem, used as an interface to kernel data structures
swap Pseudo filesystem, Linux swap area
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13/167 Swap
Swap
In Linux, the swap space is a virtual memory area (a file or a partition) used as RAM extension. Usually a partition is
preferred because of better performances concerning fragmentation and disk speed. Although listed as filesystem type
0x82, the swap partition is not a filesystem but a raw addressable memory with no structure; therefore it is not shown in
the output of mount or df commands.
The fdisk tool can be used to create a swap partition.
dd if=/dev/zero of=/swapfile \
bs=1024 count=512000
Create a 512-Mb swap file
mkswap /swapfile Initialize a (already created) swap file or partition
swapon /swapfile Enable a swap file or partition, thus telling the kernel that it can use it now
swapoff /swapfile Disable a swap file or partition
swapon -s
cat /proc/swaps
cat /proc/meminfo
free
top
Show the sizes of total and used swap areas
How to extend a LVM swap partition
1. lvs Determine the name of the swap Logical Volume
2. swapoff /dev/volgroup0/swap_lv Turn off the swap volume
3. lvresize -L+1G /dev/volgroup0/swap_lv Extend the swap volume with an additional 1 Gb of space
4. mkswap /dev/volgroup0/swap_lv Format the swap volume
5. swapon /dev/volgroup0/swap_lv Turn on the swap volume
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14/167 /etc/fstab
/etc/fstab
/etc/fstab Filesystems information
# <filesystem> <mount point> <type> <options> <dump> <pass>
/dev/sda2 / ext2 defaults 0 1
/dev/sdb1 /home ext2 defaults 1 2
/dev/cdrom /media/cdrom auto ro,noauto,user,exec 0 0
/dev/fd0 /media/floppy auto rw,noauto,user,sync 0 0
proc /proc proc defaults 0 0
/dev/hda1 swap swap pri=42 0 0
nfsserver:/dirs /mnt nfs intr 0 0
//smbserver/jdoe /shares/jdoe cifs auto,credentials=/etc/smbcreds 0 0
LABEL=/boot /boot ext2 defaults 0 0
UUID=652b786e-b87f-49d2-af23-8087ced0c667 /test ext4 errors=remount-ro,noatime 0 0
filesystem Device or partition. The filesystem can be identified either by its name, label, or UUID
mount point Directory on which the partition will be mounted
type Filesystem type, or auto if detected automatically
options
defaults Use the default options: rw, suid, dev, auto, nouser, exec, async
ro Mount read-only
rw Mount read-write (default)
suid Permit SUID and SGID bit operations (default)
nosuid Do not permit SUID and SGID bit operations
dev Interpret block special devices on the filesystem (default)
nodev Do not interpret block special devices on the filesystem
auto Mount automatically at bootup, or when command mount -a is given (default)
noauto Mount only if explicitly demanded
user Partition can be mounted by any user
nouser Partition can be mounted only by the root user (default)
exec Binaries contained on the partition can be executed (default)
noexec Binaries contained on the partition cannot be executed
sync Write files immediately to the partition
async Buffer write operations and commit them at once later, or when device is
unmounted (default)
noatime Do not update atime (i.e. access time) information for the filesystem. This can
improve performances because the system does not need anymore to do
filesystem writes for files which are just being read
context="context"Apply a specific SELinux context to the mount
Other specific options apply to specific partition types (e.g. NFS or Samba)
dump Options for the dump backup utility. 0 = do not backup
pass Order in which the filesystem must be checked by fsck. 0 = do not check
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15/167 Filesystem operations
Filesystem operations
df Report filesystem disk space usage
df -h Report filesystem disk space usage in human-readable output
df directory Shows on which device the specified directory is mounted
du directory Report disk usage as size of each file inside directory
du -s directory Report the sum of all files contained inside directory
du -sh directory Report the sum of all files contained inside directory in human-readable output
ncdu Disk usage analyzer with ncurses UI
resize2fs options device size Resize an ext2/ext3/ext4 filesystem
lsblk List information about all available block devices
lsscsi List information about all SCSI devices
blockdev --getbsz /dev/sda1 Get the block size of the specified partition
sync Flush the buffer and commit all pending writes.
To improve performance of Linux filesystems, many write operations are buffered in
RAM and written at once; writes are done in any case before unmount, reboot, or
shutdown
chroot /mnt/sysimage Start a shell with /mnt/sysimage as filesystem root.
Useful during system recovery when the machine has been booted from a removable
media (which hence is defined as the filesystem root)
mknod /dev/sda Create a directory allocating the proper inode.
Useful during system recovery when experiencing filesystem problems
hdparm Get/set drive parameters for SATA/IDE devices
hdparm -g /dev/hda Display drive geometry (cylinders, heads, sectors) of /dev/hda
hdparm -i /dev/hda Display identification information for /dev/hda
hdparm -tT /dev/hda Perform disk read benchmarks on the /dev/hda drive
hdparm -p 12 /dev/hda Reprogram IDE interface chipset of /dev/hda to mode 4. Potentially dangerous!
sdparm Access drive parameters for SCSI devices
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16/167 Filesystem maintenance
Filesystem maintenance
fsck device Check and repair a Linux filesystem (which must be unmounted).
Corrupted files will be placed into the /lost+found directory of the partition.
The exit code returned is the sum of the following conditions:
0 No errors
1 File system errors corrected
2 System should be rebooted
4 File system errors left uncorrected
8 Operational error
16 Usage or syntax error
32 Fsck canceled by user
128 Shared library error
Fsck is a wrapper utility for the actual filesystem-specific checker commands:
fsck.ext2 aka e2fsck
fsck.ext3 aka e2fsck
fsck.ext4 aka e2fsck
fsck.msdos
fsck.vfat
fsck.cramfs
fsck
fsck -As
Check and repair serially all filesystems listed in /etc/fstab
fsck -f /dev/sda1 Force a filesystem check on /dev/sda1 even if it thinks is not necessary
fsck -y /dev/sda1 During filesystem repair, do not ask questions and assume that the answer is always yes
fsck.ext2 -c /dev/sda1
e2fsck -c /dev/sda1
Check an ext2 filesystem, running the badblocks command to mark all bad blocks and
add them to the bad block inode so they will not be allocated to files or directories
touch /forcefsck (Red Hat) Force a filesystem check after next reboot
tune2fs options device Adjust tunable filesystem parameters on ext2/ext3/ext4 filesystems
tune2fs -l /dev/sda1 List the contents of the filesystem superblock
tune2fs -j /dev/sda1 Add a journal to this ext2 filesystem, making it an ext3
tune2fs -m 1 /dev/sda1 Reserve 1% of the partition size to privileged processes. This space (5% by default, but
can be reduced on modern filesystems) is reserved to avoid filesystem fragmentation
and to allow privileged processes to continue to run correctly when the partition is full
tune2fs -C 7 /dev/sda1 Set the mount count of the filesystem to 7
tune2fs -c 20 /dev/sda1 Set the filesystem to be checked by fsck after 20 mounts
tune2fs -i 15d /dev/sda1 Set the filesystem to be checked by fsck each 15 days
Both mount-count-dependent and time-dependent checking are enabled by default for all hard drives on Linux, to avoid the
risk of filesystem corruption going unnoticed.
dumpe2fs options device Dump ext2/ext3/ext4 filesystem information
dumpe2fs -h /dev/sda1 Display filesystem's superblock information (e.g. number of mounts, last
checks, UUID)
dumpe2fs /dev/sda1 | grep -i superblock Display locations of superblock (primary and backup) of filesystem
dumpe2fs -b /dev/sda1 Display blocks that are marked as bad in the filesystem
debugfs device Interactive ext2/ext3/ext4 filesystem debugger
debugfs -w /dev/sda1 Debug /dev/sda1 in read-write mode
(by default, debugfs accesses the device in read-only mode)
Many hard drives feature the Self-Monitoring, Analysis and Reporting Technology (SMART) whose purpose is to monitor the
reliability of the drive, predict drive failures, and carry out different types of drive self-tests.
The smartd daemon attempts to poll this information from all drives every 30 minutes, logging all data to syslog.
smartctl -a /dev/sda Print SMART information for drive /dev/sda
smartctl -s off /dev/sda Disable SMART monitoring and log collection for drive /dev/sda
smartctl -t long /dev/sda Begin an extended SMART self-test on drive /dev/sda
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17/167 XFS, ReiserFS, CD-ROM fs
XFS, ReiserFS, CD-ROM fs
xfs_growfs options mountpoint Expand an XFS filesystem. For this, there must be at least one spare
new disk partition available. A XFS filesystem cannot be shrunk
xfs_info /dev/sda1
xfs_growfs -n /dev/sda1
Print XFS filesystem geometry
xfs_check options device Check XFS filesystem consistency
xfs_repair options device Repair a damaged or corrupt XFS filesystem
xfsdump -v silent -f /dev/tape / Dump the root of a XFS filesystem to tape, with lowest level of verbosity.
Incremental and resumed dumps are stored in the inventory database
/var/lib/xfsdump/inventory
xfsrestore -f /dev/tape / Restore a XFS filesystem from tape
xfsdump -J - / | xfsrestore -J - /new Copy the contents of a XFS filesystem to another directory (without
updating the inventory database)
reiserfstune options device Adjust tunable filesystem parameters on ReiserFS filesystem
debugreiserfs device Interactive ReiserFS filesystem debugger
mkisofs -r -o cdrom.img data/ Create a CD-ROM image from the contents of the target directory.
Enable Rock Ridge extension and set all content on CD to be public
readable (instead of inheriting the permissions from the original files)
CD-ROM filesystems
Filesystem Commands
ISO9660 mkisofs Create a ISO9660 filesystem
UDF (Universal Disk Format)
mkudffs Create a UDF filesystem
udffsck Check a UDF filesystem
wrudf Maintain a UDF filesystem
cdrwtool Manage CD-RW drives (e.g. disk format, read/write speed)
HFS (Hierarchical File System)
CD-ROM filesystem extensions
Rock Ridge Contains the original file information (e.g. permissions, filename) for MS Windows 8.3 filenames
MS Joliet Used to create more MS Windows friendly CD-ROMs
El Torito Used to create bootable CD-ROMs
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18/167 AutoFS
AutoFS
AutoFS is a client-side service that permits automounting of filesystems, even for nonprivileged users.
AutoFS is composed of the autofs kernel module that monitors specific directories for attempts to access them; in this case,
the kernel module signals the automount userspace daemon which mounts the directory when it needs to be accessed and
unmounts it when is no longer accessed.
Mounts managed by AutoFS should not be mounted/unmounted manually or via /etc/fstab, to avoid inconsistencies.
AutoFS configuration files
/etc/sysconfig/autofs AutoFS configuration file
/etc/auto.master Master map file for AutoFS. Each line is an indirect map, and each map file stores the
configuration for the automounting of the subdir.
# mount point map options
/net -hosts
/- /etc/auto.direct
/misc /etc/auto.misc
/home /etc/auto.home --timeout=60
The -hosts map tells AutoFS to mount/unmount automatically any export from the NFS
server nfsserver when the directory /net/nfsserver/ is accessed.
AutoFS map files
/etc/auto.direct Direct map file for automounting of a NFS share.
# dir filesystem
/mydir nfsserver1.foo.org:/myshare
/etc/auto.misc Indirect map file for automounting of directory /misc .
# subdir options filesystem
public -ro,soft,intr ftp.example.org:/pub
cd -fstype=iso9660,ro,nosuid,nodev :/dev/cdrom
/etc/auto.home Indirect map file for automounting of directory /home on a NFS share.
The * wildcard matches any subdir the system attempts to access, and the & variable takes
the value of the match.
# subdir options filesystem
* -rw,soft,intr nfsserver2.bar.org:/home/&
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19/167 RAID
RAID
RAID levels
Level Description Storage capacity
RAID 0 Striping (data is written across all member disks).
High I/O but no redundancy
Sum of the capacity of member disks
RAID 1 Mirroring (data is mirrored on all disks).
High redundancy but high cost
Capacity of the smaller member disk
RAID 4 Parity on a single disk.
I/O bottleneck unless coupled to write-back caching
Sum of the capacity of member disks,
minus one
RAID 5 Parity distributed across all disks.
Can sustain one disk crash
Sum of the capacity of member disks,
minus one
RAID 6 Double parity distributed across all disks.
Can sustain two disk crashes
Sum of the capacity of member disks,
minus two
RAID 10 (1+0) Striping + mirroring.
High redundancy but high cost
Capacity of the smaller member disk
Linear RAID Data written sequentially across all disks.
No redundancy
Sum of the capacity of member disks
mdadm -C /dev/md0 -l 5 \
-n 3 /dev/sdb1 /dev/sdc1 /dev/sdd1 \
-x 1 /dev/sde1
Create a RAID 5 array from three partitions and a spare.
Partitions type must be set to 0xFD.
Once the RAID device has been created, it must be formatted e.g. via
mke2fs -j /dev/md0
mdadm --manage /dev/md0 -f /dev/sdd1 Mark a drive as faulty, before removing it
mdadm --manage /dev/md0 -r /dev/sdd1 Remove a drive from the RAID array.
The faulty drive can now be physically removed
mdadm --manage /dev/md0 -a /dev/sdd1 Add a drive to the RAID array.
To be run after the faulty drive has been physically replaced
mdadm --misc -Q /dev/sdd1 Display information about a device
mdadm --misc -D /dev/md0 Display detailed information about the RAID array
mdadm --misc -o /dev/md0 Mark the RAID array as readonly
mdadm --misc -w /dev/md0 Mark the RAID array as read & write
/etc/mdadm.conf Configuration file for the mdadm command.
DEVICE /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1
ARRAY /dev/md0 level=raid5 num-devices=3
UUID=0098af43:812203fa:e665b421:002f5e42
devices=/dev/sdb1,/dev/sdc1,/dev/sdd1,/dev/sde1
cat /proc/mdstat Display information about RAID arrays and devices
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20/167 Bootloader
Bootloader
Non-GRUB bootloaders
LILO
(Linux Loader)
Obsolete. Small bootloader that can be placed in the MBR or the boot sector of a partition.
The configuration file is /etc/lilo.conf (run /sbin/lilo afterwards to validate changes).
SYSLINUX
SYSLINUX Able to boot from FAT and NTFS filesystems e.g. floppy disks and USB drives.
Used for boot floppy disks, rescue floppy disks, and Live USBs.
ISOLINUX Able to boot from CD-ROM ISO 9660 filesystems.
Used for Live CDs and bootable install CDs.
The CD must contain the following files:
isolinux/isolinux.bin ISOLINUX image, from the SYSLINUX distro
boot/isolinux/isolinux.cfg ISOLINUX configuration
images/ Floppy images to boot
kernel/memdisk
The CD can be burnt with the command:
mkisofs -o output.iso -b isolinux/isolinux.bin -c isolinux/boot.cat \
-no-emul-boot -boot-load-size 4 -boot-info-table CDrootdir
PXELINUX Able to boot from PXE (Pre-boot eXecution Environment). PXE uses DHCP or BOOTP to enable
basic networking, then uses TFTP to download a bootstrap program that loads and configures
the kernel.
Used for Linux installations from a central server or network boot of diskless workstations.
The boot TFTP server must contain the following files:
/tftpboot/pxelinux.0 PXELINUX image, from the SYSLINUX distro
/tftpboot/pxelinux.cfg/ Directory containing a configuration file for each machine.
A machine with Ethernet MAC address 88:99:AA:BB:CC:DD
and IP address 192.0.2.91 (C000025B in hexadecimal) will
search for its configuration filename in this order:
01-88-99-aa-bb-cc-dd
C000025B
C000025
C00002
C0000
C000
C00
C0
C
default
EXTLINUX General-purpose bootloader like LILO or GRUB. Now merged with SYSLINUX.
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21/167 GRUB 2 configuration
GRUB 2 configuration
GRUB (Grand Unified Bootloader) is the standard boot manager on modern Linux distros. The latest version is GRUB 2; the
older version is GRUB Legacy.
GRUB Stage 1 (446 bytes), as well as the partition table (64 bytes) and the boot signature (2 bytes), is stored in the 512-
byte MBR. It then accesses the GRUB configuration and commands available on the filesystem, usually on /boot/grub .
/boot/grub/grub.cfg or /boot/grub2/grub.cfg GRUB 2 configuration file
# Linux Red Hat
menuentry "Fedora 2.6.32" { # Menu item to show on GRUB bootmenu
set root=(hd0,1) # root filesystem is /dev/hda1
linux /vmlinuz-2.6.32 ro root=/dev/hda5 mem=2048M
initrd /initrd-2.6.32
}
# Linux Debian
menuentry "Debian 2.6.36-experimental" {
set root=(hd0,1)
linux (hd0,1)/bzImage-2.6.36-experimental ro root=/dev/hda6
}
# Windows
menuentry "Windows" {
set root=(hd0,2)
chainloader +1
}
The GRUB 2 configuration file must not be edited manually. Instead, edit the files in /etc/grub.d/ (these are scripts that
will be run in order) and the file /etc/default/grub (the configuration file for menu display settings), then run update-
grub (Debian) or grub2-mkconfig (Red Hat) which will recreate this configuration file.
Common
kernel
parameters:
root= Specify the location of the filesystem root. This is a required parameter
ro Mount read-only on boot
quiet Disable non-critical kernel messages during boot
debug Enable kernel debugging
splash Show splash image
single Boot in single-user mode (runlevel 1)
emergency Emergency mode: after the kernel is booted, run sulogin (single-user login)
which asks for the root password for system maintenance, then run a Bash shell.
Does not load init or any daemon or configuration setting.
init=/bin/bash Run a Bash shell (may also be any other executable) instead of init
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GRUB 2 usage
The GRUB menu, presented at startup, allows choosing the OS or kernel to boot:
Boot the currently selected GRUB entry
Get a GRUB command line
Edit the selected GRUB entry (e.g. to edit kernel parameters in order to boot in single-user emergency mode,
or to change IRQ or I/O port of a device driver compiled in the kernel)
Boot the currently selected GRUB entry (this is usually done after finishing modifying it)
Bring up the GRUB password prompt (necessary if a GRUB password has been set)
grub-install /dev/sda Install GRUB on first SATA drive
grub Access the GRUB shell
grub2-set-default 1 Set GRUB to automatically boot the second entry in the GRUB menu
grub2-editenv list Display the current GRUB menu entry that is automatically booted
/boot/grub/device.map This file can be created to map Linux device filenames to BIOS drives:
(fd0) /dev/fd0
(hd0) /dev/hda
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
ENTER
C
E
B
P
23/167 GRUB Legacy
GRUB Legacy
GRUB Legacy shell commands
blocklist file Print the block list notation of a file kernel file Load a kernel
boot Boot the loaded OS lock Lock a GRUB menu entry
cat file Show the contents of a file makeactive Set active partition on root disk to
GRUB's root device
chainloader file Chainload another bootloader map drive1 drive2 Map a drive to another drive
cmp file1 file2 Compare two files md5crypt Encrypt a password in MD5 format
configfile file Load a configuration file module file Load a kernel module
debug Toggle debugging mode modulenounzip file Load a kernel module without
decompressing it
displayapm Display APM BIOS information pause message Print a message and wait for a key
press
displaymem Display memory configuration quit Quit the GRUB shell
embed stage device Embed Stage 1.5 in the device reboot Reboot the system
find file Find a file read address Read a 32-bit value from memory
and print it
fstest Toggle filesystem test mode root device Set the current root device
geometry drive Print information on a drive
geometry
rootnoverify device Set the current root device without
mounting it
halt Shut down the system savedefault Save current menu entry as the
default entry
help command Show help for a command, or the
available commands
setup device Install GRUB automatically on the
device
impsprobe Probe the Intel Multiprocessor
Specification
testload file Test the filesystem code on a file
initrd file Load an initial ramdisk image file testvbe mode Test a VESA BIOS EXTENSION
mode
install options Install GRUB (deprecated, use
setup instead)
uppermem kbytes Set the upper memory size (only
for old machines)
ioprobe drive Probe I/O ports used for a drive vbeprobe mode Probe a VESA BIOS EXTENSION
mode
/boot/grub/menu.lst or /boot/grub/grub.conf GRUB Legacy configuration file
timeout 10 # Boot the default kernel after 10 seconds
default 0 # Default kernel is 0
# Section 0: Linux boot
title Debian # Menu item to show on GRUB bootmenu
root (hd0,0) # root filesystem is /dev/hda1
kernel /boot/vmlinuz-2.6.24-19-generic root=/dev/hda1 ro quiet splash
initrd /boot/initrd.img-2.6.24-19-generic
# Section 1: Windows boot
title Microsoft Windows XP
root (hd0,1) # root filesystem is /dev/hda2
savedefault
makeactive # set the active flag on this partition
chainloader +1 # read 1 sector from start of partition and run
# Section 2: Firmware/BIOS update from floppy disk
title Firmware update
kernel /memdisk # boot a floppy disk image
initrd /floppy-img-7.7.7
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Low-level package managers
Low-level package managers Debian Red Hat
Install a package file dpkg -i package.deb rpm -i package.rpm
rpm -i ftp://host/package.rpm
rpm -i http://host/package.rpm
Remove a package dpkg -r package rpm -e package
Upgrade a package
(and remove old versions)
rpm -U package.rpm
Upgrade a package
(only if an old version is already installed)
rpm -F package.rpm
List installed packages and their state dpkg -l rpm -qa
List installed packages and their installation
date, from newest to oldest
rpm -qa --last
List the content of an installed package dpkg -L package rpm -ql package
List the content of a package file dpkg -c package.deb rpm -qpl package.rpm
Show the package containing a specific file dpkg -S file rpm -qf file
Verify an installed package rpm -V package
Reconfigure a package dpkg-reconfigure package
Install a package source file rpm -i package.src.rpm
Compile a package source file rpm -ba package.spec
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High-level package managers
High-level package managers Debian Red Hat
Install a package apt-get install package yum install package
Install a package file yum install package.rpm
yum localinstall package.rpm
Remove a package apt-get remove package yum remove package
Upgrade an installed package yum update package
Upgrade all installed packages apt-get upgrade yum update
Upgrade all installed packages and handle
dependencies with new versions
apt-get dist-upgrade
Replace a package with another yum swap packageout packagein
Get the source code for a package apt-get source package
Check for broken dependencies and update
package cache
apt-get check
Fix broken dependencies apt-get install -f
Update information on available packages apt-get update
List all installed and available packages yum list
List installed and available packages that
match the search term
yum list searchterm
List installed packages yum list installed
List packages available for install yum list available
Search for a package apt-cache search package
Search for packages that match the search
term in the package name or summary
yum search searchterm
Search for packages that match the search
term in the package name, summary, or
description
yum search all searchterm
Show package dependencies apt-cache depends package yum deplist package
Show package records apt-cache show package yum list package
Show information about a package apt-cache showpkg package yum info package
Show the installation history (installs,
updates, etc.)
yum history
yum history list
Show the installation history about a package yum history package package
yum history list package package
Update information about package contents apt-file update
List the content of an uninstalled package apt-file list package
Show which package provides a specific file apt-file search file yum whatprovides file
Add a CD-ROM to the sources list apt-cdrom add
Download package and all its dependencies yumdownloader --resolve package
Show URLs that would be downloaded yumdownloader --urls package
Try to complete unfinished or aborted package
installations
yum-complete-transaction
Execute the command but only considering a
specific repository
yum command --disablerepo="*"
--enablerepo="repository"
Print list of available repositories cat /etc/apt/sources.list yum repolist
cat /etc/yum.repos.d/*.repo
Package format compressed with ar compressed with cpio
High-level package managers are able to install remote packages and automatically solve dependencies.
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Package management tools
GUI package managers Debian Red Hat
Manage packages and dependencies using a
graphical or text-based UI
aptitude pirut
dselect
synaptic
Package management utilities Debian Red Hat
Convert a RPM package to DEB and install it.
May break the package system!
alien -i package.rpm
Convert a RPM package to a cpio archive rpm2cpio package.rpm
Add a key to the list of keys used to
authenticate packages
apt-key add keyfile
Create an XML file of repository metadata
from the set of RPMs contained in directory
createrepo directory
Show a tree with all dependencies of package repoquery --tree-requires package
Register a system to the RHSM (Red Hat
Subscription Management) portal
subscription-manager register
Attach a RHSM subscription to a registered
system
subscription-manager attach
/etc/yum.repos.d/foobar.repo Configuration file for a "foobar" repository (Red Hat)
[foobar] Repository ID
name=Foobar $releasever - $basearch Repository name
baseurl=http://download.foobarproject.org/pub/linux/\
releases/$releasever/Everything/$basearch/os/
http://foo.org/linux/$releasever/$basearch/
http://bar.org/linux/$releasever/$basearch/
List of URLs to the repository's repodata
directory. Can be any of these types:
file:/// local file
file:// NFS
http:// HTTP
https:// HTTPS
ftp:// FTP
enabled=1 Whether this repository is enabled
gpgcheck=1 Whether to perform a GPG signature check on
the packages downloaded from this repository
failovermethod=priority Makes yum try the baseurls in the order they're
listed. By default, if more than one baseurl is
specified, yum chooses one randomly
metalink=https://mirrors.foobarproject.org/metalink?repo=\
foobar-$releasever&arch=$basearch
URL to a metalink file that specifies the list of
mirrors to use. Can be used with or in
alternative to a baseurl
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-foobar-\
$releasever-$basearch
ASCII-armored GPG public key file of the
repository
The manpage man yum.conf lists all repository configuration options.
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
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Backup
dd if=/dev/sda of=/dev/sdb
cat /dev/sda > /dev/sdb
Copy the content of one hard disk over another, byte by byte
dd if=/dev/sda1 of=sda1.img Generate the image file of a partition
dd if=/dev/cdrom of=cdrom.iso bs=2048 Create an ISO file from a CD-ROM, using a block size transfer of 2 Kb
dd if=install.iso of=/dev/sdc bs=512k Write an installation ISO file to a device (e.g. a USB thumb drive)
It is recommended not to use dd on a mounted block device because of write cache issues.
rsync -rzv /home /tmp/bak
rsync -rzv /home/ /tmp/bak/home
Synchronize the content of the home directory with the temporary
backup directory. Use recursion, compression, and verbosity.
For all transfers subsequent to the first, rsync only copies the blocks that
have changed, making it a very efficient backup solution in terms of
speed and bandwidth
rsync -avz /home root@10.0.0.7:/backup/ Synchronize the content of the home directory with the backup directory
on the remote server, using SSH. Use archive mode (i.e. operates
recursively and preserves owner, group, permissions, timestamps, and
symlinks)
burp Backup and restore program
Tape libraries
Devices
/dev/st0 First SCSI tape device
/dev/nst0 First SCSI tape device (no-rewind device file)
Utility for magnetic tapes mt -f /dev/nst0 asf 3 Position the tape at the start of 3rd file
Utility for tape libraries
mtx -f /dev/sg1 status Display status of tape library
mtx -f /dev/sg1 load 3 Load tape from slot 3 to drive 0
mtx -f /dev/sg1 unload Unload tape from drive 0 to original slot
mtx -f /dev/sg1 transfer 3 4 Transfer tape from slot 3 to slot 4
mtx -f /dev/sg1 inventory Force robot to rescan all slots and drives
mtx -f /dev/sg1 inquiry Inquiry about SCSI media device
(Medium Changer = tape library)
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Archive formats
cpio
ls | cpio -o > archive.cpio
ls | cpio -oF archive.cpio
Create a cpio archive of all files in the current directory
find /home/ | cpio -o > archive.cpio Create a cpio archive of all users' home directories
cpio -id < archive.cpio Extract all files, recreating the directory structure
cpio -i -t < archive.cpio List the contents of a cpio archive file
gzip
gzip file Compress a file with gzip
gzip < file > file.gz Compress a file with gzip, leaving the original file into place
gunzip file.gz Decompress a gzip-compressed file
gunzip -tv file.gz Test the integrity of a gzip-compressed file
zcat file.gz Read a gzip-compressed text file
zgrep pattern file.gz grep for a gzip-compressed text file
zless file.gz less for a gzip-compressed text file
zmore file.gz more for a gzip-compressed text file
bzip2
bzip2 file Compress a file with bzip2
bunzip2 file.bz2 Decompress a bzip2-compressed file
bzcat file.bz2 Read a bzip2-compressed text file
7-Zip 7z a -t7z archive.7z dir/Create a 7-Zip archive (has the highest compression ratio)
xz
xz file Compress a file with xz
unxz file.xz
xz -d file.xz
Decompress a xz-compressed file
xzcat file.xz Read a xz-compressed file
LZMA
lzma file
xz --format=lzma file
Compress a file with LZMA
unlzma file.lzma
xz --format=lzma -d file.lzma
Decompress a LZMA-compressed file
lzcat file.lzma
xz --format=lzma --d --stdout file.lzma
Read a LZMA-compressed file
rar
rar a archive.rar dir/Create a RAR archive
unrar x archive.rar Extract a RAR archive
tar
tar cf archive.tar dir/Create a tarred archive (bundles multiple files in a single one)
tar czf archive.tar.gz dir/ Create a tarred gzip-compressed archive
tar xzf archive.tar.gz Extract a tarred gzip-compressed archive
tar cjf archive.tar.bz2 dir/Create a tarred bzip2-compressed archive
tar xjf archive.tar.bz2 Extract a tarred bzip2-compressed archive
tar cJf archive.tar.xz dir/Create a tarred xz-compressed archive
tar xJf archive.tar.xz Extract a tarred xz-compressed archive
tar tf archive.tar List the contents of a tarred archive
star
star -c -f=archive.star dir/Create a star archive
star -x -f=archive.star Extract a star archive
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
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Documentation
man command Show the manpage for a command
man 7 command Show section 7 of the command manpage
man man Show information about manpages' sections:
1 - Executable programs or shell commands
2 - System calls (functions provided by the kernel)
3 - Library calls (functions within program libraries)
4 - Special files
5 - File formats and conventions
6 - Games
7 - Miscellaneous
8 - System administration commands (usually only for root)
9 - Kernel routines
mandb Generate or refresh the search database for manpage entries. This must be done after
installing new packages, in order to obtain meaningful results from apropos or man -k
apropos keyword
man -k keyword
Show the commands whose manpage's short description matches the keyword.
Inverse of the whatis command
apropos -r regex
man -k regex
Show the commands whose manpage's short description matches the regex
man -K regex Show the commands whose manpage's full text matches the regex
whatis command Show the manpage's short description for a command
info command Show the Info documentation for a command
help Show the list of available shell commands and functions
help command Show help about a shell command or function
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Shell basics
history Show the history of command lines executed up to this moment.
Commands prepended by a space will be executed but will not show up in the history.
After the user logs out from Bash, history is saved into ~/.bash_history
!nExecute command number n in the command line history
history -c Clear the command line history
history -d nDelete command number n from the command line history
alias ls='ls -lap' Set up an alias for the ls command
alias Show defined aliases
unalias ls Remove the alias for the ls command
\ls
/bin/ls
Run the non-aliased version of the ls command
Almost all Linux commands accept the option -v (verbose), and some commands also accept the options -vv or -vvv
(increasing levels of verbosity).
All Bash built-in commands, and many other commands, accept the flag -- which denotes the end of options and the start
of positional parameters:
grep -- -i file Search for the string "-i" in file
rm -- -rf Delete a file called "-rf"
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Text filters
cat file Print a text file
cat file1 file2 > file3 Concatenate text files
cat file1 > file2
> file2 < file1 cat
Copy file1 to file2. The cat command is able to operate on binary streams as well
and therefore it works also with binary files (e.g. JPG images)
cat > file <<EOF
line 1
line 2
line 3
EOF
Create a Here Document, storing the lines entered in input to file
command <<< 'string'Create a Here String, passing string as input to command
tac file Print or concatenate text files in opposite order line-wise, from last line to first line
rev file Print a text file with every line reversed character-wise, from last char to first char
head file
head -n 10 file
Print the first 10 lines of a text file
tail file
tail -n 10 file
Print the last 10 lines of a text file
tail -f file Output appended data as the text file grows. Useful to read a logfile in real-time
column file Format a text file into columns
pr file Format a text file for a printer
fmt -w 75 file Format a text file so that each line has a max width of 75 characters
fold -w40 file Wrap each line of a text file to 40 characters
nl file Prepend line numbers to a text file
wc file Print the number of lines, words, and bytes of a text file
join file1 file2 Join lines of two text files on a common field
paste file1 file2 Merge lines of text files
split -l 1 file Split a text file into 1-line files (named xaa, xab, xac, and so on)
uniq file Print the unique lines of a text file, omitting consecutive identical lines
sort file Sort alphabetically the lines of a text file
shuf file Shuffle randomly the lines of a text file
expand file Convert tabs into spaces
unexpand file Convert spaces into tabs
od file Dump a file into octal (or other formats)
diff file1 file2 Compare two text files line by line and print the differences
cmp file1 file2 Compare two files and print the differences
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
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Advanced text filters
cut -d: -f3 file Cut the lines of a file, considering : as the delimiter and printing only the 3rd field
cut -d: -f1 /etc/passwd Print the list of user accounts in the system
cut -c3-50 file Print character 3 to 50 of each line of a file
sed 's/foo/bar/' file Stream Editor: Replace the first occurrence on a line of "foo" with "bar" in file, and
print on stdout the result
sed -i 's/foo/bar/' file Replace "foo" with "bar", overwriting the results in file
sed 's/foo/bar/g' file Replace all occurrences of "foo" with "bar"
sed '0,/foo/s//bar/' file Replace only the first line match
sed -n '7,13p' file Print line 7 to 13 of a text file
sed "s/foo/$var/" file Replace "foo" with the value of variable $var.
The double quotes allow for variable expansion
tr a-z A-Z <file
tr [:lower:] [:upper:] <file
Translate characters: Convert all lowercase into uppercase in a text file
tr -d 0-9 <file
tr -d [:digit:] <file
Delete all digits from a text file
awk Interpreter for the AWK programming language, designed for text processing and
data extraction
grep foo file Print the lines of a file containing "foo"
grep -v foo file Print the lines of a file not containing "foo"
grep -e foo -e bar file
grep -E 'foo|bar' file
Print the lines of a file containing "foo" or "bar"
grep -v -e foo -e bar file Print the lines of a file containing neither "foo" nor "bar"
grep -E regex file
egrep regex file
Print the lines of a file matching the given Extended Regex
tidy Validate, correct, and tidy up the markup of HTML, XHTML, and XML files
tidy -asxml -xml \
-indent -wrap 2000 -quiet \
--hide-comments yes file.xml
Strip out comments from an XML file
strings file Show all printable character sequences at least 4-character long that are inside a
file
antiword docfile Show text and images from a MS Word document
catdoc docfile Output plaintext from a MS Word document
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Regular expressions
^Beginning of a line
$End of a line
\< \> Word boundaries (beginning of line, end of line, space, or punctuation mark)
.Any character except newline
[abc] Any of the characters specified
[a-z] Any of the characters in the specified range
[^abc] Any character except those specified
*Zero or more times the preceding regex
+One or more times the preceding regex
?Zero or one time the preceding regex
{5} Exactly 5 times the preceding regex
{5,} 5 times or more the preceding regex
{,10} At most 10 times the preceding regex
{5,10} Between 5 and 10 times the preceding regex
|The regex either before or after the vertical bar
( ) Grouping, to be used for back-references. \1 expands to the 1st match, \2 to the 2nd, and so on until \9
The symbols above are used in POSIX EREs (Extended Regular Expressions).
In POSIX BREs (Basic Regular Expressions), the symbols ? + { | ( ) need to be escaped (i.e. prepended with a backslash
character \).
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File management
cp file file2 Copy a file
Common options:
-i Prompt before overwriting/deleting files (interactive)
-f Don't ask before overwriting/deleting files (force)
cp file dir/ Copy a file to a directory
cp -ar /dir1/. /dir2/Copy a directory recursively
mv file file2 Rename a file
mv file dir/ Move a file to a directory
rm file Delete a file
pv file > file2 Copy a file, monitoring the progress of data through a pipe
touch file Change access timestamp and modify timestamp of a file as now.
If the file does not exist, it is created
mktemp Create a temporary file or directory, using tmp.XXXXXXXXXX as filename template
ls List the contents of the current directory
ls -d */ List only directories contained on the current directory
ls -lap --sort=v List files, sorted by version number
stat file Display file or filesystem status
stat -c %A file Display file permissions
stat -c %s file Display file size, in bytes
shred /dev/hda Securely wipe the contents of a device
shred -u file Securely delete a file
fdupes dir Examines a directory for duplicate files in it. To consider files a duplicate, first compares file
sizes and MD5 signatures, then compares the file contents byte-by-byte
tmpwatch Remove files which have not been accessed for a period of time
lsof List all open files
lsof -u user List all files currently open by user
lsof -i List open files and their sockets (equivalent to netstat -ap)
lsof -i :80 List connections of local processes on port 80
lsof -i@10.0.0.3 List connections of local processes to remote host 10.0.0.3
lsof -i@10.0.0.3:80 List connections of local processes to remote host 10.0.0.3 on port 80
lsof -c mysqld List all files opened by mysqld, the MySQL daemon
lsof file List all processes using a specific file
lsof +L1 List all processes using an unlinked file. These processes, until killed or restarted, hold the
file open preventing it from being deleted (and freeing disk space)
lslocks List information about all currently held file locks
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Directory management
cd directory Change to the specified directory
cd - Change to the previously used directory
pwd Print the current working directory
mkdir dir Create a directory
mkdir -m 755 dir Create a directory with mode 755
mkdir -p /dir1/dir2/dir3 Create a directory, creating also the parent directories if they don't exist
rmdir dir Delete a directory (which must be empty)
tree List directories and their contents in hierarchical format
pushd dir Add a directory to the top of the directory stack and make it the current working
directory
popd Remove the top directory from the directory stack and change to the new top directory
dirs Display the directory stack (i.e. the list of remembered directories)
dirname file Output the directory path in which the file is located, stripping any non-directory suffix
from the filename
Bash directory shortcuts
.Current directory
.. Parent directory
~Home directory of current user
~jdoe Home directory of user jdoe
~- Previously used directory
File-naming wildcards (globbing)
*Matches zero or more characters
?Matches one character
[kxw] Matches k, x, or w
[!kxw] Matches any character except k, x, or w
[a-z] Matches any character between a and z
Brace expansion
cp foo.{txt,bak} Copy file "foo.txt" to "foo.bak"
touch foo_{a,b,c}
touch foo_{a..c} Create files "foo_a", "foo_b", "foo_c"
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I/O streams
In Linux, everything is (displayed as) a file. File descriptors are automatically associated to any process launched.
File descriptors
# Name Type Default device Device file
0 Standard input (stdin) Input text stream Keyboard /dev/stdin
1 Standard output (stdout) Output text stream Terminal /dev/stdout
2 Standard error (stderr) Output text stream Terminal /dev/stderr
cat /etc/passwd | wc -l Pipe the stdout of command cat to the stdin of command wc (hence printing the number
of accounts in the system). Piped commands run concurrently
ls > file
ls 1> file
Redirect the stdout of command ls to file (hence writing on file the content of the
current directory). This overwrites file if it already exists, unless the Bash noclobber
option is set (via set -o noclobber). The redirection is handled by the shell, not by the
command invoked
ls >| file Redirect the stdout of command ls to file, even if noclobber is set
ls >> file
ls 1>> file
Append the stdout of command ls to file
ls 2> file Redirect the stderr of command ls to file (hence writing any error encountered by the
command to file)
ls 2>> file Append the stderr of command ls to file
ls 2> /dev/null Silence any error coming from command ls
mail user@foo.com < file Redirect file to the stdin of command mail (hence sending via e-mail the contents of file
to the specified email address)
echo "$(sort file)" > file
echo "`sort file`" > file
sort file | sponge file
Sort the contents of file and write the output to the file itself.
sort file > file would not produce the desired result, because the stdout destination
is created (and therefore the content of the preexisting file is deleted) before the sort
command is run
ls 2>&1 Redirect stderr of command ls to stdout
ls > file 2>&1
ls &> file †
ls >& file †
Redirect both stdout and stderr of command ls to file
† = non-POSIX standard and therefore not recommended
> file Create an empty file. If the file exists, its content will be deleted
ls | tee file tee reads from stdin and writes both to stdout and file (hence writing content of current
directory to screen and to file at the same time)
ls | tee -a file tee reads from stdin and appends both to stdout and file
ls foo* | xargs cat xargs calls the cat command multiple times for each argument found on stdin
(hence printing the content of every file whose name starts by "foo")
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read and echo
while read -r line
do
echo "Hello $line"
done < file
Process a text file line by line, reading from file.
If file is /dev/stdin, reads from standard input instead
read MYVAR Read a variable from standard input
read -n 8 MYVAR Read only max 8 chars from standard input
read -t 60 MYVAR Read a variable from standard input, timing out after one minute
read -s MYVAR Read a variable from standard input without echoing to terminal (silent mode)
echo $MYVAR Print a variable on screen
echo -n "message"
printf "message"
Print message onscreen without a trailing line feed
echo -e '\a' Produce an alert sound (BEL sequence)
pv -qL10 <<< "message"Print message onscreen, one character at a time
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
38/167 Processes
Processes
Any application, program, or script that runs on the system is a process. Signals are used for inter-process communication.
Each process has a unique PID (Process ID) and a PPID (Parent Process ID); when a process spawns a child, the process
PID is assigned to the child's PPID.
The /sbin/init process, run at bootup, has PID 1. It is the ancestor of all processes and becomes the parent of any
orphaned process. It is also unkillable; should it die, the kernel will panic.
When a child process dies, its status becomes EXIT_ZOMBIE and a SIGCHLD is sent to the parent. The parent should then
call the wait() system call to read the dead process' exit status and other info; until that moment, the child process
remains a zombie.
ps -ef (UNIX options)
ps aux (BSD options)
List all processes
pstree PID Display all processes in hierarchical format.
The process tree is rooted at PID, or at init if PID is omitted
pidof process Show PID of process
top Monitor processes in real-time
htop Monitor processes in real-time (ncurses UI)
ipcs Show IPC facilities information (shared memory, message queues, and semaphores)
pmap PID Display the memory map of process PID
kill -9 1138 Send a signal 9 (SIGKILL) to process 1138, hence killing it
killall -9 sshd Kill processes whose name is "sshd"
pgrep sshd
ps -ef | grep "[s]shd"
Show processes whose name is "sshd"
pgrep -u root sshd Show processes whose name is "sshd" and are owned by root
pkill -9 -u root sshd Kill processes whose name is "sshd" and are owned by root
xkill Interactive program to kill a process by its X GUI resource
strace command Trace the execution of command, intercepting and printing the system calls called by a
process and the signals received by a process
jobs List all jobs (i.e. processes whose parent is a Bash shell)
Suspend a job, putting it in the stopped state (send a SIGTSTP)
bg %1 Put job #1 in the background (send a SIGCONT)
fg %1 Resume job #1 in the foreground and make it the current job (send a SIGCONT)
kill %1 Kill job #1
To each process is associated a niceness value: the higher the niceness, the lower the priority.
The niceness value ranges from -20 to 19, and a newly created process has a default niceness of 0.
Unprivileged users can modify a process' niceness only within the range from 1 to 19.
nice -n -5 command Start a command with a niceness of -5. If niceness is omitted, a default value of 10 is used
renice -5 command Change the niceness of a running command to -5
( command )& pid=$!; sleep n; kill -9 $pid Run a command and kill it after n seconds
:(){ :|:& };: Fork bomb: starts a process that continually replicates itself, slowing
down or crashing the system because of resource starvation
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CTRL Z
39/167 Signals
Signals
Most frequently used signals
Signal number Signal name Meaning
1 SIGHUP Used by many daemons to reload their configuration
2 SIGINT Interrupt, stop
9 SIGKILL Kill unconditionally (this signal cannot be ignored)
15 SIGTERM Terminate gracefully
18 SIGCONT Continue execution
20 SIGTSTP Stop execution
The manpage man 7 signal lists all signal numbers and names.
kill -l List all available signal names
kill -l nPrint the name of signal number n
trap action condition Trap a signal
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40/167 Resource monitoring
Resource monitoring
vmstat Print a report about virtual memory statistics: processes, memory, paging, block I/O,
traps, disks, and CPU activity
iostat Print a report about CPU utilization, device utilization, and network filesystem.
The first report shows statistics since the system boot; subsequent reports will show
statistics since the previous report
mpstat Print a report about processor activities
vmstat 2 5
iostat 2 5
mpstat 2 5
Print the relevant report every 2 seconds, for 5 times
iotop Display I/O usage by processes in the system
atop Advanced system monitor that displays the load on CPU, RAM, disk, and network
free Show the amount of free and used memory in the system
uptime Show how long the system has been up, how many users are connected, and the system
load averages for the past 1, 5, and 15 minutes
time command Execute command and, at its completion, write to stderr timing statistics about the run:
elapsed real time between invocation and termination, user CPU time, system CPU time
sar Show reports about system activity.
Reports are generated from data collected via the cron job sysstat and stored in
/var/log/sa/sn, where n is the day of the month
sar -n DEV Show reports about network activity (received and transmitted packets per second)
sar -f /var/log/sa/s19 \
-s 06:00:00 -e 06:30:00
Show reports for system activity from 6 to 6:30 AM on the 19th of the month
powertop Power consumption and power management diagnosis tool
sysbench Multi-threaded benchmark tool able to monitor different OS parameters: file I/O,
scheduler, memory allocation, thread implementation, databases
inxi Debugging tool to rapidly and easily gather system information and configuration
Linux monitoring tools
collectd System statistics collector
Nagios System monitor and alert
MRTG Network load monitor
Cacti Network monitor
Munin System and network monitor and alert
Zabbix System and network monitor and alert
Centreon System and network monitor and alert
netdata Real-time performance and health monitor
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41/167 vmstat and free
vmstat and free
Output of command vmstat
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
r b swpd free buff cache si so bi bo in cs us sy id wa st
0 0 0 296724 267120 3393400 0 0 17 56 0 3 2 2 95 1 0
procs rNumber of runnable processes (running or waiting for run time)
bNumber of processes in uninterruptible sleep
memory
swpd Virtual memory used (swap)
in Kb
free Free memory (idle)
buff Memory used as buffers
cache Memory used as cache
swap si Memory swapped in from disk in Kb/second
so Memory swapped out to disk
io bi Blocks received in from a block device in blocks/second
bo Blocks sent out to a block device
system in Number of interrupts per second
cs Number of context switches
cpu
us Time spent running user code (non-kernel)
in percentage of total CPU time
sy Time spent running system code (kernel)
id Time spent idle
wa Time spent waiting for I/O
st Time stolen from a virtual machine
Output of command free
total used free shared buff/cache available
Mem: 16344088 2273312 11531400 776228 2539376 12935112
Swap: 1048572 0 1048572
total used free shared buffers cached
Mem: 1504544 1491098 13021 0 91112 764542
-/+ buffers/cache: 635212 869498
Swap: 2047686 7667 2040019
Mem
total Total configured amount of memory
used Used memory
free Unused memory
shared Memory used by tmpfs, 0 if not available
buff/cache Memory used by kernel buffers, page cache, and slabs
available Memory available for new applications (without using swap) *
-/+ buffers/cache used Memory used by kernel buffers
free Memory available for new applications (without using swap) *
Swap
total Total configured amount of swap space
used Used swap space
free Free swap space *
* These are the true values indicating the free system resources available. All values are in Kb, unless options are used.
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
42/167 File permissions
File permissions
- r w x r w x r w x
Permission Octal value Command Effect on file Effect on directory
Read
user: 400 chmod u+r
Can open and read the file Can list directory contentgroup: 40 chmod g+r
others: 4 chmod o+r
Write
user: 200 chmod u+w
Can modify the file Can create, delete, and rename files in
the directory
group: 20 chmod g+w
others: 2 chmod o+w
Execute
user: 100 chmod u+x
Can execute the file (binary
or script)
Can enter the directory, and search files
within (by accessing a file's inode)
group: 10 chmod g+x
others: 1 chmod o+x
SetUID (SUID) 4000 chmod u+s Executable is run with the
privileges of the file's owner No effect
SetGID (SGID) 2000 chmod g+s Executable is run with the
privileges of the file's group
All new files and subdirectories inherit
the directory's group ID
Sticky 1000 chmod +t No effect Files inside the directory can be deleted
or moved only by the file's owner
chmod 711 file
chmod u=rwx,go=x file
Set read, write, and execute permission to user; set execute permission to group and others
chmod u+wx file Add write and execute permission to user
chmod -x file Remove execute permission from everybody (user, group, and others)
chmod -R g+x /path Set the group execute bit recursively on path and every dir and file underneath
find /path -type d \
-exec chmod g+x {} \;
Set the group execute bit recursively on path and every dir, but not file, underneath
chown user file Change the owner of the file to user
chown user:group file Change the owner of the file to user, and group ownership of the file to group
chown :group file
chgrp group file
Change group ownership of the file to group
umask 022 Set the permission mask to 022, hence masking write permission for group and others.
Linux default permissions are 0666 for files and 0777 for directories. These base
permissions are ANDed with the inverted umask value to calculate the final permissions of a
new file or directory
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
- = regular file
d = directory
l = symbolic link
s = Unix domain socket
p = named pipe
c = character device file
b = block device file
- = regular file
d = directory
l = symbolic link
s = Unix domain socket
p = named pipe
c = character device file
b = block device file
user (owner)
r = read
w = write
x = execute
s = setUID and execute
S = setUID and not execute
user (owner)
r = read
w = write
x = execute
s = setUID and execute
S = setUID and not execute
group
r = read
w = write
x = execute
s = setGID and execute
S = setGID and not execute
group
r = read
w = write
x = execute
s = setGID and execute
S = setGID and not execute
others
r = read
w = write
x = execute
t = sticky and execute
T = sticky and not execute
others
r = read
w = write
x = execute
t = sticky and execute
T = sticky and not execute
43/167 File attributes
File attributes
chattr +attribute file Add a file or directory attribute
chattr -attribute file Remove a file or directory attribute
chattr =attribute file Set a file or directory attribute, removing all other attributes
lsattr file List file or directory attributes
Attribute Effect
aFile can only be opened in append mode for writing
AWhen file is accessed, its atime record is not modified
cFile is automatically compressed on-the-fly on disk by the kernel
CFile is not subject to copy-on-write updates. This applies only to filesystems which perform copy-on-write
dFile will not be backed up by the dump program
DWhen directory is modified, changes are written synchronously on disk. Equivalent to dirsync mount option
eFile is using extents for mapping the blocks on disk
ECompression error on file. This attribute is used by experimental compression patches
hFile stores its blocks in units of filesystem blocksize instead of in units of sectors, and is larger than 2 Tb
iFile is immutable i.e. cannot be modified, linked, or changed permissions
IDirectory is being indexed using hashed trees
jAll file data is written to the ext3 or ext4 journal before being written to the file itself
NFile has data stored inline within the inode itself
sFile will be securely wiped by zeroing when deleted
S When file is modified, changes are written synchronously on disk. Equivalent to sync mount option
tFile will not have EOF partial block fragment merged with other files. This applies only to filesystems with
support for tail-merging
TDirectory is the top of directory hierarchies for the purpose of the Orlov block allocator
uAfter file is deleted, it can be undeleted
XRaw contents of compressed file can be accessed directly. This attribute is used by experimental
compression patches
ZCompressed file is dirty. This attribute is used by experimental compression patches
Timestamp Value tracked Command to show
mtime Time of last modification to file contents (data itself) ls -l
ctime Time of last change to file contents or metadata (owner, group, or permissions) ls -lc
atime Time of last access to file for reading contents ls -lu
The POSIX standard does not define a timestamp for file creation. Some filesystems (e.g. ext4, JFS, Btrfs) store this value,
but currently there is no Linux kernel API to access it.
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44/167 ACLs
ACLs
Access Control Lists (ACLs) provide a fine-grained set of permissions that can be applied to files and directories.
An access ACL is set on an individual file or directory; a default ACL is set on a directory, and applies to all files and
subdirs created inside it that don't have an access ACL.
The final permissions are the intersection of the ACL with the chmod/umask value.
A partition must have been mounted with the acl option in order to support ACLs on files.
setfacl -m u:user:permissions file Set an access ACL on a file for an user
setfacl -m g:group:permissions file Set an access ACL on a file for a group
setfacl -m m:permissions file Set the effective rights mask on a file
setfacl -m o:permissions file Set the permissions on a file for other users
setfacl -x u:user file Remove an access ACL from a file for an user
setfacl -x g:group file Remove an access ACL from a file for a group
The permissions are standard Unix permissions specified as any combination of r w x.
setfacl -m d:u:user:permissions dir
setfacl -d -m u:user:permissions dir
As above, but set a default ACL instead of an access ACL.
This applies to all commands above
getfacl file Display the access (and default, if any) ACL for a file
getfacl file1 | setfacl --set-file=- file2 Copy the ACL of file1 and apply it to file2
getfacl --access dir | setfacl -d -M- dir Copy the access ACL of a directory and set it as default ACL
chacl options Change an ACL. This is an IRIX-compatibility command
man acl Show the manpage about ACLs
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45/167 Links
Links
A Linux directory contains a list of structures which are associations between a filename and an inode.
An inode contains all file metadata: file type, permissions, owner, group, size, access/change/modification/deletion times,
number of links, attributes, ACLs, and address where the actual file content (data) is stored.
An inode does not contain the name of the file; this information is stored in the directory where the file is.
ls -i Show a listing of the directory with the files' inode numbers
df -i Report filesystem inode usage
Hard link Soft or symbolic link
Definition A link to an already existing inode A path to a filename; a shortcut
Command to create it ln file hardlink ln -s file symlink
Link is still valid if the original file
is moved or deleted
Yes (because the link references the
inode the original file pointed to)
No (because the path now references a
non-existent file)
Can link to a file in another
filesystem
No (because inode numbers make sense
only within a determinate filesystem) Yes
Can link to a directory No Yes
Link permissions Reflect the original file's permissions,
even when these are changed rwxrwxrwx
Link attributes - (regular file) l (symbolic link)
Inode number The same as the original file A new inode number
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46/167 Find system files
Find system files
find / -name "foo*"
find / -name "foo*" -print
Find all files, starting from the root dir, whose name start with foo
find / -name "foo*" -exec chmod 700 {} \; Find all files whose name start with "foo" and apply permission 700 to
all of them
find / -name "foo*" -ok chmod 700 {} \; Find all files whose name start with "foo" and apply permission 700 to
all of them, asking for confirmation before each file
find / -size +128M Find all files larger than 128 Mb
find / -ctime +10 Find all files created more than 10 days ago
find / -perm -4000 -type f Find all files of type file (i.e. not directories) and with SUID set
(a possible security risk, because a shell with SUID root is a backdoor)
find / -perm -2000 -type f Find all files with SGID set
find /home/jdoe/path -type f \
-newermt "May 4 14:50" -delete
Find and delete all files newer than the specified datetime.
Using -delete is preferable to using -exec rm {} \;
find . -type f -print -exec cat {} \; Print all files in the current directory with a filename header
locate command
slocate command
Locate command by searching the file index /etc/updatedb.conf,
not by actually walking the filesystem. The search is fast but will only
held results relative to the last rebuilding of the file index
updatedb Rebuild the file index
which command Locate a binary executable command within the PATH
which -a command Locate all matches of a command, not only the first one
whereis command Locate the binary, source, and manpage files for a command
whereis -b command Locate the binary files for a command
whereis -s command Locate the source files for a command
whereis -m command Locate the manpage files for a command
type command Determine if a command is a program or a built-in (i.e. an internal
feature of the shell)
file file Analyze the content of a file or directory, and display the kind of file
(e.g. executable, text file, program text, swap file)
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47/167 Shell variables
Shell variables
The scope of variables is the current shell only, while environment variables are visible within the current shell as well as
within all subshells and Bash child processes spawned by the shell.
Environment variables are set in /etc/environment in the form variable=value.
set Display all variables
env Display all environment variables
export MYVAR Export a variable, making it an environment variable
MYVAR=value
((MYVAR=value))
let "MYVAR=value"
Set a variable
echo $MYVAR
echo ${MYVAR}
Use a variable (in this case, echo it to screen).
If other characters follow the variable name, it is necessary to specify the boundaries of
the variable name via {} to make it unambiguous
command "$MYVAR" Pass a variable as argument to command.
It is recommended to double quote a variable when referencing it, to prevent
interpretation of special characters (except \ $ ` ), and avoid word splitting if the
variable contains spaces
MYVAR=$((2+2))
MYVAR=$[2+2]
FOO=$((BAR + 42))
FOO=`expr $BAR + 42`
Evaluate a numeric expression and assign the result to another variable
MYVAR=`date`
MYVAR=$(date)
Assign to a variable the output resulting from a command
for i in /path/*
do
echo "Filename: $i"
done
Loop and operate through all the output tokens (in this case, files in the path).
Note: looping over the output of $(ls) is unnecessary and harmful, as filenames
containing whitespace or glob characters may have unintended results
unset MYVAR Delete a variable
set ${MYVAR:=value}
MYVAR=${MYVAR:-value}
Set a variable, only if it is not already set (i.e. does not exist) or is null
echo ${MYVAR:-message}If variable exists and is not null, print its value, otherwise print message
echo ${MYVAR:+message}If variable exists and is not null, print message, otherwise print nothing
echo ${MYVAR,,} Print a string variable in lowercase
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48/167 Shell operations
Shell operations
Bash built-in variables
$0 Script name
$nnth argument passed to the script or function
$@ All arguments passed to the script or function; each argument is a separate word
$* All arguments passed to the script or function, as a single word
$# Number of arguments passed to the script or function
$? Exit status of the last recently executed command
${PIPESTATUS[n]} Exit status of the nth command in the executed pipeline
$$ PID of the script in which this variable is called
$! PID of the last recently executed background command
$SHLVL Deepness level of current shell, starting with 1
Bash shell options
set -option
set -o longoption Enable a Bash option
set +option
set +o longoption Disable a Bash option
set -o Show the status of all Bash options
set -v
set -o verbose Enable printing of shell input lines as they are read
set -x
set -o xtrace Enable printing of command traces before execution of each command (debug mode)
set -u
set -o nounset Treat expansion of unset variables as an error
To run a script with a Bash option enabled, do one of the following:
- Run the script with bash -option scriptfile.sh
- Specify the shebang line as #!/bin/bash -option
- Add the command set -option at the beginning of the script
Bash shell event Files run
When a login shell is launched
/etc/profile
/etc/profile.d/*.sh
~/.bash_profile
~/.bash_login
~/.profile
The shell executes the system-wide
profile files, then the first of the 3
user files that exists and is readable
When a login shell exits ~/.bash_logout
When a non-login shell is launched
/etc/bash.bashrc
/etc/bashrc
~/.bashrc
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49/167 Shell scripting
Shell scripting
Bash shell scripts must start with the shebang line #!/bin/bash indicating the location of the script interpreter.
Script execution
source myscript.sh
. myscript.sh
Script execution takes place in the same shell. Variables defined and
exported in the script are seen by the shell when the script exits
bash myscript.sh
./myscript.sh (file must be executable) Script execution spawns a new shell
command & Execute command in the background
command1; command2 Execute command 1 and then command 2
command1 && command2 Execute command 2 only if command 1 executed successfully (exit status = 0)
command1 || command2 Execute command 2 only if command 1 did not execute successfully (exit status > 0)
(command1 && command2)Group commands together for evaluation priority
(command)Run command in a subshell. This is used to isolate command's effects, as variable
assignments and other changes to the shell environment operated by command will
not remain after command completes
exit Terminate a script
exit nTerminate a script with the specified exit status number n. By convention, a 0 exit
status is used if the script executed successfully, non-zero otherwise
command || exit 1 (To be used inside a script.) Exit the script if command fails
/bin/true Do nothing and return immediately a status code of 0 (indicating success)
/bin/false Do nothing and return immediately a status code of 1 (indicating failure)
if command
then
echo "Success"
else
echo "Failure"
fi
Run a command, then evaluate whether it exited successfully or failed
if [ $? -eq 0 ]
then
echo "Success"
else
echo "Failure"
fi
Evaluate whether the last executed command exited successfully or failed
function myfunc { commands }
myfunc() { commands }
Define a function. A function must be defined before it can be used in a Bash script.
An advantage of functions over aliases is that functions can be passed arguments
myfunc arg1 arg2 ... Call a function
typeset -f Show functions defined in the current Bash session
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50/167 Script execution
Script execution
watch command Execute command every 2 seconds
watch -d -n 1 command Execute command every second, highlighting the differences in the output
timeout 30s command Execute command and kill it after 30 seconds
command | ts Prepend a timestamp to each line of the output of command
sleep 5 Pause for 5 seconds
usleep 5000 Pause for 5000 microseconds
getopts Parse positional parameters in a shell script
script Generate a typescript of a terminal session
expect Dialogue with interactive programs according to a script, analyzing what can be expected
from the interactive program and replying accordingly
parallel command Run a command in parallel. This is used to operate on multiple inputs, similarly to xargs
zenity Display GTK+ graphical dialogs for user messages and input
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51/167 Tests
Tests
test "$MYVAR" operator "value" && command
[ "$MYVAR" operator "value" ] && command
if [ "$MYVAR" operator "value" ]; then command; fi
Perform a test; if it results true, command is executed
Test operators
Integer operators File operators Expression operators
-eq Equal to -e or -a Exists -a Logical AND
-ne Not equal to -d Is a directory -o Logical OR
-lt Less than -b Is a block special file !Logical NOT
-le Less than or equal to -c Is a character special file \( \) Priority
-gt Greater than -f Is a regular file
-ge Greater than or equal to -r Is readable
String operators -w Is writable
-z Is zero length -x Is executable
-n or nothing Is non-zero length -s Is non-zero length
= or == Is equal to -u Is SUID
!= Is not equal to -g Is SGID
<Is alphabetically before -k Is sticky
>Is alphabetically after -h Is a symbolic link
Evaluation operators
=Equal to +Plus string : regex
match string regex String matches regex
!= Not equal to -Minus
<Less than \* Multiplied by substr string pos length Substring
<= Less than or equal to /Divided by index string chars Index of any chars in string
>Greater than %Remainder length string String length
>= Greater than or equal to
expr "$MYVAR" = "39 + 3"
MYVAR=$((39 + 3)) Evaluate an expression (in this case, assigns the value 42 to the variable)
expr string : regex Return the length of the substring matching the regex
expr string : \(regex\) Return the substring matching the regex
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52/167 Flow control
Flow control
Tests
if [test 1]
then
[command block 1]
elif [test 2]
then
[command block 2]
else
[command block 3]
fi
case $STRING in
pattern1)
command1
command1bis
;;
pattern2)
command2
;;
*)
defaultcommand
;;
esac
Loops
while [test]
do [command block]
done
until [test]
do [command block]
done
for I in [list]
do [command block]
done
The command block executes
as long as test is true
The command block executes
as long as test is false
The command block executes
for each I in list
i=0
while [ $i -le 7 ]
do
echo $i
let i++
done
i=0
until [ $i -gt 7 ]
do
echo $i
let i++
done
for i in 0 1 2 3 4 5 6 7
do
echo $i
done
for i in {0..7}
do
echo $i
done
start=0
end=7
for i in $(seq $start $end)
do
echo $i
done
start=0
end=7
for ((i = start; i <= end; i++))
do
echo $i
done
break Exit a loop
continue Jump to the next iteration
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53/167 Text processors
Text processors
vi Vi, text editor
vim Vi Improved, an advanced text editor
gvim Vim with GUI
vimdiff file1 file2 Compare two text files in Vim
pico Pico, simple text editor
nano Nano, simple text editor (a GNU clone of Pico)
emacs GNU Emacs, a GUI text editor
gedit GUI text editor
ed Line-oriented text editor
more Text pager (obsolete)
less Text pager
less pager commands
hHelp
gGo to the first line in the file
GGo to the last line in the file
FGo to the end of the file, and move forward automatically as the file grows
Stop moving forward
-N Show line numbers
-n Don't show line numbers
=Show information about the file
Show current and total line number, byte, and percentage of the file read
:n When reading multiple files, go to the next file
:p When reading multiple files, go to the previous file
qQuit
less pager options
--follow-name Attempts periodically to reopen the file by name. Useful to keep reading, via
the F command, a logfile that is being rotated. Note that, by default, less
continues to read the original input file even if it has been renamed
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
CTRL G
CTRL C
54/167 Vi commands
Vi commands
Go to Command mode
iInsert text before cursor
and go to Insert mode
IInsert text after line
aAppend text after cursor
AAppend text after line
vGo to Visual mode, character-wise then use the arrow keys to select a block of text
VGo to Visual mode, line-wise
dDelete selected block gu Switch block to lowercase
yCopy (yank) selected block into buffer gU Switch block to uppercase
wMove to next word $Move to end of line
bMove to beginning of word 1G Move to line 1 i.e. beginning of file
eMove to end of word GMove to end of file
0Move to beginning of line z Make current line the top line of the screen
Show current line and column number
ma Mark position "a". Marks a-z are local to current file, while marks A-Z are global to a specific file
'a Go to mark "a". If using a global mark, it also opens the specific file
y'a Copy (yank) from mark "a" to current line, into the buffer
d'a Delete from mark "a" to current line
pPaste buffer after current line yy Copy current line
PPaste buffer before current line yyp Duplicate current line
xDelete current character DDelete from current character to end of line
XDelete before current character dd Delete current line
7dd Delete 7 lines. Almost any command can be prepended by a number to repeat it a number of times
uUndo last command. Vi can undo the last command only, Vim is able to undo several commands
.Repeat last text-changing command
/string Search for string forward nSearch for next match of string
?string Search for string backwards NSearch for previous match of string
:s/s1/s2/Replace the first occurrence of s1 with s2 in the current line
:s/s1/s2/g Replace globally every occurrence of s1 with s2 in the current line
:%s/s1/s2/g Replace globally every occurrence of s1 with s2 in the whole file
:%s/s1/s2/gc Replace globally every occurrence of s1 with s2 in the whole file, asking for confirmation
:5,40s/^/#/ Add a hash character at the beginning of each line, from line 5 to 40
!!program Replace line with output from program
:r file Read file and insert it after current line
:X Encrypt current document. Vi will automatically prompt for the password to encrypt and decrypt
:w file Write to file
:wq
:x
ZZ
Save changes and quit
:q Quit (fails if there are unsaved changes) :q! Abandon all changes and quit
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
ESC
CTRL G
RETURN
55/167 Vi options
Vi options
Option Effect
ai Turn on auto indentation
all Display all options
ap Print a line after the commands d c J m :s t u
aw Automatic write on commands :n ! e# ^^ :rew ^} :tag
bf Discard control characters from input
dir=tmpdir Set tmpdir as directory for temporary files
eb Precede error messages with a bell
ht=8 Set terminal tab as 8 spaces
ic Ignore case when searching
lisp Modify brackets for Lisp compatibility
list Show tabs and EOL characters
set listchars=tab:>- Show tab as > for the first char and as - for the following chars
magic Allow pattern matching with special characters
mesg Enable UNIX terminal messaging
nu Show line numbers
opt Speed up output by eliminating automatic Return
para=LIlPLPPPQPbpP Set macro to start paragraphs for { } operators
prompt Prompt : for command input
re Simulate smart terminal on dumb terminal
remap Accept macros within macros
report Show largest size of changes on status line
ro Make file readonly
scroll=12 Set screen size as 12 lines
sh=/bin/bash Set shell escape to /bin/bash
showmode Show current mode on status line
slow Postpone display updates during inserts
sm Show matching parentheses when typing
sw=8 Set shift width to 8 characters
tags=/usr/lib/tags Set path for files checked for tags
term Print terminal type
terse Print terse messages
timeout Eliminate 1-second time limit for macros
tl=3 Set significance of tags beyond 3 characters (0 = all)
ts=8 Set tab stops to 8 for text input
wa Inhibit normal checks before write commands
warn Warn "No write since last change"
window=24 Set text window as 24 lines
wm=0 Set automatic wraparound 0 spaces from right margin
:set option turn on an option
:set nooption turn off an option
Options can also be permanently set by including them in ~/.exrc (Vi) or ~/.vimrc (Vim)
vi -R file Open file in read-only mode
cat file | vi - Open file in read-only mode (this is done by having Vi read from stdin)
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
56/167 SQL
SQL
SHOW DATABASES; Show all existing databases
SHOW TABLES; Show all tables from the selected database
USE CompanyDatabase; Choose which database to use
SELECT DATABASE(); Show which database is currently selected
CREATE TABLE customers (
cusid INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
firstname VARCHAR(32), lastname VARCHAR(32), dob DATE,
city VARCHAR(24), zipcode VARCHAR(5));
CREATE TABLE payments (
payid INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
date DATE, fee INT, bill VARCHAR(128), cusid INT,
CONSTRAINT FK1 FOREIGN KEY (cusid) REFERENCES customers(cusid));
Create tables
INSERT INTO customers (firstname,lastname,dob)
VALUES ('Arthur','Dent',1959-08-01), ('Trillian','',1971-03-19);
Insert new records in a table
DELETE FROM customers WHERE firstname LIKE 'Zaphod'; Delete some records in a table
UPDATE customers SET city = 'London' WHERE zipcode = '00789'; Modify records in a table
CREATE INDEX lastname_index ON customers(lastname);
ALTER TABLE customers ADD INDEX lastname_index (lastname);
Create an index for faster searches
DESCRIBE customers; Describe the columns of a table
SHOW CREATE TABLE customers; Show the code used to create a table
SHOW INDEXES FROM customers; Show primary key and indexes of a table
DROP TABLE customers; Delete a table
DROP DATABASE CompanyDatabase; Delete a database
ALTER TABLE customers MODIFY city VARCHAR(32); Modify the type of a column
CREATE VIEW cust_view AS
SELECT * FROM customers WHERE city != 'London';
Create a view. Views are used similarly to
tables
COMMIT; Commit changes to the database
ROLLBACK; Rollback the current transaction, canceling
any changes done during it
START TRANSACTION;
BEGIN;
Disable autocommit for this transaction,
until a COMMIT or ROLLBACK is issued
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
57/167 SQL SELECT
SQL SELECT
SELECT * FROM customers; Select all columns from the
customers table
SELECT firstname, lastname FROM customers LIMIT 5; Select first and last name of
customers, showing 5 records only
SELECT firstname, lastname FROM customers WHERE zipcode = '00123'; Select first and last name of
customers whose zip code is 00123
SELECT firstname, lastname FROM customers WHERE zipcode IS NOT NULL; Select first and last name of
customers with a recorded zip code
SELECT * FROM customers ORDER BY lastname, firstname; Select customers in alphabetical
order by last name, then first name
SELECT * FROM customers ORDER by zipcode DESC; Select customers, sorting them by zip
code in reverse order
SELECT firstname, lastname,
TIMESTAMPDIFF(YEAR,dob,CURRENT_DATE) as age FROM customers;
Select first name, last name, and
calculated age of customers
SELECT DISTINCT city FROM customers; Show all cities but retrieving each
unique output record only once
SELECT city, COUNT(*) FROM customers GROUP BY city; Show all cities and the number of
customers in each city. NULL values
are not counted
SELECT cusid, SUM(fee) FROM payments GROUP BY cusid; Show all fee payments grouped by
customer ID, summed up
SELECT cusid, AVG(fee) FROM payments GROUP BY cusid
HAVING AVG(fee)<50;
Show the average of fee payments
grouped by customer ID, where this
average is less than 50
SELECT MAX(fee) FROM payments; Show the highest fee in the table
SELECT COUNT(*) FROM customers; Show how many rows are in the table
SELECT cusid FROM payments t1 WHERE fee =
(SELECT MAX(t2.fee) FROM payments t2 WHERE t1.cusid=t2.cusid);
Show the customer ID that pays the
highest fee (via a subquery)
SELECT @maxfee:=MAX(fee) FROM payments;
SELECT cusid FROM payments t1 WHERE fee = @maxfee;
Show the customer ID that pays the
highest fee (via a user set variable)
SELECT cusid FROM payments WHERE fee >
ALL (SELECT fee FROM payments WHERE cusid = 4242001;
Show the customer IDs that pay fees
higher than the highest fee paid by
customer ID 4242001
SELECT * FROM customers WHERE firstname LIKE 'Trill%'; Select customers whose first name
matches the expression:
% any number of chars, even zero
_ a single char
SELECT * FROM customers WHERE firstname REGEXP '^Art.*r$'; Select customers whose first name
matches the regex
SELECT firstname, lastname FROM customers WHERE zipcode = '00123'
UNION
SELECT firstname, lastname FROM customers WHERE cusid > 4242001;
Select customers that satisfy any of
the two requirements
SELECT firstname, lastname FROM customers WHERE zipcode = '00123'
INTERSECT
SELECT firstname, lastname FROM customers WHERE cusid > 4242001;
Select customers that satisfy both of
the two requirements
SELECT firstname, lastname FROM customers WHERE zipcode = '00123'
EXCEPT
SELECT firstname, lastname FROM customers WHERE cusid > 4242001;
Select customers that satisfy the first
requirement but not the second
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
58/167 SQL JOIN
SQL JOIN
SQL MySQL Operation
SELECT customers.name, payments.bill
FROM customers, payments
WHERE customers.cusid = payments.cusid;
SELECT customers.name, payments.bill
FROM customers NATURAL JOIN payments;
SELECT customers.name, payments.bill
FROM customers JOIN payments
USING (cusid);
SELECT customers.name, payments.bill
FROM customers JOIN payments
ON customers.cusid = payments.cusid;
SELECT customers.name, payments.bill
FROM customers
[ JOIN | INNER JOIN | CROSS JOIN ]
payments
ON customers.cusid = payments.cusid;
SELECT customers.name, payments.bill
FROM customers
[ JOIN | INNER JOIN | CROSS JOIN ]
payments
USING (cusid);
Perform a join (aka inner
join) of two tables to select
data that are in a relationship
SELECT customers.name, payments.bill
FROM customers CROSS JOIN payments;
SELECT customers.name, payments.bill
FROM customers JOIN payments;
Perform a cross join (aka
Cartesian product) of two
tables
SELECT customers.name, payments.bill
FROM customers LEFT JOIN payments
ON customers.cusid = payments.cusid;
Perform a left join (aka
left outer join) of two
tables, returning records
matching the join condition
and also records in the left
table with unmatched values
in the right table
SELECT customers.name, payments.bill
FROM customers RIGHT JOIN payments
ON customers.cusid = payments.cusid;
Perform a right join (aka
right outer join) of two
tables, returning records
matching the join condition
and also records in the right
table with unmatched values
in the left table
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
59/167 MySQL
MySQL
MySQL is the most used open source RDBMS (Relational Database Management System). It runs on TCP port 3306.
On RHEL 7 it is replaced by its fork MariaDB, but the names of the client and of most tools remain unchanged.
mysqld_safe Start the MySQL server (mysqld) with safety features
such as restarting the server if errors occur and logging
runtime information to the error logfile. Recommended
mysql_install_db (deprecated)
mysqld --initialize
Initialize the MySQL data directory, create system
tables, and set up an administrative account.
To be run just after installing the MySQL server
mysql_secure_installation Set password for root, remove anonymous users, disable
remote root login, and remove test database.
To be run just after installing the MySQL server
mysql -u root -p Login to MySQL as root and prompt for the password
mysql -u root -ppassword Login to MySQL as root with the specified password
mysql -u root -p -h host -P port Login to the specified remote MySQL server and port
mysql -u root -p -eNB'SHOW DATABASES' Run a SQL command via MySQL. Flags are:
e Run in batch mode
N Do not print table header
B Do not print table decoration characters +-|
mysqldump -u root -p --all-databases > alldbs.sql Backup all databases to a dump file
mysqldump -u root -p MyDatabase > mydb.sql Backup a database to a dump file
mysqldump -u root -p --databases MyDb1 MyDb2 > dbs.sql Backup several databases to a dump file
mysqldump -u root -p MyDatabase t1 t2 > tables.sql Backup some tables of a database to a dump file
mysql -u root -p < alldbsbak.sql Restore all databases from a dump file (which contains a
complete dump of a MySQL server)
mysql -u root -p MyDatabase < mydbbak.sql Restore a specific database from a dump file (which
contains one database)
mysql_upgrade -u root -p Check all tables in all databases for incompatibilities with
the current version of MySQL
mysqlcheck options Perform table maintenance. Each table is locked while is
being processed. Options are:
--check Check table for errors (default)
--analyze Analyze table
--optimize Optimize table
--repair Repair table; can fix almost all problems
except unique keys that are not unique
mysqlcheck --check db table Check the specified table of the specified database
mysqlcheck --check --databases db1 db2 Check the specified databases
mysqlcheck --check --all-databases Check all databases
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
60/167 MySQL tools
MySQL tools
mysqlslap Tool for MySQL stress tests
mysqltuner.pl Review the current MySQL installation configuration for performances and stability
mysqlreport (obsolete) Generate a user-friendly report of MySQL status values
mytop Monitor MySQL processes and queries
innotop Monitor MySQL InnoDB transactions
dbs="$(mysql -uroot -ppassword -Bse'SHOW DATABASES;')"
for db in $dbs
do
[operation on $db]
done
Perform an operation on each database name
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
61/167 MySQL syntax
MySQL syntax
SELECT Host, User FROM mysql.user; List all MySQL users
CREATE USER 'john'@'localhost' IDENTIFIED BY 'p4ssw0rd'; Create a MySQL user and set his
password
DROP USER 'john'@'localhost'; Delete a MySQL user
SET PASSWORD FOR 'john'@'localhost' = PASSWORD('p4ssw0rd');
SET PASSWORD FOR 'john'@'localhost' = '*7E684A3DF6273CD1B6DE53';
Set a password for a MySQL user.
The password can be specified either in
plaintext or by its hash value
SHOW GRANTS FOR 'john'@'localhost'; Show permissions for a user
GRANT ALL PRIVILEGES ON MyDatabase.* TO 'john'@'localhost'; Grant permissions to a user
REVOKE ALL PRIVILEGES ON MyDatabase.* FROM 'john'@'localhost'; Revoke permissions from a user; must
match the already granted permission on
the same database or table
GRANT SELECT ON *.* TO 'john'@'localhost' IDENTIFIED BY 'p4ssw0rd';
GRANT SELECT ON *.* TO 'john'@'localhost' IDENTIFIED BY PASSWORD
'*7E684A3DF6273CD1B6DE53';
Create a MySQL user and set his grants
FLUSH PRIVILEGES; Reload and commit the grant tables; must
be run after any GRANT command
SELECT * INTO OUTFILE '/tmp/mytable.csv'
FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"'
LINES TERMINATED BY '\n' FROM MyDatabase.mytable;
Export a table to a CSV file
USE MyDatabase; SOURCE mydbbak.sql; Restore a database from a dump file
USE MyDatabase; LOAD DATA LOCAL INFILE 'foofile' INTO TABLE foo; Populate a table with data from file (one
record per line, values separated by tabs)
DO SLEEP(n);
SELECT SLEEP(n);
Sleep for n seconds
SET PROFILING=1; Enable profiling
SHOW PROFILE; Show the profile of the last executed
query, with detailed steps and their timing
statement;
statement\g
Send a SQL statement to the server
statement\G Display result in vertical format, showing
each record in multiple rows
SELECT /*!99999 comment*/ * FROM MyDatabase.mytable; Insert a comment
SELECT /*!n statement*/ * FROM MyDatabase.mytable; The commented statement is executed
only if MySQL is version n or higher
\c Cancel current input
\! command Run a shell command
TEE logfile Log all I/O of the current MySQL session
to the specified logfile
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
62/167 MySQL status
MySQL status
SHOW VARIABLES;
SHOW SESSION VARIABLES;
SHOW LOCAL VARIABLES;
Print session variables (affecting current connection only)
SHOW GLOBAL VARIABLES; Print global variables (affecting global operations on the server)
SHOW VARIABLES LIKE '%query%'; Print session variables that match the given pattern
SHOW VARIABLES LIKE 'hostname';
SELECT @@hostname;
Print a session variable with the given name
SET sort_buffer_size=10000;
SET SESSION sort_buffer_size=10000;
SET LOCAL sort_buffer_size=10000;
SET @@sort_buffer_size=10000;
SET @@session.sort_buffer_size=10000;
SET @@local.sort_buffer_size=10000;
Set a session variable
SET GLOBAL sort_buffer_size=10000;
SET @@global.sort_buffer_size=10000;
Set a global variable
SHOW STATUS;
SHOW SESSION STATUS;
SHOW LOCAL STATUS;
Print session status (concerning current connection only)
SHOW GLOBAL STATUS; Print global status (concerning global operations on the server)
SHOW STATUS LIKE '%wsrep%'; Print session status values that match the given pattern
SHOW WARNINGS; Print warnings, errors and notes resulting from the most recent
statement in the current session that generated messages
SHOW ERRORS; Print errors resulting from the most recent statement in the
current session that generated messages
SHOW TABLE STATUS; Print information about all tables of the current database e.g.
engine (InnoDB or MyISAM), rows, indexes, data length
SHOW ENGINE INNODB STATUS; Print statistics concerning the InnoDB engine
SELECT * FROM information_schema.processlist;
SHOW FULL PROCESSLIST;
Print the list of threads running in your local session; if run as
root, print the list of threads running on the system
SELECT * FROM information_schema.processlist
WHERE user='you';
Print the list of threads running in your local session and all your
other logged-in sessions
SHOW CREATE TABLE table;
SHOW CREATE VIEW view;
Print the CREATE statement that created table or view
SELECT VERSION(); Print the version of the MySQL server
SELECT CURDATE();
SELECT CURRENT_DATE;
Print the current date
SELECT CURTIME();
SELECT CURRENT_TIME;
Print the current time
SELECT NOW(); Print the current date and time
SELECT USER(); Print the current user@hostname that is logged in
\s Print status information about server and current connection
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
63/167 MySQL recipes
MySQL recipes
SELECT table_schema AS "Name",
SUM(data_length+index_length)/1024/1024 AS "Size in Mb"
FROM information_schema.tables GROUP BY table_schema;
Display the sizes of all databases in the
system (counting data + indexes)
SELECT table_schema AS "Name",
SUM(data_length+index_length)/1024/1024 AS "Size in Mb"
FROM information_schema.tables WHERE table_schema='database';
Display the size of database
SELECT table_name AS "Name",
ROUND(((data_length)/1024/1024),2) AS "Data size in Mb",
ROUND(((index_length)/1024/1024),2) AS "Index size in Mb"
FROM information_schema.TABLES WHERE table_schema='database'
ORDER BY table_name;
Display data and index size of all tables of
database
SELECT table_name, table_rows
FROM information_schema.tables WHERE table_schema='database';
Print an estimate of the number of rows of
each table of database
SELECT SUM(data_length+index_length)/1024/1024 AS "InnoDB Mb"
FROM information_schema.tables WHERE engine='InnoDB';
Display the amount of InnoDB data in all
databases
SELECT table_name, engine
FROM information_schema.tables WHERE table_schema = 'database';
Print name and engine of all tables in
database
SELECT CONCAT('KILL ',id,';')
FROM information_schema.processlist WHERE user='user'
INTO OUTFILE '/tmp/killuser'; SOURCE /tmp/killuser;
Kill all connections belonging to user
SELECT COUNT(1) SlaveThreadCount
FROM information_schema.processlist WHERE user='system user';
Distinguish between master and slave server;
returns 0 on a master, >0 on a slave
SELECT ROUND(SUM(CHAR_LENGTH(field)<40)*100/COUNT(*),2)
FROM table;
Display the percentage of rows on which the
string field is shorter than 40 chars
SELECT CHAR_LENGTH(field) AS Length, COUNT(*) AS Occurrences
FROM table GROUP BY CHAR_LENGTH(field);
Display all different lengths of string field and
the number of times they occur
SELECT MAX(CHAR_LENGTH(field)) FROM table;Display the longest string stored in field
SHOW FULL TABLES IN database WHERE table_type LIKE 'VIEW'; Display the list of views in database
SELECT "Table 1" AS `set`, t1.* FROM table1 t1 WHERE
ROW(t1.col1, t1.col2, t1.col3) NOT IN (SELECT * FROM table2)
UNION ALL
SELECT "Table 2" AS `set`, t2.* FROM table2 t2 WHERE
ROW(t2.col1, t2.col2, t2.col3) NOT IN (SELECT * FROM table1)
Display the differences between the contents
of two tables table1 and table2 (assuming
they're composed of 3 columns each)
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
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MySQL operations
How to resync a master-slave replication
1. On the master, on terminal 1: mysql -uroot -p
RESET MASTER;
FLUSH TABLES WITH READ LOCK;
SHOW MASTER STATUS;
Note the values of MASTER_LOG_FILE and MASTER_LOG_POS; these values will need
to be copied on the slave
2. On the master, on terminal 2: mysqldump -uroot -p --all-databases > /root/dump.sql
It is not necessary to wait until the dump completes
3. On the master, on terminal 1: UNLOCK TABLES;
4. Transfer the dump file from
the master to the slave
5. On the slave: mysql -uroot -p
STOP SLAVE;
SOURCE /root/dump.sql;
RESET SLAVE;
CHANGE MASTER TO MASTER_LOG_FILE='mysql-bin.nnnnnn', MASTER_LOG_POS=mm;
START SLAVE;
SHOW SLAVE STATUS;
How to recover the MySQL root password
1. Stop the MySQL server
2. Restart the MySQL server
skipping the grant tables:
mysqld_safe --skip-grant-tables --skip-networking &
3. Connect to the MySQL server
passwordlessly:
mysql -uroot
4. Reload the grant tables: FLUSH PRIVILEGES;
5. Change the root password: SET PASSWORD FOR 'root'@'localhost' = PASSWORD('s3cr3t');
6. Stop the MySQL server and
restart it normally
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
65/167 PostgreSQL
PostgreSQL
PostgreSQL (aka Postgres) is an open source object-relational database. By default it listens for connections on TCP port
5432.
\list
\l List all databases
\list+
\l+ List all databases, displaying database size and description
\connect database
\c database Connect to database
\q Quit
How to setup PostgreSQL with a database owned by user
1. Set up PostgreSQL: postgresql-setup initdb
2. Change the password of the
postgres shell user:
passwd postgres
3. Create the user shell user: useradd user
4. Switch to the postgres shell user
and connect to PostgreSQL:
su - postgres
psql -U postgres
5. Create the user PostgreSQL user: CREATE ROLE user WITH LOGIN;
\password user
\q
6. Create a database owned by user:createdb -E utf8 -l C -T template0 database -O user
7. Switch to the postgres shell user
and connect to PostgreSQL:
su - postgres
psql -U postgres
8. Grant the necessary privileges: GRANT ALL PRIVILEGES ON DATABASE database TO user;
\q
9. Verify that user can login to
PostgreSQL:
su - user
psql -U user -W
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
66/167 X
X
The X Window System (aka X11 or X) is a windowing system for Linux and UNIX-like OSes, providing a basic framework
for GUI applications via a client-server model. A display manager provides a login screen to enter an X session and
introduces the user to the desktop environment (e.g. GNOME, KDE, CDE, Enlightenment).
Display Manager Configuration files Display Manager greeting screen
xdm
X
Display
Manager
/etc/x11/xdm/Xaccess Control inbound requests from
remote hosts
Defined in /etc/x11/xdm/Xresources
by the line:
xlogin*greeting: \
Debian GNU/Linux (CLIENTHOST)
/etc/x11/xdm/Xresources Configuration settings for X
applications and the login screen
/etc/x11/xdm/Xservers
Association of X displays with
local X server software, or with X
terminals via XDMCP
/etc/x11/xdm/Xsession Script launched by xdm after
login
/etc/x11/xdm/Xsetup_0 Script launched before the
graphical login screen
/etc/x11/xdm/xdm-config Association of all xdm
configuration files
gdm
GNOME
Display
Manager
/etc/gdm/gdm.conf or /etc/gdm/custom.conf Configured via gdmsetup
kdm
KDE
Display
Manager
/etc/kde/kdm/kdmrc Configured via kdm_config
/etc/init.d/xdm start
/etc/init.d/gdm start
/etc/init.d/kdm start
Start the appropriate Display Manager
xorgconfig (Debian)
Xorg -configure (Red Hat)
Configure X (text mode)
xorgcfg (Debian)
system-config-display (Red Hat)
Configure X (graphical mode)
X -version Show which version of X is running
xdpyinfo Display information about the X server
xwininfo Display information about windows
xhost + 10.3.3.3
xhost - 10.3.3.3
Add or remove 10.3.3.3 to the list of hosts allowed to make X connections to
the local machine
switchdesk gde Switch to the GDE Display Manager at runtime
gnome-shell --version Show which version of GNOME is running
/etc/X11/xorg.conf Configuration file for X
~/.Xresources Configuration settings for X applications, in the form
program*resource: value
$DISPLAY Environment variable defining the display name of the X server, in the form
hostname:displaynumber.screennumber
The following line in /etc/inittab instructs init to launch XDM at runlevel 5:
x:5:respawn:/usr/X11R6/bin/xdm -nodaemon
The following lines in /etc/sysconfig/desktop define GNOME as the default Display Environment and Display Manager:
desktop="gde"
displaymanager="gdm"
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67/167 X tools
X tools
xdotool X automation tool
xdotool getwindowfocus Get the ID of the currently focused window (if run in command line, it is
the terminal where this command is typed)
xdotool selectwindow Pop up an X cursor and get the ID of the window selected by it
xdotool key --window 12345678 Return Simulate a keystroke inside window ID 12345678
xprop X property displayer. Pops up a cursor to select a window
xprop | grep WM_CLASS Get process name and GUI application name of the selected window
xrandr
xrandr -q
Show screen(s) size and resolution
xrandr --output eDP1 --right-of VGA1 Extend the screen on an additional VGA physical screen situated to the left
xsel Manipulate the X selection (primary, secondary, and clipboard)
xsel -b < file Copy the contents of a file to the X clipboard
xsel -b -a < file Append the contents of a file to the X clipboard
xsel -b -o Output onscreen the contents of the X clipboard
cat file | xclip -i Copy the contents of a file to the X clipboard
mkfontdir Catalog the newly installed fonts in the new directory
xset fp+ /usr/local/fonts Dynamically add new installed fonts in /usr/local/fonts to the X server
xfs Start the X font server
fc-cache Install fonts and build font information cache
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
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68/167 X keysim codes
X keysim codes
Main Latin 1 Latin 2
BackSpace ff08
Tab ff09
Linefeed ff0a
Clear ff0b
Return ff0d
Pause ff13
Scroll_Lock ff14
Sys_Req ff15
Escape ff1b
Delete ffff
space 0020
exclam 0021
quotedbl 0022
numbersign 0023
dollar 0024
percent 0025
ampersand 0026
apostrophe 0027
quoteright 0027
parenleft 0028
parenright 0029
asterisk 002a
plus 002b
comma 002c
minus 002d
period 002e
slash 002f
0 - 9 0030 - 0039
colon 003a
semicolon 003b
less 003c
equal 003d
greater 003e
question 003f
at 0040
A - Z 0041 - 005a
bracketleft 005b
backslash 005c
bracketright 005d
asciicircum 005e
underscore 005f
grave 0060
quoteleft 0060
a - z 0061 - 007a
braceleft 007b
bar 007c
braceright 007d
asciitilde 007e
nobreakspace 00a0
exclamdown 00a1
cent 00a2
sterling 00a3
currency 00a4
yen 00a5
brokenbar 00a6
section 00a7
diaeresis 00a8
copyright 00a9
ordfeminine 00aa
guillemotleft 00ab
notsign 00ac
hyphen 00ad
registered 00ae
macron 00af
degree 00b0
plusminus 00b1
twosuperior 00b2
threesuperior 00b3
acute 00b4
mu 00b5
paragraph 00b6
periodcentered 00b7
cedilla 00b8
onesuperior 00b9
masculine 00ba
guillemotright 00bb
onequarter 00bc
onehalf 00bd
threequarters 00be
questiondown 00bf
Agrave 00c0
Aacute 00c1
Acircumflex 00c2
Atilde 00c3
Adiaeresis 00c4
Aring 00c5
AE 00c6
Ccedilla 00c7
Egrave 00c8
Eacute 00c9
Ecircumflex 00ca
Ediaeresis 00cb
Igrave 00cc
Iacute 00cd
Icircumflex 00ce
Idiaeresis 00cf
ETH 00d0
Eth 00d0
Ntilde 00d1
Ograve 00d2
Oacute 00d3
Ocircumflex 00d4
Otilde 00d5
Odiaeresis 00d6
multiply 00d7
Oslash 00d8
Ooblique 00d8
Ugrave 00d9
Uacute 00da
Ucircumflex 00db
Udiaeresis 00dc
Yacute 00dd
THORN 00de
Thorn 00de
ssharp 00df
agrave 00e0
aacute 00e1
acircumflex 00e2
atilde 00e3
adiaeresis 00e4
aring 00e5
ae 00e6
ccedilla 00e7
egrave 00e8
eacute 00e9
ecircumflex 00ea
ediaeresis 00eb
igrave 00ec
iacute 00ed
icircumflex 00ee
idiaeresis 00ef
eth 00f0
ntilde 00f1
ograve 00f2
oacute 00f3
ocircumflex 00f4
otilde 00f5
odiaeresis 00f6
division 00f7
oslash 00f8
ooblique 00f8
ugrave 00f9
uacute 00fa
ucircumflex 00fb
udiaeresis 00fc
yacute 00fd
thorn 00fe
ydiaeresis 00ff
Aogonek 01a1
breve 01a2
Lstroke 01a3
Lcaron 01a5
Sacute 01a6
Scaron 01a9
Scedilla 01aa
Tcaron 01ab
Zacute 01ac
Zcaron 01ae
Zabovedot 01af
aogonek 01b1
ogonek 01b2
lstroke 01b3
lcaron 01b5
sacute 01b6
caron 01b7
scaron 01b9
scedilla 01ba
tcaron 01bb
zacute 01bc
doubleacute 01bd
zcaron 01be
zabovedot 01bf
Racute 01c0
Abreve 01c3
Lacute 01c5
Cacute 01c6
Ccaron 01c8
Eogonek 01ca
Ecaron 01cc
Dcaron 01cf
Dstroke 01d0
Nacute 01d1
Ncaron 01d2
Odoubleacute 01d5
Rcaron 01d8
Uring 01d9
Udoubleacute 01db
Tcedilla 01de
racute 01e0
abreve 01e3
lacute 01e5
cacute 01e6
ccaron 01e8
eogonek 01ea
ecaron 01ec
dcaron 01ef
dstroke 01f0
nacute 01f1
ncaron 01f2
odoubleacute 01f5
rcaron 01f8
uring 01f9
udoubleacute 01fb
tcedilla 01fe
abovedot 01ff
Cursor control
Home ff50
Left ff51
Up ff52
Right ff53
Down ff54
Prior ff55
Page_Up ff55
Next ff56
Page_Down ff56
End ff57
Begin ff58
Misc functions
Select ff60
Print ff61
Execute ff62
Insert ff63
Undo ff65
Redo ff66
Menu ff67
Find ff68
Cancel ff69
Help ff6a
Break ff6b
Mode_switch ff7e
script_switch ff7e
Num_Lock ff7f
Modifiers
Shift_L ffe1
Shift_R ffe2
Control_L ffe3
Control_R ffe4
Caps_Lock ffe5
Shift_Lock ffe6
Meta_L ffe7
Meta_R ffe8
Alt_L ffe9
Alt_R ffea
Super_L ffeb
Super_R ffec
Hyper_L ffed
Hyper_R ffee
This is an excerpt of keysymdef.h which defines keysym codes (i.e. characters or functions associated with each key in X11)
as XK_key and the key hex value. These keys can be used as argument for the xdotool key command.
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
69/167 /etc/passwd
/etc/passwd
/etc/passwd User accounts
root:x:0:0:/root:/bin/bash
bin:x:1:1:/bin:/bin/bash
jdoe:x:500:100:John Doe,,555-1234,,:/home/jdoe:/bin/bash
1 2 3 4 5 6 7
1Login name
2Hashed password (obsolete), or x if password is in /etc/shadow
3UID – User ID
4GID – Default Group ID
5GECOS field – Information about the user: Full name, Room number, Work phone, Home phone, Other
6Home directory of the user
7Login shell (if set to /sbin/nologin or /bin/false, user will be unable to log in)
/etc/shadow User passwords
root:$6$qk8JmJHf$X9GfOZ/i9LZP4Kldu6.D3cx2pXA:15537:0:99999:7:::
bin:*:15637:0:99999:7:::
jdoe:!$6$YOiH1otQ$KxeeUKHExK8e3jCUdw9Rxy3Wu53:15580:0:99999:7::15766:
1 2 a b c 3 4 5 6 7 8 9
1Login name
2Hashed password (* if account is disabled, ! or !! if no password is set, prefixed by ! if the account is locked).
Composed of the following subfields separated by $:
a Hashing algorithm: 1 = MD5, 2a = Blowfish, 5 = SHA256, 6 = SHA512 (recommended)
b Random salt, up to 16 chars long. This is to thwart password cracking attempts based on rainbow tables
c String obtained by hashing the user's plaintext password concatenated to the stored salt
3Date of last password change (in number of days since 1 January 1970)
4Days before password may be changed; if 0, user can change the password at any time
5Days after which password must be changed
6Days before password expiration that user is warned
7Days after password expiration that account is disabled
8Date of account disabling (in number of days since 1 January 1970)
9Reserved field
/etc/group Group accounts
root:x:0:root
jdoe:x:501
staff:x:530:jdoe,asmith
1 2 3 4
1Group name
2Encrypted password, or x if password is in /etc/gshadow
3GID – Group ID
4Group members (if this is not their Default Group)
/etc/gshadow Group passwords
root::root:root
jdoe:!::
staff:0cfz7IpLhW19i::root,jdoe
1 2 3 4
1Group name
2Encrypted password, or ! if no password set (default)
3Group administrators
4Group members
/etc/shadow and /etc/gshadow are mode 000 and therefore readable only by the root user.
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
70/167 User management
User management
useradd -m jdoe Create a user account, creating and populating his homedir from /etc/skel
useradd -mc "John Doe" jdoe Create a user account, specifying his full name
useradd -ms /bin/ksh jdoe Create a user account, specifying his login shell
useradd -D Show default values for user account creation, as specified in /etc/login.defs and
/etc/default/useradd
usermod -c "Jonas Doe" jdoe Modify the GECOS field of a user account
usermod -L jdoe Lock a user account
usermod -U jdoe Unlock a user account
Most options for usermod and useradd are the same.
userdel -r jdoe Delete a user and his homedir
chfn jdoe Change the GECOS field of a user
chsh jdoe Change the login shell of a user
passwd jdoe Change the password of a user
passwd -l jdoe Lock a user account
passwd -S jdoe Show information about a user account: username, account status (L=locked,
P=password, NP=no password), date of last password change, min age, max age,
warning period, inactivity period in days
chage -E 2022-02-14 jdoe Change the password expiration date; account will be locked at that date
chage -d 13111 jdoe Change the date (in number of days since 1 January 1970) of last password change
chage -d 0 jdoe Force the user to change password at his next login
chage -M 30 jdoe Change the max number of days during which a password is valid
chage -m 7 jdoe Change the min number of days between password changes
chage -W 15 jdoe Change the number of days before password expiration that the user will be warned
chage -I 3 jdoe Change the number of days after password expiration before the account is locked
chage -l jdoe List password aging information for a user
groupadd staff Create a group
groupmod -n newstaff staff Change a group name
groupdel staff Delete a group
gpasswd staff Set or change the password of a group
gpasswd -a jdoe staff Add a user to a group
gpasswd -d jdoe staff Delete a user from a group
gpasswd -A jdoe staff Add a user to the list of administrators of the group
adduser
deluser
addgroup
delgroup
(Debian) User-friendly front-end commands for user and group management
system-config-users (Red Hat) GUI for user and group management
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
71/167 UID and GID
UID and GID
UID 0 is assigned to the superuser.
UIDs from 0 to 99 should* be reserved for static allocation by the system and not be created by applications.
UIDs from 100 to 499 should* be reserved for dynamic allocation by the superuser and post-install scripts.
UIDs for user accounts start from 500 (Red Hat) or 1000 (SUSE, Debian).
* as recommended by the Linux Standard Base core specifications
A process has an effective, saved, and real UID and GID:
Effective UID Used for most access checks, and as the owner for files created by the process. An unprivileged process
can change its effective UID only to either its saved UID or its real UID.
Saved UID Used when a process running with elevated privileges needs to temporarily lower its privileges. The
process changes its effective UID (usually root) to an unprivileged one, and its privileged effective UID is
copied to the saved UID. Later, the process can resume its elevated privileges by resetting its effective
UID back to the saved UID.
Real UID Used to identify the real owner of the process and affect the permissions for sending signals. An
unprivileged process can signal another process only if the sender’s real or effective UID matches the
receiver's real or saved UID. Child processes inherit the credentials from the parent, so they can signal
each other.
/etc/login.defs Definition of default values (UID and GID ranges, mail directory, account validity,
password encryption method, and so on) for user account creation
whoami Print your username (as effective UID)
id Print your real and effective UID and GID, and the groups you are a member of
id -u Print your effective UID
id user Print UID, GID, and groups information about a user
who Print the list of users logged into the system
wPrint the list of users logged into the system, and what they are doing
last Print the list of users that logged in and out. Searches through the file /var/log/wtmp
lastb Print the list of bad login attempts. Searches through the file /var/log/btmp
fail2ban Scan authentication logs and temporarily ban IP addresses (via firewall rules) that have
too many failed password logins
/var/log/auth.log Logfile containing user logins and authentication mechanisms
/var/log/pwdfail Logfile containing failed authentication attempts
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
72/167 su and sudo
su and sudo
runuser -u user command Run command as user. Can be launched only by the superuser
su user Run a shell as user
su
su root
Run a shell as root
su -c "fdisk -l" Pass a single command to the shell
su -
su -l
Ensure that the spawned shell is a login shell, hence running login scripts and setting
the correct environment variables. Recommended option
sudo -uuser command Run command as user
sudo command
sudo -uroot command
Run command as root
sudo -l List the allowed commands for the current user
sudo !! Run again the last command, but this time as root
sudoedit /etc/passwd
sudo -e /etc/passwd
Edit a protected file. It is recommended to use this instead of allowing users to sudo
text editors as root, which will cause security problems if the editor spawns a shell
visudo Edit /etc/sudoers, the configuration file that specifies access rights to sudo
Sudo commands are logged via syslog on /var/log/auth.log (Debian) or /var/log/secure (Red Hat).
sudo su -
sudo -i
Login on an interactive shell as the superuser
gksu -u root -l
gksudo -u root guicommand
GUI front-ends to su and sudo used to run an X Window command as root. Pops up a
requester prompting the user for root's password
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
73/167 Terminals
Terminals
chvt nMake /dev/ttyn the foreground terminal
vlock
away
Lock the virtual console (terminal)
tty Print your terminal device (e.g. /dev/tty1, /dev/pts/1)
stty Change or display terminal line settings
stty -ixon Disable XON/XOFF flow control
nohup script.sh Prevent a process from terminating (receiving a SIGHUP) when its parent
Bash dies.
When a Bash shell is terminated cleanly via exit, its jobs will become child of
the Bash's parent and will continue running. When a Bash shell is killed
instead, it issues a SIGHUP to his children which will terminate
screen Screen manager that multiplexes a single virtual VT100/ANSI terminal
between multiple processes or shells.
When the connection to a terminal is lost (e.g. because the terminal is closed
manually, the user logs out, or the remote SSH session goes into timeout), a
SIGHUP is sent to the shell and from there to all running child processes
which are therefore terminated. The screen command starts an interactive
shell screen session, to which you will be able to reattach later
screen -S sessionname Start a screen session with the specified session name
screen command Start the specified command in a screen session; session will end when the
command exits
screen -list Show the list of detached screen sessions
screen -r pid.tty.host
screen -r sessionowner/pid.tty.host
Resume a detached screen session
screen -R Resume the last detached screen session
screen -d -R sessionname Detach a remote screen session and reattach your current terminal to it
Send a command to the window manager:
0 ... 9 Switch between screen sessions
c Create a new screen session
? Show help
How to detach an already running job that was not started in a screen session
1. Suspend the job
2. bg Send the job to background
3. jobs Show the number (say n) of the backgrounded job
4. disown -h %nMark job n so it will not receive a SIGHUP from its parent shell
or
1. screen Start a screen session
2. reptyr pid Attach the job with process ID pid to the new terminal (screen session)
Now, when the terminal is closed, the job will not be killed.
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
CTRL A
CTRL Z
CTRL ALT Fn
74/167 Messaging
Messaging
write user Write interactively a message to the terminal of user (must be logged in)
wall Write interactively a message to the terminal of all logged in users
echo "Hello" | write user Write a message to the terminal of user (must be logged in)
echo "Hello" | wall Write a message to the terminal of all logged in users
talk user Open an interactive chat session with user (must be logged in)
mesg y
chmod g+w $(tty)
Allow the other users to message you via write, wall, and talk
mesg n
chmod g-w $(tty)
Disallow the other users to message you via write, wall, and talk
mesg Display your current message permission status
mesg works by enabling/disabling the group write permission of your terminal device, which is owned by system group tty.
The root user is always able to message users.
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
75/167 cron
cron
cron is used for repeated scheduled execution of commands.
If /etc/cron.allow exists, only users listed therein can access the service.
If /etc/cron.deny exists, all users except those listed therein can access the service.
If none of these files exist, all users can access the service.
It is not necessary to restart crond after the modification of a crontab file, as the changes will be reloaded automatically.
crontab -e Edit your user crontab file
crontab -l List the contents of your crontab file
crontab -e -u jdoe Edit the crontab file of another user (command available only to the superuser)
/etc/crontab System-wide crontab file; this is the list of commands to execute periodically
/etc/cron.d/ Directory containing commands to execute periodically, one command per file
(which must have the same syntax as /etc/crontab)
/etc/cron.hourly/
/etc/cron.daily/
/etc/cron.weekly/
/etc/cron.monthly/
Scripts placed in these directories will be automatically executed on the
specified periods
/var/spool/cron/user Crontab of user
/etc/crontab
# m h dom mon dow user command
25 6 * * 1 root foo.sh every Monday at 6:25 AM
*/5 16 * * * root /opt/myscript.sh from 4:00 to 4:55 PM every 5 minutes everyday
0,30 7 25 12 * jdoe /home/jdoe/bar.sh at 7:00 and 7:30 AM on 25th December
3 17 * * 1-5 root baz.sh at 5:03 PM everyday, from Monday to Friday
mminutes
hhours
dom day of month (1-31)
mon month (1-12 or jan-dec)
dow day of week (0-7 or sun-sat; 0=7=Sunday)
user User as whom the command will be executed
command Command that will be executed at the specified times
The crond daemon checks /etc/crontab every minute and runs the command as the specified user at the specified times.
Each user may also set his own crontab scheduling, which will result in a file /var/spool/cron/user; this user's crontab file
has the same format as the system-wide crontab file, except that the user field is not present.
/etc/anacrontab
# period delay job-identifier command
7 10 cron.weekly /opt/myscript.sh If the job has not been run in the last 7 days,
wait 10 minutes and then execute the command
period period, in days, during which the command was not executed
delay delay to wait, in minutes, before execution of the command
job-identifier job identifier in anacron messages; should be unique for each anacron job
command command that will be executed
Anacron jobs are run by crond, and permit the execution of periodic jobs on a machine that is not always powered on, such
as a laptop.
Only the superuser can schedule anacron jobs, which have a granularity of one day (vs one minute for cron jobs).
The file /var/spool/anacron/job_identifier contains the date of the last execution of the specified anacron job.
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
76/167 at
at
at is used for scheduled execution of commands that must run only once.
If /etc/at.allow exists, only users listed therein can access the service.
If /etc/at.deny exists, all users except those listed therein can access the service.
If none of these files exist, no user except root can access the service.
at 5:00pm tomorrow myscript.sh
at -f mylistofcommands.txt 5:00pm tomorrow
echo "rm file" | at now+2 minutes
Execute a command once at the specified time (absolute or relative)
at -l
atq
List the scheduled jobs
at -d 3
atrm 3
Remove job number 3 from the list
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
77/167 Utilities
Utilities
bc Calculator
factor Print the prime factors of an integer number
units Converter of quantities between different units
cal Calendar
banner Print a text in large letters made of the character #
figlet Print a text in large letters, in a specific font
toilet Print a text in large colorful letters, in a specific font
lolcat Print a text in rainbow coloring
fortune Print a random aphorism, like those found in fortune cookies
sensors Print sensor chips information (e.g. temperature)
beep Produce a beep from the machine's speakers
speaker-test Speaker test tone generator for the ALSA (Advanced Linux Sound Architecture) framework
on_ac_power Return 0 (true) if machine is connected to AC power, 1 (false) if on battery. Useful for laptops
ipcalc IP addresses calculator
pwgen Password generator
uuidgen Generate a UUID value, random or time-based
aspell Spell checker
cloc Count lines of source code
gnome-terminal GNOME shell terminal
conky Highly configurable system monitor widget with integration for audio player, email, and news
gkrellm System monitor widget
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
78/167 Localization
Localization
Locale environment variables
LANG
LANGUAGE
Language, stored in /etc/default/locale.
When scripting, it is recommended to set LANG=C because this specifies the minimal locale
environment for C translation, and guarantees a standard collation and formats for the execution
of scripts
LC_CTYPE Character classification and case conversion
LC_NUMERIC Non-monetary numeric formats
LC_TIME Date and time formats
LC_COLLATE Alphabetical order
LC_MONETARY Monetary formats
LC_MESSAGES Language and encoding of system messages and user input
LC_PAPER Paper size
LC_NAME Personal name formats
LC_ADDRESS Geographic address formats
LC_TELEPHONE Telephone number formats
LC_MEASUREMENT Measurement units (metric or others)
LC_IDENTIFICATION Metadata about locale
LC_ALL Special variable overriding all others
The values of these locale environment variables are in the format language_territory.encoding e.g. en_US.UTF-8.
The list of supported locales is stored in /usr/share/i18n/SUPPORTED.
locale Show locale environment variables
locale-gen it_IT.UTF-8 Generate a locale (in this case IT) by compiling a list of locale
definition files
apt-get install manpages-it language-pack-it Install a different locale (in this case IT); this affects system
messages and manpages
iconv -f IS6937 -t IS8859 filein > fileout Convert a text file from a codeset to another
ISO/IEC-8859 is a standard for 8-bit encoding of printable characters.
The first 256 characters in ISO/IEC-8859-1 (Latin-1) are identical to those in Unicode.
UTF-8 encoding can represent every character in the Unicode set, and was designed for backward compatibility with ASCII.
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
79/167 System time
System time
date Show current date and time
date -d "9999 days ago"
date -d "1970/01/01 + 4242" Calculate a date and show it
date +"%F %H:%M:%S" Show current date in the format specified
date +"%s" Show current date in Unix time format (seconds elapsed since 00:00:00 1/1/1970)
date -s "20130305 23:30:00" Set the date
date 030523302013 Set the date, in the format MMDDhhmmYYYY
timedatectl Show current date and time
timedatectl set-time 2013-03-05
timedatectl set-time 23:30 Set the date
timedatectl list-timezones List all possible timezones
zdump GMT Show current date and time in the GMT timezone
tzselect
tzconfig
dpkg-reconfigure tzdata (Debian)
timedatectl set-timezone timezone (Red Hat)
Set the timezone
/etc/timezone (Debian) Timezone
/etc/localtime (Red Hat) Timezone, a symlink to the appropriate timezone file in /usr/share/zoneinfo/
ntpd NTP daemon, keeps the clock in sync with Internet time servers
ntpd -q Synchronize the time once and quit
ntpd -g Force NTP to start even if clock is off by more than the panic threshold (1000 secs)
ntpd -nqg Start NTP as a non-daemon, force synchronization of the clock, and quit.
The NTP daemon must not be running when this command is launched
ntpq -p timeserver Print the list of peers for the time server
ntpdate timeserver Synchronizes the clock with the specified time server
ntpdate -b timeserver Brutally set the clock, without waiting for a slow adjusting
ntpdate -q timeserver Query the time server without setting the clock
The ntpdate command is deprecated; to synchronize the clock, use ntpd instead.
chronyd Daemon of chrony, a versatile NTP client/server
chronyc Command line interface for the chrony daemon
hwclock --show
hwclock -r Show the hardware clock
hwclock --hctosys
hwclock -s Set the system time from the hardware clock
hwclock --systohc
hwclock -w Set the hardware clock from system time
hwclock --utc Indicate that the hardware clock is kept in Coordinated Universal Time
hwclock --localtime Indicate that the hardware clock is kept in local time
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
80/167 syslog
syslog
Syslog logging facility:
syslogd
rsyslogd (Ubuntu 14) Daemon logging events from user processes
klogd Daemon logging events from kernel processes
/etc/syslog.conf
# facility.level action
*.info;mail.none;authpriv.none /var/log/messages
authpriv.* /var/log/secure
mail.* /var/log/maillog
*.alert root
*.emerg *
local5.* @10.7.7.7
local7.* /var/log/boot.log
Facility
Creator of the message
Level
Severity of the message
Action
Destination of the message
auth or security†
authpriv
cron
daemon
kern
lpr
mail
mark (for syslog internal use)
news
syslog
user
uucp
local0 ... local7 (custom)
emerg or panic† (highest)
alert
crit
err or error†
warning or warn†
notice
info
debug (lowest)
none (facility disabled)
file message is written into a log file
@host message is sent to a logger
server host (via UDP port 514)
user1,user2,user3 message is sent to users'
consoles
*message is sent to all logged-in
users' consoles
† = deprecated
Facilities and levels are listed in the manpage man 3 syslog.
logger -p auth.info "Message"Send a message to syslogd with facility "auth" and priority "info"
logrotate Rotate logs. It gzips, renames, and eventually deletes old logfiles according to the
configuration file /etc/logrotate.conf
tail -f logfile
less +F logfile
Display the logs in real-time. Prints the end of the log file, showing new entries
and moving forward in the file as soon as they appear
/var/log/messages Global system logfile
/var/log/dmesg Kernel ring buffer information
/var/log/kern.log Kernel log
/var/log/boot.log Information logged during boot
/var/log/secure Information about failed authentication and authorization (e.g. sshd failed logins)
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
81/167 E-mail
E-mail
e.g. Pine, Mutt e.g. Sendmail, Exim, Postfix, qmail e.g. Procmail, SpamAssassin
~/.forward Mail address(es) to which forward the user's mail, or mail commands
/etc/aliases
/etc/mail/aliases Aliases database for users on the local machine. Each line has syntax alias: user
/var/spool/mail/user Inbox for user on the local machine
/var/log/mail.log (Debian)
/var/log/maillog (Red Hat) Mail logs
mail
mailx Commands to send mail
mailx -s "Subject" \
-S smtp="mailserver.foobar.com:25" \
jdoe@example.org < messagefile
Send a mail message to jdoe@example.org, using an
external SMTP server
uuencode binaryfile | mail jdoe@example.org Send a binary file to jdoe@example.org (not recommended
because many mailclients will display the received
attachment inline)
mutt -a binaryfile -- jdoe@example.org < /dev/null Send a binary file to jdoe@example.org using the Mutt MUA
Mailbox formats
mbox
Each mail folder is a single file, storing multiple email messages.
Advantages: universally supported, fast search inside a mail folder.
Disadvantages: issues with file locking, possible mailbox corruption.
$HOME/Mail/folder
Maildir
Each mail folder is a directory, and contains the subdirectories /cur, /new, and /tmp.
Each email message is stored in its own file with a unique filename ID.
The process that delivers an email message writes it to a file in the tmp/ directory,
and then moves it to new/. The moving is commonly done by hard linking the file to
new/ and then unlinking the file from tmp/, which guarantees that a MUA will not see
a partially written message as it never looks in tmp/.
When the MUA finds mail messages in new/ it moves them to cur/.
Advantages: fast location/retrieval/deletion of a specific mail message, no file locking
needed, can be used with NFS.
Disadvantages: some filesystems may not efficiently handle a large number of small
files, searching text inside all mail messages is slow
$HOME/Mail/folder/
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
MUA
(Mail User Agent)
mailclient of sender
MTA
(Mail Transfer Agent)
SMTP server of sender
MTA
(Mail Transfer Agent)
remote host
MDA
(Mail Delivery Agent)
mailserver of recipient
MUA
(Mail User Agent)
mailclient of recipient
82/167 SMTP
SMTP
SMTP commands
220 smtp.example.com ESMTP Postfix (server)
HELO xyz.linux.org (client)
250 Hello xyz.linux.org, glad to meet you
MAIL FROM: alice@linux.org
250 Ok
RCPT TO bob@foobar.com
250 Ok
RCPT TO carol@quux.net
250 Ok
DATA
354 End data with <CR><LF>.<CR><LF>
From: Alice <alice@linux.org>
To: Bob <bob@foobar.com>
Cc: Carol <carol@quux.net>
Date: Wed, 13 August 2014 18:02:43 -0500
Subject: Test message
This is a test message.
.
250 OK id=1OjReS-0005kT-Jj
QUIT
221 Bye
HELO xyz.linux.org Initiate the conversation and
identify client host to server
EHLO xyz.linux.org Like HELO, but tell server to
use Extended SMTP
MAIL FROM: alice@linux.org Specify mail sender
RCPT TO: bob@foobar.com Specify mail recipient
DATA Specify data to send. Ended
with a dot on a single line
QUIT
RSET Disconnect
HELP List all available commands
NOOP Empty command
VRFY alice@linux.org
Verify the existence of an e-
mail address (this command
should not be implemented,
for security reasons)
EXPN mailinglist Check mailing list membership
SMTP response codes
first digit
1Command accepted, but not processed until client sends confirmation
2Command successfully completed
3Command accepted, but not processed until client sends more information
4Command failed due to temporary errors
5Command failed due to permanent errors
second digit
0Syntax error or command not implemented
1Informative response in reply to a request for information
2Connection response in reply to a data transmission
5Status response in reply to a mail transfer operation
third digit Specifies further the response
211
214
220
221
250
251
354
421
450
451
452
500
501
502
503
504
550
551
552
553
554
System status or help reply
Help message
The server is ready
The server is ending the conversation
The requested action was completed
The specified user is not local, but the server will forward the mail message
Reply to the DATA command. After getting this, start sending the message body
The mail server will be shut down, try again later
The mailbox that you are trying to reach is busy, try again later
The requested action was not done. Some error occurred in the mail server
The requested action was not done. The mail server ran out of system storage
The last command contained a syntax error or the command line was too long
The parameters or arguments in the last command contained a syntax error
The last command is not implemented in the mail server
The last command was sent out of sequence
One of the parameters of the last command is not implemented by the server
The mailbox that you are trying to reach can't be found or you don't have access rights
The specified user is not local; part of message text will contain a forwarding address
The mailbox that you are trying to reach has run out of space, try again later
The mail address that you specified was not syntactically correct
The mail transaction has failed for unknown causes
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
83/167 Sendmail
Sendmail
Sendmail is a MTA distributed as a monolithic binary file.
Previous versions used to run SUID root, which caused many security problems; recent versions run SGID smmsp, the group
that has write access on the mail queue.
Sendmail uses smrsh, a restricted shell, to run some external programs.
/etc/mail/submit.cf Sendmail local mail transfer configuration file
/etc/mail/sendmail.cf Sendmail MTA configuration file
The .cf configuration files must not be edited by hand and are generated from editable .mc text files via the m4 command,
e.g. m4 /etc/mail/submit.mc > /etc/mail/submit.cf
/etc/mail/access.db Access control file to allow or deny access to systems or users
/etc/mail/local-host-names.db List of domains that must be considered as local accounts
/etc/mail/virtusertable.db Map for local accounts, used to distribute incoming email
/etc/mail/mailertable.db Routing table, used to dispatch emails from remote systems
/etc/mail/domaintable.db Domain table, used for transitions from an old domain to a new one
/etc/mail/genericstable.db Map for local accounts, used to specify a different sender for outgoing mail
/etc/mail/genericsdomain.db Local FQDN
The .db database files must not be edited by hand and are generated from editable text files via the makemap command,
e.g. makemap hash /etc/mail/access.db < /etc/mail/access
/var/spool/mqueue/ Temporary mailqueue files (where nnn is the Message ID):
dfnnn Mail body
qfnnn Message envelope with headers and routing information
Qfnnn Message envelope if abandoned
hfnnn Message envelope if held / quarantined by a milter (i.e. mail filter)
tfnnn Temporary file
lfnnn Lock file
nfnnn Backup file
xfnnn Transcript of delivery attempts
newaliases
sendmail -bi Update the aliases database; must be run after any change to /etc/aliases
mailq
sendmail -bp Examine the mail queue
sendmail -bt Run Sendmail in test mode
sendmail -q Force a queue run
hoststat Print statistics about remote hosts usage
purgestat Clear statistics about remote host usage
mailstats Print statistics about the mailserver
praliases Display email aliases
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
84/167 Exim
Exim
Exim is a free MTA, distributed under open source GPL license.
/etc/exim.conf
/usr/local/etc/exim/configure (FreeBSD) Exim4 configuration file
exim4 -bp Examine the mail queue
exim4 -M messageID Attempt delivery of message
exim4 -Mrm messageID Remove a message from the mail queue
exim4 -Mvh messageID See the headers of a message in the mail queue
exim4 -Mvb messageID See the body of a message in the mail queue
exim4 -Mvc messageID See a message in the mail queue
exim4 -qf domain Force a queue run of all queued messages for a domain
exim4 -Rff domain Attempt delivery of all queued messages for a domain
exim4 -bV Show version and other info
exinext Give the times of the next queue run
exigrep Search through Exim logfiles
exicyclog Rotate Exim logfiles
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
85/167 Postfix
Postfix
Postfix is a fast, secure, easy to configure, open source MTA intended as a replacement for Sendmail. It is implemented as
a set of small helper daemons, most of which run in a chroot jail with low privileges. The main ones are:
master Postfix master daemon, always running; starts the other daemons when necessary
nqmgr Queue manager for incoming and outgoing mail, always running
smtpd SMTP daemon for incoming mail
smtp SMTP daemon for outgoing mail
bounce Manager of bounce messages
cleanup Daemon that verifies the syntax of outgoing messages before they are handed to the queue manager
local Daemon that handles local mail delivery
virtual Daemon that handles mail delivery to virtual users
/var/spool/postfix/incoming Incoming queue.
All new mail entering the Postfix queue is written here by the cleanup daemon.
Under normal conditions this queue is nearly empty
/var/spool/postfix/active Active queue.
Contains messages ready to be sent. The queue manager places messages here
from the incoming queue as soon as they are available
/var/spool/postfix/deferred Deferred queue.
A message is placed here when all its deliverable recipients are delivered, and for
some recipients delivery failed for a transient reason. The queue manager scans
this queue periodically and puts some messages into the active queue for a retry
/var/spool/postfix/bounce Message delivery status report about why mail is bounced (non-delivered mail)
/var/spool/postfix/defer Message delivery status report about why mail is delayed (non-delivered mail)
/var/spool/postfix/trace Message delivery status report (delivered mail)
postfix reload Reload configuration
postconf -e 'mydomain = example.org' Edit a setting in the Postfix configuration
postconf -l List supported mailbox lock methods
postconf -m List supported database types
postconf -v Increase logfile verbosity
postmap dbtype:textfile Manage Postfix lookup tables, creating a hashed map file of database
type dbtype from textfile
postmap hash:/etc/postfix/transport Regenerate the transport database
postalias Convert /etc/aliases into the aliases database file /etc/aliases.db
postsuper Operate on the mail queue
postqueue Unprivileged mail queue manager
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
86/167 Postfix configuration
Postfix configuration
/etc/postfix/main.cf Postfix main configuration file
mydomain = example.org This system's domain
myorigin = $mydomain Domain from which all sent mail will appear to originate
myhostname = foobar.$mydomain This system's hostname
inet_interfaces = all Network interface addresses that this system receives mail on.
Value can also be localhost, all, or loopback-only
proxy_interfaces = 1.2.3.4 Network interface addresses that this system receives mail on
by means of a proxy or NAT unit
mynetworks = 10.3.3.0/24 !10.3.3.66 Networks the SMTP clients are allowed to connect from
mydestination = $myhostname, localhost,
$mydomain, example.com,
hash:/etc/postfix/otherdomains
Domains for which Postfix will accept received mail.
Value can also be a lookup database file e.g. a hashed map
relayhost = 10.6.6.6 Relay host to which Postfix should send all mail for delivery,
instead of consulting DNS MX records
relay_domains = $mydestination Sources and destinations for which mail will be relayed.
Can be empty if Postfix is not intended to be a mail relay
virtual_alias_domains = virtualex.org
virtual_alias_maps = /etc/postfix/virtual
or
virtual_alias_domains = hash:/etc/postfix/virtual
Set up Postfix to handle mail for virtual domains too.
The /etc/postfix/virtual file is a hashed map, each line of
the file containing the virtual domain email address and the
destination real domain email address:
jdoe@virtualex.org john.doe@example.org
ksmith@virtualex.org kim.smith
@virtualex.org root
The last line is a catch-all specifying that all other email
messages to the virtual domain are delivered to the root user
on the real domain
mailbox_command = /usr/bin/procmail Use Procmail as MDA
A line beginning with whitespace or tab is a continuation of the previous line.
A line beginning with a # is a comment. The # is not a comment delimiter when not placed at the beginning of a line.
/etc/postfix/master.cf Postfix master daemon configuration file
# service type private unpriv chroot wakeup maxproc command + args
smtp inet n - - - - smtpd
pickup fifo n - - 60 1 pickup
cleanup unix n - - - 0 cleanup
qmgr fifo n - - 300 1 qmgr
rewrite unix - - - - - trivial-rewrite
bounce unix - - - - 0 bounce
defer unix - - - - 0 bounce
flush unix n - - 1000? 0 flush
smtp unix - - - - - smtp
showq unix n - - - - showq
error unix - - - - - error
local unix - n n - - local
virtual unix - n n - - virtual
lmtp unix - - n - - lmtp
service Name of the service
type Transport mechanism used by the service
private Whether the service is accessible only by Postfix daemons and not by the whole system. Default is yes
unprivileged Whether the service is unprivileged i.e. not running as root. Default is yes
chroot Whether the service is chrooted. Default is yes
wakeup How often the service needs to be woken up by the master daemon. Default is never
maxproc Max number of simultaneous processes providing the service. Default is 50
command Command used to start the service
The - indicates that an option is set to its default value.
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
87/167 Procmail
Procmail
Procmail is a regex-based MDA whose main purpose is to preprocess and sort incoming email messages.
It is able to work both with the standard mbox format and the Maildir format.
To have all email processed by Procmail, the ~/.forward file may be edited to contain:
"|exec /usr/local/bin/procmail || exit 75"
/etc/procmailrc System-wide recipes
~/.procmailrc User's recipes
procmail -h List all Procmail flags for recipes
formail Utility for email filtering and editing
lockfile Utility for mailbox file locking
mailstat Utility for generation of reports from Procmail logs
/etc/procmailrc and ~/.procmailrc Procmail recipes
PATH=$HOME/bin:/usr/bin:/bin:/usr/sbin:/sbin
MAILDIR=$HOME/Mail
DEFAULT=$MAILDIR/Inbox
LOGFILE=$HOME/.procmaillog
Common parameters, non specific to Procmail
:0h: or :0:
* ^From: .*(alice|bob)@foobar\.org
$DEFAULT
Flag: match headers (default) and use file locking (highly
recommended when writing to a file or a mailbox in mbox format)
Condition: match the header specifying the sender address
Destination: default mailfolder
:0:
* ^From: .*owner@listserv\.com
* ^Subject:.*Linux
$MAILDIR/Geekstuff1
Conditions: match sender address and subject headers
Destination: specified mailfolder, in mbox format
:0
* ^From: .*owner@listserv\.com
* ^Subject:.*Linux
$MAILDIR/Geekstuff2/
Flag: file locking not necessary because using Maildir format
Conditions: match sender address and subject headers
Destination: specified mailfolder, in Maildir format
# Blacklisted by SpamAssassin
:0
* ^X-Spam-Status: Yes
/dev/null
Flag: file locking not necessary because blackholing to /dev/null
Condition: match SpamAssassin's specific header
Destination: delete the message
:0B:
* hacking
$MAILDIR/Geekstuff
Flag: match body of message instead of headers
:0HB:
* hacking
$MAILDIR/Geekstuff
Flag: match either headers or body of message
:0:
* > 256000
| /root/myprogram
Condition: match messages larger than 256 Kb
Destination: pipe message through the specified program
:0fw
* ^From: .*@foobar\.org
| /root/myprogram
Flags: use the pipe as a filter (modifying the message), and tell
Procmail to wait that the filter finished processing the message
:0c
* ^Subject:.*administration
! secretary@domain.com
:0:
$MAILDIR/Forwarded
Flag: copy the message and proceed with next recipe
Destination: forward to specified email address, and (as ordered
by the next recipe) save in the specified mailfolder
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
88/167 Courier POP configuration
Courier POP configuration
The Courier MTA provides modules for ESMTP, IMAP, POP3, webmail, and mailing list services in a single framework.
To use Courier, you must first launch the courier-authlib service, then launch the desired mail service e.g. courier-imap
for the IMAP service.
/usr/lib/courier-imap/etc/
or
/etc/courier/
imapd Courier IMAP daemon configuration
imapd-ssl Courier IMAPS daemon configuration
pop3d Courier POP3 daemon configuration
pop3d-ssl Courier POP3S daemon configuration
/usr/lib/courier-imap/share/ Directory for public and private keys
mkimapdcert Generate a certificate for the IMAPS service
mkpop3dcert Generate a certificate for the POP3 service
makealiases Create system aliases in /usr/lib/courier/etc/aliases.dat , which is
made by processing a /usr/lib/courier/etc/aliases/system text file:
root : postmaster
mailer-daemon : postmaster
MAILER-DAEMON : postmaster
uucp : postmaster
postmaster : admin
/usr/lib/courier-imap/etc/pop3d Courier POP configuration file
ADDRESS=0 Address to listen on. 0 means all addresses
PORT=127.0.0.1.900,192.168.0.1.900 Port number connections are accepted on. Accept connections on
port 900 on IP addresses 127.0.0.1 and 192.168.0.1
POP3AUTH="LOGIN CRAM-MD5 CRAM-SHA1" POP authentication advertising SASL (Simple Authentication and
Security Layer) capability, with CRAM-MD5 and CRAM-SHA1
POP3AUTH_TLS="LOGIN PLAIN" Also advertise SASL PLAIN if SSL is enabled
MAXDAEMONS=40 Maximum number of POP3 servers started
MAXPERIP=4 Maximum number of connections to accept from the same IP address
PIDFILE=/var/run/courier/pop3d.pid PID file
TCPDOPTS="-nodnslookup -noidentlookup" Miscellaneous couriertcpd options that shouldn't be changed
LOGGEROPTS="-name=pop3d" courierlogger options
POP3_PROXY=0 Enable or disable proxying
PROXY_HOSTNAME=myproxy Override value from gethostname() when checking if a proxy
connection is required
DEFDOMAIN="@example.com" Optional default domain. If the username does not contain the first
character of DEFDOMAIN, then it is appended to the username. If
DEFDOMAIN and DOMAINSEP are both set, then DEFDOMAIN is appended
only if the username does not contain any character from DOMAINSEP
POP3DSTART=YES Flag intended to be read by the system startup script
MAILDIRPATH=Maildir Name of the maildir directory
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
89/167 Courier IMAP configuration
Courier IMAP configuration
/usr/lib/courier-imap/etc/imapd Courier IMAP configuration file
ADDRESS=0 Address on which to listen. 0 means all addresses
PORT=127.0.0.1.900,192.168.0.1.900 Port number on which connections are accepted. Accepts connections
on port 900 on IP addresses 127.0.0.1 and 192.168.0.1
AUTHSERVICE143=imap Authenticate using a different service parameter depending on the
connection's port. This only works with authentication modules that
use the service parameter, such as PAM
MAXDAEMONS=40 Maximum number of IMAP servers started
MAXPERIP=20 Maximum number of connections to accept from the same IP address
PIDFILE=/var/run/courier/imapd.pid File where couriertcpd will save its process ID
TCPDOPTS="-nodnslookup -noidentlookup" Miscellaneous couriertcpd options that shouldn't be changed
LOGGEROPTS="-name=imapd" courierlogger options
DEFDOMAIN="@example.com" Optional default domain. If the username does not contain the first
character of DEFDOMAIN, then it is appended to the username. If
DEFDOMAIN and DOMAINSEP are both set, then DEFDOMAIN is appended
only if the username does not contain any character from DOMAINSEP
IMAP_CAPABILITY="IMAP4rev1 UIDPLUS \
CHILDREN NAMESPACE THREAD=ORDEREDSUBJECT \
THREAD=REFERENCES SORT QUOTA IDLE"
Specifies what most of the response should be to the CAPABILITY
command
IMAP_KEYWORDS=1 Enable or disable custom IMAP keywords. Possible values are:
0 disable keywords
1 enable keywords
2 enable keywords with a slower algorithm
IMAP_ACL=1 Enable or disable IMAP ACL extension
SMAP_CAPABILITY=SMAP1 Enable the experimental Simple Mail Access Protocol extensions
IMAP_PROXY=0 Enable or disable proxying
IMAP_PROXY_FOREIGN=0 Proxying to non-Courier servers. Resends the CAPABILITY command
after logging in to remote server. May not work with all IMAP clients
IMAP_IDLE_TIMEOUT=60 How often, in seconds, the server should poll for changes to the folder
while in IDLE mode
IMAP_CHECK_ALL_FOLDERS=0 Enable or disable server check for mail in every folder
IMAP_UMASK=022 Set the umask of the server process. This value is passed to the
umask command. This feature is mostly useful for shared folders,
where the file permissions of the messages may be important
IMAP_ULIMITD=131072 Set the upper limit of the size of the data segment of the server
process, in Kb. This value is passed to the ulimit -d command.
This feature is used as an additional safety check that should stop any
potential DoS attacks that exploit any kind of a memory leak to
exhaust all the available memory on the server
IMAP_USELOCKS=1 Enable or disable dot-locking to support concurrent multiple access to
the same folder. Strongly recommended when using shared folders
IMAP_SHAREDINDEXFILE=\
/etc/courier/shared/index
Index of all accessible folders.
Normally, this setting should not be changed
IMAP_TRASHFOLDERNAME=Trash Name of the trash folder
IMAP_EMPTYTRASH=Trash:7,Sent:30 Purge folders i.e. delete all messages from the specified folders after
the specified number of days
IMAP_MOVE_EXPUNGE_TO_TRASH=0 Enable or disable moving expunged messages to the trash folder
(instead of directly deleting them)
HEADERFROM=X-IMAP-Sender Make the return address, $SENDER, being saved in the
X-IMAP-Sender mail header. This header is added to the sent
message, but not in the copy of the message saved in the folder
MAILDIRPATH=Maildir Name of the mail directory
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
90/167 Dovecot
Dovecot
Dovecot is an open source, security-hardened, fast, and efficient IMAP and POP3 server.
By default it uses PAM authentication. The script mkcert.sh can be used to create self-signed SSL certificates.
/etc/dovecot.conf Dovecot configuration file
base_dir = /var/run/dovecot/ Base directory where to store runtime data
protocols = imaps pop3s Protocols to serve. If Dovecot should use dovecot-auth, this can be set
to none
listen = *, [::] Network interfaces to accept connections on.
Here, listen to all IPv4 and IPv6 interfaces
disable_plaintext_auth = yes Disable LOGIN command and all other plaintext authentications unless
SSL/TLS is used (LOGINDISABLED capability)
shutdown_clients = yes Kill all IMAP and POP3 processes when Dovecot master process shuts
down. If set to no, Dovecot can be upgraded without forcing existing
client connections to close
log_path = /dev/stderr Log file to use for error messages, instead of sending them to syslog.
Here, log to stderr
info_log_path = /dev/stderr Log file to use for informational and debug messages. Default value is
the same as log_path
syslog_facility = mail Syslog facility to use if logging to syslog
login_dir = /var/run/dovecot/login Directory where the authentication process places authentication UNIX
sockets, to which the login process needs to be able to connect
login_chroot = yes Chroot login process to the login_dir
login_user = dovecot User to use for the login process. This user is used to control access for
authentication process, and not to access mail messages
login_process_size = 64 Maximum login process size, in Mb
login_process_per_connection = yes If yes, each login is processed in its own process (more secure); if no,
each login process processes multiple connections (faster)
login_processes_count = 3 Number of login processes to keep for listening for new connections
login_max_processes_count = 128 Maximum number of login processes to create
login_max_connections = 256 Maximum number of connections allowed per each login process.
This setting is used only if login_process_per_connection = no; once
the limit is reached, the process notifies master so that it can create a
new login process
login_greeting = Dovecot ready. Greeting message for clients
login_trusted_networks = \
10.7.7.0/24 10.8.8.0/24
Trusted network ranges (usually IMAP proxy servers).
Connections from these IP addresses are allowed to override their IP
addresses and ports, for logging and authentication checks.
disable_plaintext_auth is also ignored for these networks
mbox_read_locks = fcntl
mbox_write_locks = dotlock fcntl
Locking methods to use for locking mailboxes in mbox format.
Possible values are:
dotlock Create mailbox.lock file; oldest and NSF-safe method
dotlock_try Same as dotlock, but skip if failing
fcntl Recommended; works with NFS too if lockd is used
flock May not exist in all systems; doesn't work with NFS
lockf May not exist in all systems; doesn't work with NFS
maildir_stat_dirs = no Option for mailboxes in Maildir format. If no (default), the LIST
command returns all entries in the mail directory beginning with a dot.
If yes, returns only entries which are directories
dbox_rotate_size = 2048
dbox_rotate_min_size = 16
Maximum and minimum file size, in Kb, of a mailbox in dbox format
until it is rotated
!include /etc/dovecot/conf.d/*.conf Include configuration file
!include_try /etc/dovecot/extra.conf Include optional configuration file, do not give error if file not found
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91/167 Dovecot mailbox configuration
Dovecot mailbox configuration
/etc/dovecot.conf Dovecot configuration file
mail_location = \
mbox:~/mail:INBOX=/var/spool/mail/%u
or
mail_location = maildir:~/Maildir
Mailbox location, in mbox or Maildir format. Variables:
%u username
%n user part in user@domain, same as %u if there is no domain
%d domain part in user@domain, empty if there is no domain
%h home directory
namespace shared { Definition of a shared namespace, for accessing other users' mailboxes
that have been shared.
Private namespaces are for users' personal emails.
Public namespaces are for shared mailboxes managed by root user
separator = / Hierarchy separator to use. Should be the same for all namespaces; it
depends on the underlying mail storage format
prefix = shared/%%u/ Prefix required to access this namespace; must be different for each.
Here, mailboxes are visible under shared/user@domain/ ; the variables
%%n, %%d and %%u are expanded to the destination user
location = maildir:%%h/Maildir:\
INDEX=~/Maildir/shared/%%u
Mailbox location for other users' mailboxes; it is in the same format as
mail_location which is also the default for it.
%variable and ~/ expand to the logged in user's data;
%%variable expands to the destination user's data
inbox = no There can be only one INBOX, and this setting defines which
namespace has it
hidden = no Define whether the namespace is hidden i.e. not advertised to clients
via NAMESPACE extension
subscriptions = no Namespace handles its own subscriptions; if set to no, the parent
namespace handles them and Dovecot uses the default namespace for
saving subscriptions. If prefix is empty, this should be set to yes
list = children Show the mailboxes under this namespace with LIST command,
making the namespace visible for clients that do not support the
NAMESPACE extension.
Here, lists child mailboxes but hide the namespace prefix; list the
namespace only if there are visible shared mailboxes
}
mail_uid = 666
mail_gid = 666
UID and GID used to access mail messages
mail_privileged_group = mail Group to enable temporarily for privileged operations; currently this is
used only with INBOX when its initial creation or a dotlocking fails
mail_access_groups = tmpmail Supplementary groups to grant access to for mail processes; typically
these are used to set up access to shared mailboxes
lock_method = fcntl Locking method for index files. Can be fcntl, flock, or dotlock
first_valid_uid = 500
last_valid_uid = 0
Valid UID range for users; default is 500 and above. This makes sure
that users cannot login as daemons or other system users.
Denying root login is hardcoded to Dovecot and cannot be bypassed
first_valid_gid = 1
last_valid_gid = 0
Valid GID range for users; default is non-root/wheel. Users having
non-valid primary GID are not allowed to login
max_mail_processes = 512 Maximum number of running mail processes. When this limit is
reached, new users are not allowed to login
mail_process_size = 256 Maximum mail process size, in Mb
valid_chroot_dirs = List of directories under which chrooting is allowed for mail processes
mail_chroot = Default chroot directory for mail processes. Usually not needed as
Dovecot does not allow users to access files outside their mail directory
mailbox_idle_check_interval = 30 When IDLE command is running, mailbox is checked once in a while to
see if there are any new mails or other changes. This setting defines
the minimum time to wait between these checks, in seconds
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
92/167 Dovecot POP and IMAP configuration
Dovecot POP and IMAP configuration
/etc/dovecot.conf Dovecot configuration file
protocol pop3 { Block with options for the POP3 protocol
listen = *:110 Network interfaces to accept POP3 connections on
login_executable = /usr/libexec/dovecot/pop3-login Location of the POP3 login executable
mail_executable = /usr/libexec/dovecot/pop3 Location of the POP3 mail executable
pop3_no_flag_updates = no If set to no, do not try to set mail messages non-recent
or seen with POP3 sessions, to reduce disk I/O.
With Maildir format do not move files from new/ to cur/,
with mbox format do not write Status- headers
pop3_lock_session = no Whether to keep the mailbox locked for the whole POP3
session
pop3_uidl_format = %08Xu%08Xv POP3 UIDL (Unique Mail Identifier) format to use
}
protocol imap { Block with options for the IMAP protocol
listen = *:143
ssl_listen = *:993
Network interfaces to accept IMAP and IMAPS
connections on
login_executable = /usr/libexec/dovecot/imap-login Location of the IMAP login executable
mail_executable = /usr/libexec/dovecot/imap Location of the IMAP mail executable
mail_max_userip_connections = 10 Maximum number of IMAP connections allowed for a
user from each IP address
imap_idle_notify_interval = 120 How many seconds to wait between "OK Still here"
notifications when client is IDLE
}
ssl = yes SSL/TLS support.
Possible values are yes, no, required
ssl_cert_file = /etc/ssl/certs/dovecot-cert.pem Location of the SSL certificate
ssl_key_file = /etc/ssl/private/dovecot-key.pem Location of private key
ssl_key_password = b1gs3cr3t Password of private key, if it is password-protected.
Since /etc/dovecot.conf is usually world-readable, it is
better to place this setting into a root-owned 0600 file
instead and include it via the setting
!include_try /etc/dovecot/dovecot-passwd.conf.
Alternatively, Dovecot can be started with
dovecot -p b1gs3cr3t
ssl_ca_file = /etc/dovecot/cafile.pem List of trusted SSL certificate authorities; the file
contains the CA certificates followed by the CRLs
ssl_verify_client_cert = yes Request client to send a certificate
ssl_cipher_list = ALL:!LOW:!SSLv2 List of SSL ciphers to use
verbose_ssl = yes Show protocol level SSL errors
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
93/167 Dovecot authentication
Dovecot authentication
/etc/dovecot.conf Dovecot configuration file
auth_executable = /usr/libexec/dovecot/dovecot-auth Location of the authentication executable
auth_process_size = 256 Max authentication process size, in Mb
auth_username_chars = abcde ... VWXYZ01234567890.-_@ List of allowed characters in the username. If the
username entered by the user contains a character not
listed in here, the login automatically fails. This is to
prevent a user exploiting any potential quote-escaping
vulnerabilities with SQL/LDAP databases
auth_realms = List of realms for SASL authentication mechanisms that
need them. If empty, multiple realms are not supported
auth_default_realm = example.org Default realm/domain to use if none was specified
auth_anonymous_username = anonymous Username to assign to users logging in with ANONYMOUS
SASL mechanism
auth_verbose = no Whether to log unsuccessful authentication attempts and
the reasons why they failed
auth_debug = no Whether to enable more verbose logging (e.g. SQL
queries) for debugging purposes
auth_failure_delay = 2 Delay before replying to failed authentications, in seconds
auth default {
mechanisms = plain login cram-md5 Accepted authentication mechanisms
passdb passwd-file {
args = /etc/dovecot.deny
deny = yes
}
Deny login to the users listed in /etc/dovecot.deny (file
contains one user per line)
passdb pam {
args = cache_key=%u%r dovecot
}
PAM authentication block.
Enables authentication matching (username and remote
IP address) for PAM
passdb passwd {
blocking = yes
args =
}
System users e.g. NSS or /etc/passwd
passdb shadow {
blocking = yes
args =
}
Shadow passwords for system users e.g. NSS or
/etc/passwd
passdb bsdauth {
cache_key = %u
args =
}
PAM-like authentication for OpenBSD
passdb sql {
args = /etc/dovecot/dovecot-sql.conf
}
SQL database
passdb ldap {
args = /etc/dovecot/dovecot-ldap.conf
}
LDAP database
socket listen {
master {
path = /var/run/dovecot/auth-master
mode = 0600
user =
group =
}
client {
path = /var/run/dovecot/auth-client
mode = 0660
}
}
Export the authentication interface to other programs.
Master socket provides access to userdb information; it is
typically used to give Dovecot's local delivery agent
access to userdb so it can find mailbox locations. The
default user/group is the one who started dovecot-auth
(i.e. root).
The client socket is generally safe to export to everyone.
Typical use is to export it to the SMTP server so it can do
SMTP AUTH lookups using it
}
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
94/167 FTP
FTP
Active mode (default)
1. Client connects to FTP server on port 21 (control channel) and sends second unprivileged port number
2. Server acknowledges
3. Server connects from port 20 (data channel) to client's second unprivileged port number
4. Client acknowledges
Passive mode (more protocol-compliant, because it is the client that initiates the connection)
1. Client connects to FTP server on port 21 and requests passive mode via the PASV command
2. Server acknowledges and sends unprivileged port number via the PORT command
3. Client connects to server's unprivileged port number
4. Server acknowledges
FTP servers
Very Secure FTP A hardened and high-performance FTP implementation. The vsftpd daemon operates with multiple
processes that run as a non-privileged user in a chrooted jail.
Pure-FTP A free, easy-to-use FTP server.
pure-ftpd Pure-FTP daemon
pure-ftpwho Show clients connected to the Pure-FTP server
pure-mrtginfo Show connections to the Pure-FTP server as a MRTG graph
pure-statsdecode Show Pure-FTP log data
pure-pw Manage Pure-FTP virtual accounts
pure-pwconvert Convert the system user database to a Pure-FTP virtual accounts database
pure-quotacheck Manage Pure-FTP quota database
pure-uploadscript Run a command on the Pure-FTP server to process an uploaded file
FTP clients
ftp Standard FTP client.
lftp A sophisticated FTP client with support for HTTP and BitTorrent.
lftp ftpserver.domain.org Connect to a FTP server and tries an anonymous login
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
95/167 vsftpd
vsftpd
/etc/vsftpd/vsftpd.conf Very Secure FTP server configuration file
listen=NO Run vsftpd in standalone mode (i.e. not via inetd)?
local_enable=YES Allow local system users (i.e. in /etc/passwd) to log in?
chroot_local_user=YES Chroot local users in their home directory?
write_enable=YES Allow FTP commands that write on the filesystem (i.e.
STOR, DELE, RNFR, RNTO, MKD, RMD, APPE and SITE)?
anonymous_enable=YES Allow anonymous logins? If yes, anonymous and ftp are
accepted as logins
anon_root=/var/ftp/pub After anonymous login, go to directory /var/ftp/pub
anon_upload_enable=YES Allow anonymous uploads?
chown_uploads=YES Change ownership of anonymously uploaded files?
chown_username=ftp Change ownership of anonymously uploaded files to user
ftp
anon_world_readable_only=NO Allow anonymous users to only download files which are
world readable?
ssl_enable=YES Enable SSL?
force_local_data_ssl=NO Encrypt local data?
force_local_logins_ssl=YES Force encrypted authentication?
allow_anon_ssl=YES Allow anonymous users to use SSL?
ssl_tlsv1=YES
ssl_tlsv2=NO
ssl_tlsv3=NO
Versions of SSL/TLS that are allowed
rsa_cert_file=/etc/pki/tls/certs/vsftpd.pem Location of certificate file
rsa_private_key_file=/etc/pki/tls/certs/vsftpd.pem Location of private key file
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
96/167 CUPS
CUPS
cupsd CUPS (Common Unix Printing System) daemon.
Administration of printers is done via web interface on http://localhost:631
/etc/cups/cupsd.conf CUPS configuration file
/etc/cups/printers.conf Database of available local CUPS printers
/etc/printcap Database of printer capabilities, for old printing applications
/var/spool/cups/ Printer spooler for data awaiting to be printed
/var/log/cups/error_log CUPS error log
/var/log/cups/page_log Information about printed pages
/etc/init.d/cupsys start Start the CUPS service
gnome-cups-manager Run the CUPS Manager graphical application
cupsenable printer0 Enable a CUPS printer
cupsdisable printer0 Disable a CUPS printer
cupsaccept printer0 Accept a job sent on a printer queue
cupsreject -r "Rejected" printer0 Reject a job sent on a printer queue, with an informational message
cupstestppd LEXC510.ppd Test the conformance of a PPD file to the format specification
cupsaddsmb printer0 Export a printer to Samba (for use with Windows clients)
cups-config --cflags Show the necessary compiler options
cups-config --datadir Show the default CUPS data directory
cups-config --ldflags Show the necessary linker options
cups-config --libs Show the necessary libraries to link to
cups-config --serverbin Show the default CUPS binaries directory that stores filters and backends
cups-config --serverroot Show the default CUPS configuration file directory
lpstat Show CUPS status information
lpadmin Administer CUPS printers
lpadmin -p printer0 -P LEXC750.ppd Specify a PPD (Adobe PostScript Printer Description) file to associate to a printer
lp -d printer0 file Print a file on the specified printer
lpq View the default print queue
lpq -P printer0 View a specific print queue
lpq jdoe View the print queue of a specific user
lprm -P printer0 5 Delete a specific job from a printer queue
lprm -P printer0 jdoe Delete all jobs from a specific user from a printer queue
lprm -P printer0 - Delete all jobs from a printer queue
lpc Manage print queues
a2ps file.txt Convert a text file to PostScript
ps2pdf file.ps Convert a file from PostScript to PDF
mpage file.ps Print a PostScript document on multiple pages per sheet on a PostScript printer
gv file.ps View a PostScript document (the gv software is derived from GhostView)
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
97/167 IP addressing
IP addressing
IPv4 addressing
Address range Prefix Number of addresses Reference
Classful
Class A (Unicast) 0.0.0.0 – 127.255.255.255
first octet: 0XXX XXXX
/8 128 networks ×
16,777,216 addresses
RFC 791
Class B (Unicast) 128.0.0.0 – 191.255.255.255
first octet: 10XX XXXX
/16 16,384 networks ×
65,536 addresses
RFC 791
Class C (Unicast) 192.0.0.0 – 223.255.255.255
first octet: 110X XXXX
/24 2,097,152 networks ×
256 addresses
RFC 791
Class D (Multicast) 224.0.0.0 – 239.255.255.255
first octet: 1110 XXXX
/4 268,435,456 RFC 3171
Class E (Experimental) 240.0.0.0 – 255.255.255.255
first octet: 1111 XXXX
/4 268,435,456 RFC 1166
Private
Private Class A 10.0.0.0 – 10.255.255.255 10.0.0.0/8 16,777,216 RFC 1918
Private Class B 172.16.0.0 – 172.31.255.255 172.16.0.0/12 1,048,576 RFC 1918
Private Class C 192.168.0.0 – 192.168.255.255 192.168.0.0/16 65,536 RFC 1918
Reserved
Source 0.0.0.0 – 0.255.255.255 0.0.0.0/8 16,777,216 RFC 1700
Loopback 127.0.0.0 – 127.255.255.255 127.0.0.0/8 16,777,216 RFC 1700
Autoconf 169.254.0.0 – 169.254.255.255 169.254.0.0/16 65,536 RFC 3330
TEST-NET 192.0.2.0 – 192.0.2.255 192.0.2.0/24 256 RFC 3330
6to4 relay anycast 192.88.99.0 – 192.88.99.255 192.88.99.0/24 256 RFC 3068
Device benchmarks 198.18.0.0 – 198.19.255.255 198.18.0.0/15 131,072 RFC 2544
IPv4 address: 32-bit long, represented divided in four octets (dotted-quad).
e.g. 193.22.33.44
4 × 109 total addresses
IPv4 classful addressing is obsolete and has been replaced by CIDR (Classless Inter-Domain Routing).
IPv6 addressing
Unicast
64-bit network prefix (>= 48-bit routing prefix + <= 16-bit subnet id) + 64-bit interface identifier
A 48-bit MAC address is transformed into a 64-bit EUI-64 by inserting ff:fe in the middle.
A EUI-64 is then transformed into an IPv6 interface identifier by inverting the 7th most significant bit.
Link-local fe80:0000:0000:0000 + 64-bit interface identifier
Multicast ff + 4-bit flag + 4-bit scope field + 112-bit group ID
IPv6 address: 128-bit long, represented divided in eight 16-bit groups (4 hex digits).
e.g. 2130:0000:0000:0000:0007:0040:15bc:235f which can also be written as 2130::7:40:15bc:235f
Leading zeros in each group can be deleted. A single chunk of one or more adjacent 0000 groups can be deleted.
3 × 1038 total addresses
The IANA (Internet Assigned Numbers Authority) manages the allocation of IPv4 and IPv6 addresses, assigning large blocks
to RIRs (Regional Internet Registries) which in turn allocate addresses to ISPs and other local registries.
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
98/167 Subnetting
Subnetting
VLSM chart - Last octet subnetting (CIDR notation)
Prefix: /24
Netmask: .0
00000000
1 subnet
254 hosts each
254 total hosts
Prefix: /25
Netmask: .128
10000000
2 subnets
126 hosts each
252 total hosts
Prefix: /26
Netmask: .192
11000000
4 subnets
62 hosts each
248 total hosts
Prefix: /27
Netmask: .224
11100000
8 subnets
30 hosts each
240 total hosts
Prefix: /28
Netmask: .240
11110000
16 subnets
14 hosts each
224 total hosts
Prefix: /29
Netmask: .248
11111000
32 subnets
6 hosts each
192 total hosts
Prefix: /30
Netmask: .252
11111100
64 subnets
2 hosts each
128 total hosts
.0
.0
.0
.0
.0
.0 .0
.4
.8 .8
.12
.16
.16 .16
.20
.24 .24
.28
.32
.32
.32 .32
.36
.40 .40
.44
.48
.48 .48
.52
.56 .56
.60
.64
.64
.64
.64 .64
.68
.72 .72
.76
.80
.80 .80
.84
.88 .88
.92
.96
.96
.96 .96
.100
.104 .104
.108
.112
.112 .112
.116
.120 .120
.124
.128
.128
.128
.128
.128 .128
.132
.136 .136
.140
.144
.144 .144
.148
.152 .152
.156
.160
.160
.160 .160
.164
.168 .168
.172
.176
.176 .176
.180
.184 .184
.188
.192
.192
.192
.192 .192
.196
.200 .200
.204
.208
.208 .208
.212
.216 .216
.220
.224
.224
.224 .224
.228
.232 .232
.236
.240
.240 .240
.244
.248 .248
.252
Each block of a column identifies a subnet, whose range of valid hosts addresses is [network address +1 — broadcast
address -1] inclusive.
The network address of the subnet is the number shown inside a block.
The broadcast address of the subnet is the network address of the block underneath -1 or, for the bottom block, .255.
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
99/167 Network services
Network services
Most common well-known ports
Port number Service
20 TCP FTP (data)
21 TCP FTP (control)
22 TCP SSH
23 TCP Telnet
25 TCP SMTP
53 TCP/UDP DNS
67 UDP BOOTP/DHCP (server)
68 UDP BOOTP/DHCP (client)
80 TCP HTTP
110 TCP POP3
119 TCP NNTP
123 UDP NTP
139 TCP/UDP Microsoft NetBIOS
143 TCP IMAP
161 UDP SNMP
443 TCP HTTPS (HTTP over SSL/TLS)
465 TCP SMTP over SSL
993 TCP IMAPS (IMAP over SSL)
995 TCP POP3S (POP3 over SSL)
1-1023: privileged ports, used server-side
1024-65535: unprivileged ports, used client-side
/etc/services lists all well-known ports.
Many network services are run by the xinetd super server.
ISO/OSI and TCP/IP protocol stack models
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
Physical
Data Link Network Access
Network Internet
Transport Transport
Session
Presentation
ISO/OSI TCP/IP
1
2
3
4
5
6
7Application
Application
Ethernet, Wi-Fi, PPP Frame
IPv4, IPv6, ICMP Packet
TCP, UDP Segment (TCP), Datagram (UDP)
HTTP, SMTP, POP Message
Standards (e.g.) Data transmission unitLayer
Bit
100/167 Network configuration commands
Network configuration commands
ip a
ip addr
ip addr show
ifconfig -a
Display configuration of all network
interfaces
ip link show eth0
ifconfig eth0
Display configuration of eth0
ip addr add dev eth0 10.1.1.1/8
ifconfig eth0 10.1.1.1 netmask 255.0.0.0 broadcast 10.255.255.255
Configure IP address of eth0
ifconfig eth0 hw ether 45:67:89:ab:cd:ef Configure MAC address of eth0
ip link set eth0 up
ifconfig eth0 up
ifup eth0
Activate eth0
ip link set eth0 down
ifconfig eth0 down
ifdown eth0
Shut down eth0
dhclient eth0
pump
dhcpcd eth0 (SUSE)
Request an IP address via DHCP
ip neigh
arp -a
Show the ARP cache table
ip neigh show 10.1.0.6
arp 10.1.0.6
Show the ARP cache entry for a host
ip neigh add 10.1.0.7 lladdr 01:23:45:67:89:ab dev eth0
arp -s 10.1.0.7 01:23:45:67:89:ab
Add a new ARP entry for a host
ip neigh del 10.1.0.7 dev eth0
arp -d 10.1.0.7
Delete an ARP entry
ip neigh flush all Delete the ARP table for all interfaces
hostname Get the hostname
hostname -f Get the FQDN (Fully Qualified Domain Name)
hostname mylinuxbox
hostnamectl set-hostname --static "mylinuxbox" (RHEL 7)
Set the hostname
hostnamectl (RHEL 7) Get the hostname, OS, and other information
/etc/init.d/networking restart (Debian)
/etc/init.d/network restart (Red Hat)
Restart network services
ethtool option device Query or control network driver and hardware
settings
ethtool eth0 View hardware settings of eth0
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101/167 Wireless networking
Wireless networking
iwlist wlan0 scan List all wireless devices in range, with their quality of signal and other information
iwlist wlan0 freq Display transmission frequency settings
iwlist wlan0 rate Display transmission speed settings
iwlist wlan0 txpower Display transmission power settings
iwlist wlan0 key Display encryption settings
iwgetid wlan0 option Print NWID, ESSID, AP/Cell address or other information about the wireless network
that is currently in use
iwconfig wlan0 Display configuration of wireless interface wlan0
iwconfig wlan0 option Configure wireless interface wlan0
iw dev wlan0 station dump On a wireless card configured in AP Mode, display information (e.g. MAC address,
tx/rx, bitrate, signal strength) about the clients
rfkill list List installed wireless devices
rfkill unblock nEnable wireless device number n
hcidump -i device Display raw HCI (Host Controller Interface) data exchanged with a Bluetooth device
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102/167 Network tools
Network tools
dig example.org Perform a DNS lookup for the specified domain or hostname.
Returns information in BIND zone file syntax; uses an internal
resolver and hence does not honor /etc/resolv.conf
host example.org
nslookup example.org (deprecated)
Perform a DNS lookup for the specified domain or hostname.
Does honor /etc/resolv.conf
dig @nameserver -t MX example.org
host -t example.org nameserver
Perform a DNS lookup for the MX record of the specified
domain, querying nameserver
dig example.org any
host -a example.org
Get all DNS records for a domain
dig -x a.b.c.d
host a.b.c.d
Perform a reverse DNS lookup for the IP address a.b.c.d
whois example.org Query the WHOIS service for an Internet resource, usually a
domain name
ping host Test if a remote host can be reached and measure the round-
trip time to it. This is done by sending an ICMP Echo Request
datagram and expecting an ICMP Echo Response
fping -a host1 host2 host3 Ping multiple hosts in parallel and report which ones are alive
bing host1 host2 Calculate point-to-point throughput between two remote
hosts
traceroute host Print the route, hop by hop, packets trace to a remote host.
This is done by sending a sequence of ICMP Echo Request
datagrams with increasing TTL values, starting with TTL=1,
and expecting ICMP Time Exceeded datagrams
tracepath host Simpler traceroute
mtr host traceroute and ping combined
redir --laddr=ip1 --lport=port1 \
--caddr=ip2 --cport=port2
Redirect all connections coming to local IP address ip1 and
port port1, to remote IP address ip2 and port port2
telnet host port Establish a telnet connection to the specified host and port
number. If port is omitted, uses default port 23
ftp host Establish an interactive FTP connection with the remote host
wget –-no-clobber –-html-extension \
--page-requisites --convert-links \
--recursive --domains example.org \
--no-parent www.example.org/path
Download a whole website www.example.org/path
curl www.example.org/file.html -o myfile.html Download a file via HTTP and save it locally under another
name
curl -u user:password 'ftp://ftpserver/path/file' Download a file via FTP, after logging in to the server
curl -XPUT webserver -d'data'Send a HTTP PUT command with data to webserver
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103/167 Network monitoring
Network monitoring
netstat Display network connections
netstat –-tcp
netstat -t
Display active TCP connections
netstat -l Display only listening sockets
netstat -a Display all listening and non-listening sockets
netstat -n Display network connections, without resolving hostnames or portnames
netstat -p Display network connections, with PID and name of program to which each socket
belongs
netstat -i Display network interfaces
netstat -s Display protocol statistics
netstat -r Display kernel routing tables (equivalent to route -e)
netstat -c Display network connections continuously
ss Display socket statistics (similarly to netstat)
ss -t -a Display all TCP sockets
nmap host
nmap -sS host
Scan for open TCP ports (TCP SYN scan) on remote host
nmap -sP host Do a ping sweep (ICMP ECHO probes) on remote host
nmap -sU host Scan for open UDP ports on remote host
nmap -sV host Do a service and version scan on open ports
nmap -p 1-65535 host Scan all ports (1-65535), not only the common ports, on remote host
nmap -O host Find which operating system is running on remote host (OS fingerprinting)
arp-scan Scan all hosts on the LAN. Uses ARP (Layer 2) packets and is therefore able to find
hosts that drop all IP or ICMP traffic
ngrep Filter data payload of network packets matching a specified regex
nload Display a graph of the current network usage
iptraf
iptraf-ng
IP LAN monitor (ncurses UI)
netserver Run a network performance benchmark server
netperf Do network performance benchmarks by connecting to a netserver
iperf -s Run a network throughput benchmark server
iperf -c server Perform network throughput tests in client mode, by connecting to an iperf server
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104/167 Packet sniffing
Packet sniffing
tcpdump -ni eth0 Sniff all network traffic on interface eth0,
suppressing DNS resolution
tcpdump ip host 10.0.0.2 tcp port 25 Sniff network packets on TCP port 25 from and
to 10.0.0.2
tcpdump ether host '45:67:89:ab:cd:ef' Sniff traffic from and to the network interface
having MAC address 45:67:89:ab:cd:ef
tcpdump 'src host 10.0.0.2 and (tcp port 80 or tcp port 443)' Sniff HTTP and HTTPS traffic having as source
host 10.0.0.2
tcpdump -ni eth0 not port 22 Sniff all traffic on eth0 except that belonging
to the SSH connection
tcpdump -vvnn -i eth0 arp Sniff ARP traffic on eth0, on maximum
verbosity level, without converting host IP
addresses and port numbers to names
tcpdump ip host 10.0.0.2 and not 10.0.0.9 Sniff IP traffic between 10.0.0.2 and any other
host except 10.0.0.9
dhcpdump -i eth0 Sniff all DHCP packets on interface eth0
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
105/167 netcat
netcat
nc
ncat (Red Hat)
netcat (SUSE)
Netcat, "the Swiss Army knife of networking", a very flexible
generic TCP/IP client/server
nc -z 10.0.0.7 22
ncat 10.0.0.7 22
Scan for a listening SSH daemon on remote host 10.0.0.7
nc -l -p 25 Listen for connections on port 25 (i.e. mimic a SMTP server).
Send any input received on stdin to the connected client and
dump on stdout any data received from the client
nc 10.0.0.7 389 < file Push the content of file to port 389 on remote host 10.0.0.7
echo "GET / HTTP/1.0\r\n\r\n" | nc 10.0.0.7 80 Connect to web server 10.0.0.7 and issue a HTTP GET
while true; \
do nc -l -p 80 -q 1 < page.html; done
while true; \
do echo "<html><body><h1>WWW</h1></body></html>" \
| ncat -l -p 80; done
Start a minimal web server, serving the specified HTML page
to any connected client
nc -v -n -z -w1 -r 10.0.0.7 1-1023 Run a TCP port scan against remote host 10.0.0.7.
Probes randomly all privileged ports with a 1-second timeout,
without resolving service names, and with verbose output
echo "" | nc -v -n -w1 10.0.0.7 1-1023 Retrieve the greeting banner of any network service that
might be running on remote host 10.0.0.7
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
106/167 Network settings
Network settings
/etc/hosts Mappings between IP addresses and hostnames, for name resolution
127.0.0.1 localhost.localdomain localhost
10.2.3.4 myhost.domain.org myhost
/etc/nsswitch.conf Sources that must be used by various system library lookup functions
passwd: files nisplus nis
shadow: files nisplus nis
group: files nisplus nis
hosts: files dns nisplus nis
/etc/host.conf Sources for name resolution, for systems before glibc2.
Obsolete, superseded by /etc/nsswitch.conf
order hosts,bind
multi on
/etc/resolv.conf Domain names that must be appended to bare hostnames, and DNS servers
that will be used for name resolution
search domain1.org domain2.org
nameserver 192.168.3.3
nameserver 192.168.4.4
/etc/networks Mappings between network addresses and names
loopback 127.0.0.0
mylan 10.2.3.0
/etc/services List of service TCP/UDP port numbers
/etc/protocols List of available protocols
/sys/class/net List of all network interfaces in the system
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
107/167 Network configuration
Network configuration
Red Hat
/etc/sysconfig/network Network configuration file
ADDRESS=10.2.3.4
NETMASK=255.255.255.0
GATEWAY=10.2.3.254
HOSTNAME=mylinuxbox.example.org
NETWORKING=yes
/etc/sysconfig/network-scripts/ifcfg-eth0 Configuration file for eth0.
This file is read by the ifup and ifdown scripts
DEVICE=eth0
TYPE=Ethernet
HWADDR=AA:BB:CC:DD:EE:FF
BOOTPROTO=none
ONBOOT=yes
NM_CONTROLLED=no
IPADDR=10.2.3.4
NETMASK=255.255.255.0
GATEWAY=10.2.3.254
DNS1=8.8.8.8
DNS2=4.4.4.4
USERCTL=no
/etc/sysconfig/network-scripts/ifcfg-eth0:0
/etc/sysconfig/network-scripts/ifcfg-eth0:1
/etc/sysconfig/network-scripts/ifcfg-eth0:2
Multiple configuration files for a single eth0 interface, which allows
binding multiple IP addresses to a single NIC
/etc/sysconfig/network-scripts/route-eth0 Static route configuration for eth0
default 10.2.3.4 dev eth0
10.7.8.0/24 via 10.2.3.254 dev eth0
10.7.9.0/24 via 10.2.3.254 dev eth0
/etc/ethertypes Ethernet frame types.
Lists various Ethernet protocol types used on Ethernet networks
Debian
/etc/network/interfaces List and configuration of all network interfaces
allow-hotplug eth0
iface eth0 inet static
address 10.2.3.4
netmask 255.255.255.0
gateway 10.2.3.254
dns-domain example.com
dns-nameservers 8.8.8.8 4.4.4.4
/etc/hostname Hostname of the local machine
/etc/ethers ARP mappings (i.e. MAC to IP addresses)
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108/167 nmcli
nmcli
In RHEL7 the network configuration is managed by the NetworkManager daemon.
A connection is a network configuration that applies to a device (aka network interface). A device can be included in
multiple connections, but only one of them may be active at a time.
The configuration for connection is stored in the file /etc/sysconfig/network-scripts/ifcfg-connection. Although it is
possible to set up networking by editing these configuration files, it is much easier to use the command nmcli.
nmcli device status Show all network devices
nmcli device disconnect iface Disconnects the device iface.
This command should be used instead of
nmcli connection down connection
because if connection is set to autoconnect, Network
Manager will bring it up again shortly
nmcli connection show Show all connections. Connections with an empty
device entry are inactive
nmcli connection show --active Show active connections
nmcli connection show connection Show the configuration of connection
nmcli connection add con-name connection \
type ethernet ifname iface ipv4.method manual \
ipv4.addresses 10.0.0.13/24 ipv4.gateway 10.0.0.254
Configure a new connection that uses the Ethernet
interface iface and assigns it an IPv4 address and
gateway
nmcli connection modify connection [options] Modify the configuration of connection
nmcli connection up connection Brings up a connection
nmcli connection reload Reload any manual change made to the files
/etc/sysconfig/network-scripts/ifcfg-*
The manpage man nmcli-examples contains many network configuration examples.
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
109/167 Teaming and bridging
Teaming and bridging
Network teaming allows binding together two or more network interfaces to increase throughput or provide redundancy.
RHEL7 implements network teaming via the teamd daemon.
How to set up a teaming connection
1. nmcli connection add type team con-name teamcon ifname teamif \
config '{"runner":{"name":"loadbalance"}}'
Set up a team connection teamcon and a
team interface teamif with a runner (in
JSON code) for automatic failover
2. nmcli connection modify teamcon ipv4.method manual \
ipv4.addresses 10.0.0.14/24 ipv4.gateway 10.0.0.254
Assign manually an IP address and
gateway
3. nmcli connection add type team-slave ifname iface \
master teamcon
Add an existing device iface as a slave of
team teamcon.
The slave connection will be automatically
named team-slave-iface
4. Repeat the previous step for each slave interface.
teamdctl teamif state Show the state of the team interface teamif
teamnl teamif command Debug a team interface teamif
A network bridge emulates a hardware bridge, i.e. a Layer 2 device able to forward traffic between networks based on
MAC addresses.
How to set up a bridge connection
1. nmcli connection add type bridge con-name brcon ifname brif Set up a bridge connection brcon and a
bridge interface brif
2. nmcli connection modify brcon ipv4.method manual \
ipv4.addresses 10.0.0.15/24 ipv4.gateway 10.0.0.254
Assign manually an IP address and
gateway
3. nmcli connection add type bridge-slave ifname iface \
master brcon
Add an existing device iface as a slave of
bridge brcon.
The slave connection will be automatically
named bridge-slave-iface
4. Repeat the previous step for each slave interface.
brctl show brif Display information about the bridge interface brif
The manpage man teamd.conf lists many examples of team configurations and runners.
The manpage man nmcli-examples contains, among others, examples of teaming and bridging configuration.
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110/167 TCP Wrapper
TCP Wrapper
/etc/hosts.allow
/etc/hosts.deny
Host access control files used by the TCP Wrapper system.
Each file contains zero or more daemon:client lines. The first matching line is considered.
Access is granted when a daemon:client pair matches an entry in /etc/hosts.allow.
Otherwise, access is denied when a daemon:client pair matches an entry in /etc/hosts.deny.
Otherwise, access is granted.
/etc/hosts.allow and /etc/hosts.deny lines syntax
ALL: ALL All services to all hosts
ALL: .example.edu All services to all hosts of the example.edu domain
ALL: .example.edu EXCEPT host1.example.edu All services to all hosts of example.edu, except host1
in.fingerd: .example.com Finger service to all hosts of example.com
in.tftpd: LOCAL TFTP to hosts of the local domain only
sshd: 10.0.0.3 10.0.0.4 10.1.1.0/24 SSH to the hosts and network specified
sshd: 10.0.1.0/24 SSH to 10.0.1.0/24
sshd: 10.0.1. SSH to 10.0.1.0/24
sshd: 10.0.1.0/255.255.255.0 SSH to 10.0.1.0/24
in.tftpd: ALL: spawn (/safe_dir/safe_finger \
-l @%h | /bin/mail -s %d-%h root) &
Send a finger probe to hosts attempting TFTP and
notify root user via email
portmap: ALL: (echo Illegal RPC request \
from %h | /bin/mail root) &
When a client attempts a RPC request via the
portmapper (NFS access), echo a message to the
terminal and notify root user via email
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111/167 Routing
Routing
Output of command route -en
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.3.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
0.0.0.0 192.168.3.1 0.0.0.0 UG 0 0 0 eth0
Destination
network or host destination network or host
0.0.0.0 default route
Gateway
host gateway
0.0.0.0
*no gateway needed, network is directly connected
-rejected route
Genmask
network mask network mask to apply for the destination network
255.255.255.255 destination host
0.0.0.0 default route
Flags
U route is up
Guse gateway
Htarget is host
!rejected route
Ddynamically installed by daemon
Mmodified from routing daemon
Rreinstate route for dynamic routing
ip route
route -en
route -F
netstat -rn
Display IP routing table
ip route show cache
route -C
Display kernel routing cache
ip route add default via 10.1.1.254
route add default gw 10.1.1.254
Add a default gateway
ip route add 10.2.0.1 dev eth0
ip route add 10.2.0.1 via 10.2.0.254
route add -host 10.2.0.1 gw 10.2.0.254
Add a route for a host
ip route add 10.2.0.0/16 via 10.2.0.254
route add -net 10.2.0.0 netmask 255.255.0.0 gw 10.2.0.254
Add a route for a network
ip route delete 10.2.0.1 dev eth0
route del -host 10.2.0.1 gw 10.2.0.254
Delete a route for a host
ip route flush all Delete the routing table for all interfaces
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112/167 iptables
iptables
The Netfilter framework provides firewalling capabilities in Linux. It is implemented by the user-space application programs
iptables for IPv4 (which replaced ipchains, which itself replaced ipfwadm) and ip6tables for IPv6.
iptables is implemented in the kernel and therefore does not have a daemon process or a service.
The ability to track connection state is provided by the ip_conntrack kernel module.
In RHEL 7, iptables is replaced by the firewalld daemon. It is possible, but not recommended, to use iptables anyway by
installing the package iptables-services (which provides a systemd interface for iptables) and disabling firewalld.
In Ubuntu, iptables is managed by the ufw service (Uncomplicated Firewall).
/etc/sysconfig/iptables Default file containing the firewall rules
iptables-restore < file Load into iptables the firewall rules specified in the file
iptables-save > file Save into iptables the firewall rules specified in the file
iptables rules file
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
COMMIT
Delete all rules and open the firewall to all connections
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113/167 iptables rules
iptables rules
Iptables uses tables containing sets of chains, which contain sets of rules. Each rule has a target (e.g. ACCEPT).
The "filter" table contains chains INPUT, FORWARD, OUTPUT (built-in chains); this is the default table to which all iptables
commands are applied, unless another table is specified via the -t option.
The "nat" table contains chains PREROUTING, OUTPUT, POSTROUTING.
The "mangle" table contains chains PREROUTING, OUTPUT.
When a packet enters the system, it is handed to the INPUT chain. If the destination is local, it is processed; if the
destination is not local and IP forwarding is enabled, the packet is handed to the FORWARD chain, otherwise it is dropped.
An outgoing packet generated by the system will go through the OUTPUT chain.
If NAT is in use, an incoming packet will pass at first through the PREROUTING chain, and an outgoing packet will pass last
through the POSTROUTING chain.
iptables -A INPUT -s 10.0.0.6 -j ACCEPT Add a rule to accept all packets from 10.0.0.6
iptables -A INPUT -s 10.0.0.7 -j REJECT Add a rule to reject all packets from 10.0.0.7 and send
back a ICMP response to the sender
iptables -A INPUT -s 10.0.0.8 -j DROP Add a rule to silently drop all packets from 10.0.0.8
iptables -A INPUT -s 10.0.0.9 -j LOG Add a rule to log via syslog all packets from 10.0.0.9
iptables -D INPUT -s 10.0.0.9 -j LOG Delete a specific rule
iptables -D INPUT 42 Delete rule 42 of the INPUT chain
iptables -F INPUT Flush all rules of the INPUT chain
iptables -F Flush all rules, hence disabling the firewall
iptables -t mangle -F Flush all rules of the "mangle" table
iptables -t mangle -X Delete all user-defined (not built-in) rules in the "mangle"
table
iptables -L INPUT List the rules of the INPUT chain
iptables -L -n List all rules, without translating numeric values (IP
addresses to FQDNs and port numbers to services)
iptables -N mychain Define a new chain
iptables -P INPUT DROP Define the chain policy target, which takes effect when no
rule matches and the end of the rules list is reached
iptables -A OUTPUT -d 10.7.7.0/24 -j DROP Add a rule to drop all packets with destination 10.7.7.0/24
iptables -A FORWARD -i eth0 -o eth1 -j LOG Add a rule to log all packets entering the system via eth0
and exiting via eth1
iptables -A INPUT -p 17 -j DROP
iptables -A INPUT -p udp -j DROP
Add a rule to drop all incoming UDP traffic (protocol
numbers are defined in /etc/protocols)
iptables -A INPUT --sport 1024:65535 --dport 53 \
-j ACCEPT
Add a rule to accept all packets coming from any
unprivileged port and with destination port 53
iptables -A INPUT -p icmp --icmp-type echo-request \
-m limit --limit 1/s -i eth0 -j ACCEPT
Add a rule to accept incoming pings through eth0 at a
maximum rate of 1 ping/second
iptables -A INPUT -m state --state ESTABLISHED \
-j ACCEPT
Load the module for stateful packet filtering, and add a
rule to accept all packets that are part of a
communication already tracked by the state module
iptables -A INPUT -m state --state NEW -j ACCEPT Add a rule to accept all packets that are not part of a
communication already tracked by the state module
iptables -A INPUT -m state --state RELATED -j ACCEPT Add a rule to accept all packets that are related (e.g.
ICMP responses to TCP or UDP traffic) to a communication
already tracked by the state module
iptables -A INPUT -m state --state INVALID -j ACCEPT Add a rule to accept all packets that do not match any of
the states above
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114/167 iptables NAT routing
iptables NAT routing
SNAT (Source Network Address Translation)
iptables -t nat -A POSTROUTING -s 10.0.0.0/24 -o eth1 \
-j SNAT --to-source 93.184.216.119
Map all traffic leaving the LAN to the external IP
address 93.184.216.119
iptables -t nat -A POSTROUTING -s 10.0.0.0/24 -o eth1 \
-j SNAT --to-source 93.184.216.119:93.184.216.127
Map all traffic leaving the LAN to a pool of external
IP addresses 93.184.216.119-127
iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE Map all traffic leaving the LAN to the address
dynamically assigned to eth1 via DHCP
DNAT (Destination Network Address Translation)
iptables -t nat -A PREROUTING -i eth1 -d 93.184.216.119 \
-j DNAT --to-destination 10.0.0.13
Allow the internal host 10.0.0.13 to be publicly
reachable via the external address 93.184.216.119
PAT (Port Address Translation)
iptables -t nat -A PREROUTING -i eth1 -d 93.184.216.119 \
-p tcp --dport 80 -j DNAT --to-destination 10.0.0.13:8080
Make publicly accessible a webserver that is
located in the LAN, by mapping port 8080 of the
internal host 10.0.0.13 to port 80 of the external
address 93.184.216.119
iptables -t nat -A PREROUTING -i eth0 -d ! 10.0.0.0/24 \
-p tcp --dport 80 -j REDIRECT --to-ports 3128
Redirect all outbound HTTP traffic originating from
the LAN to a proxy running on port 3128 on the
Linux box
sysctl -w net.ipv4.ip_forward=1
echo 1 > /proc/sys/net/ipv4/ip_forward
Enable IP forwarding; necessary to set up a Linux machine as a router.
(This command causes other network options to be changed as well.)
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
LAN
10.0.0.0/24
Linux box
NAT router eth1
93.184.216.119
eth0
10.0.0.1
Internet
115/167 firewalld
firewalld
In firewalld, a network interface (aka interface) or a subnet address (aka source) can be assigned to a specific zone.
To determine to which zone a packet belongs, first the zone of the source is analyzed, then the zone of the interface; if no
source or interface matches, the packet is associated to the default zone (which is "public", unless set otherwise).
If the zone is not specified (via --zone=zone), the command is applied to the default zone.
By default, commands are temporary; adding the --permanent option to a command sets it as permanent, or shows
permanent settings only.
Temporary commands are effective immediately but are canceled at reboot, firewall reload, or firewall restart.
Permanent commands are effective only after reboot, firewall reload, or firewall restart.
Firewalld zones (as obtained by firewall-cmd --get-zones)
block Rejects incoming connections with an ICMP HOST_PROHIBITED; allows only established connections
dmz Used to expose services to the public; allows only specific incoming connections
drop Drops all incoming packets; allows only outgoing connections
external Used for routing and masquerading; allows only specific connections
home Allows only specific incoming connections
internal Used to define internal networks and allow only private network traffic
public Allows only specific incoming connections. Default zone
trusted Accepts all traffic
work Used to define internal networks and allow only private network traffic
systemctl status firewalld
firewall-cmd --state Check the status of the firewall
firewall-config Firewall management GUI
firewall-cmd --reload Reload firewall configuration; this applies all permanent changes and
cancels all temporary changes. Current connections are not terminated
firewall-cmd --complete-reload Reload firewall configuration, stopping all current connections
firewall-cmd --runtime-to-permanent Transform all temporary changes to permanent
firewall-cmd --list-all-zones List all zones and their full settings
firewall-cmd --get-default-zone Show the default zone
firewall-cmd --set-default-zone=home Set "home" as the default zone
firewall-cmd --get-active-zones Show the active zones i.e. zones bound to
either an interface or a source
firewall-cmd --get-zones Show all available zones
firewall-cmd --get-zone-of-interface=eth0 Show the zone assigned to eth0
firewall-cmd --new-zone=test Create a new zone called "test"
firewall-cmd --zone=home --change-interface=eth0 Assign eth0 to the "home" zone
firewall-cmd --zone=home --list-all List temporary settings of the "home" zone
firewall-cmd --zone=home --list-all --permanent List permanent settings of the "home" zone
firewall-cmd --zone=home --add-source=10.1.1.0/24 Assign 10.1.1.0/24 to the "home" zone i.e.
route all traffic from that subnet to that zone
firewall-cmd --zone=home --list-sources List sources bound to the "home" zone
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116/167 firewalld rules
firewalld rules
firewall-cmd --zone=trusted --add-service=ssh
firewall-cmd --zone=trusted --add-port=22/tcp Add the SSH service to the "trusted" zone
firewall-cmd --zone=trusted --add-service={ssh,http,https} Add the SSH, HTTP, and HTTPS services to the
"trusted" zone
firewall-cmd --zone=trusted --list-services Show temporary and permanent services
bound to the "trusted" zone
firewall-cmd --zone=trusted --list-ports Show temporary and permanent ports open on
the "trusted" zone
firewall-cmd --get-services List all predefined services
Predefined services are configured in /usr/lib/firewalld/services/service.xml.
User-defined services are configured in /etc/firewalld/services/service.xml.
firewall-cmd --get-icmptypes Show all known types of ICMP messages
firewall-cmd --add-icmp-block=echo-reply Block a specific ICMP message type
firewall-cmd --query-icmp-block=echo-reply Tell if a specific ICMP message type is blocked
firewall-cmd --list-icmp-block Show the list of blocked ICMP message types
firewall-cmd --add-rich-rule='richrule'Set up a rich rule (for more complex and
detailed firewall configurations)
firewall-cmd --add-rich-rule='rule \
family=ipv4 source address=10.2.2.0/24 service name=tftp
log prefix=tftp level=info limit value=3/m accept'
Set up a rich rule to allow tftp connections
from subnet 10.2.2.0/24 and log them via
syslog at a rate of 3 per minute
firewall-cmd --list-rich-rules List all rich rules
The manpage man firewalld.richlanguage contains several examples of rich rules.
firewall-cmd --direct --add-rule directrule Set up a direct rule (in iptables format)
firewall-cmd --direct --add-rule \
ipv4 filter INPUT 0 -p tcp --dport 22 -j ACCEPT
Set up a direct rule to allow SSH connections
firewall-offline-cmd directrule Set up a direct rule when firewalld is not
running
firewall-cmd --direct --get-all-rules Show all direct rules
The manpage man firewalld.direct documents the syntax of direct rules.
User-defined direct rules are stored in /etc/firewalld/direct.xml.
firewall-cmd --zone=zone --add-masquerade Set up masquerading for hosts of zone;
packets originating from zone will get the
firewall's IP address on the "external" zone as
source address
firewall-cmd --zone=zone --add-rich-rule='rule \
family=ipv4 source address=10.2.2.0/24 masquerade'
Set up masquerading only for those hosts of
zone located in subnet 10.2.2.0/24
firewall-cmd --zone=zone --add-forward-port=\
port=22:proto=tcp:toport=2222:toaddr=10.7.7.7
Set up port forwarding for hosts of zone;
incoming connections to port 22 for hosts of
zone will be forwarded to port 2222 on host
10.7.7.7
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117/167 SSH
SSH
ssh user@host Connect to a remote host via SSH (Secure Shell) and login
as user.
Options:
-v -vv -vvv Increasing levels of verbosity
-p n Use port n instead of standard port 22
ssh user@host /path/to/command Execute a command on a remote host
sftp user@host FTP-like tool for secure file transfer
scp /path1/file user@host:/path2/
scp user@host:/path1/file /path2/
scp user1@host1:/path1/file user2@host2:/path2/
Non-interactive secure file copy.
Can transfer files from local to remote, from remote to
local, or between two remote hosts
sshpass -p password ssh user@host Connect to a remote host using the specified password
pssh -i -H "host1 host2 host3" /path/to/command Execute a command in parallel on a group of remote hosts
ssh-keygen -t rsa -b 2048 Generate interactively a 2048-bit RSA key pair; will
prompt for a passphrase
ssh-keygen -t dsa Generate a DSA key pair
ssh-keygen -p -t rsa Change passphrase of the private key
ssh-keygen -q -t rsa -f /etc/ssh/id_rsa -N '' -C '' Generate a RSA key with no passphrase (for non-
interactive use) and no comment
ssh-keygen -lf /etc/ssh/id_rsa.pub View key length and fingerprint of a public key
ssh-agent Echo to the terminal the environment variables that must
be set in order to use the SSH Agent
eval `ssh-agent` Start the SSH Agent daemon that caches decrypted
private keys in memory; also shows the PID of ssh-agent
and sets the appropriate environment variables.
Once ssh-agent is started, one must add the keys to cache
via the ssh-add command. The cached keys will then be
automatically used by any SSH tool e.g. ssh, sftp, scp
ssh-agent bash -c 'ssh-add /path/to/keyfile' Start ssh-agent and cache the specified key
ssh-add Add the default private keys to the ssh-agent cache
ssh-add /path/to/keyfile Add a specific private key to the ssh-agent cache
ssh-copy-id user@host Use locally available keys to authorize, via public key
authentication, login of user on a remote host.
This is done by copying the user's local public key
~/.ssh/id_rsa.pub to ~/.ssh/authorized_keys on the
remote host
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118/167 SSH operations
SSH operations
SSH port forwarding (aka SSH tunneling)
ssh -L 2525:mail.foo.com:25 user@mail.foo.com Establish a SSH encrypted tunnel from localhost to remote
host mail.foo.com, redirecting traffic from local port 2525
to port 25 of remote host mail.foo.com.
Useful if the local firewall blocks outgoing port 25. In this
case, port 2525 is used to go out; the application must be
configured to connect to localhost on port 2525 (instead of
mail.foo.com on port 25)
ssh -L 2525:mail.foo.com:25 user@login.foo.com Establish a SSH encrypted tunnel from localhost to remote
host login.foo.com.
Remote host login.foo.com will then forward, unencrypted,
all data received over the tunnel on port 2525 to remote
host mail.foo.com on port 25
SSH reverse forwarding (aka SSH reverse tunneling)
ssh -R 2222:localhost:22 user@login.foo.com Establish a SSH encrypted reverse tunnel from remote
host login.foo.com back to localhost, redirecting traffic
sent to port 2222 of remote host login.foo.com back
towards local port 22.
Useful if the local firewall blocks incoming connections so
remote hosts cannot connect back to local machine. In
this case, port 2222 of login.foo.com is opened for
listening and connecting back to localhost on port 22;
remote host login.foo.com is then able to connect to the
local machine on port 2222 (redirected to local port 22)
SSH as a SOCKS proxy
ssh -D 33333 user@login.foo.com The application supporting SOCKS must be configured to
connect to localhost on port 33333. Data is tunneled from
localhost to login.foo.com, then unencrypted to destination
X11 Forwarding
ssh -X user@login.foo.com Enable the local display to execute locally a X application
stored on a remote host login.foo.com
How to enable public key authentication
1. On remote host, set PubkeyAuthentication yes in /etc/ssh/sshd_config
2. On local machine, do ssh-copy-id you@remotehost (or copy your public key to the remote host by hand)
How to enable host-based authentication amongst a group of trusted hosts
1. On all hosts, set HostbasedAuthentication yes in /etc/ssh/sshd_config
2. On all hosts, create /etc/ssh/shosts.equiv and enter in this file all trusted hostnames
3. Connect via SSH manually from your machine on each host so that all hosts' public keys go into ~/.ssh/known_hosts
4. Copy ~/.ssh/known_hosts from your machine to /etc/ssh/ssh_known_hosts on all hosts
How to enable X11 Forwarding
1. On remote host 10.2.2.2, set X11Forwarding yes in /etc/ssh/sshd_config, and make sure that xauth is installed
2. On local host 10.1.1.1, type ssh -X 10.2.2.2, then run on remote host the graphical application e.g. xclock &
It is also possible to enable X11 Forwarding via telnet (but this is insecure and obsolete, and therefore not recommended):
1. On remote host 10.2.2.2, type export DISPLAY=10.1.1.1:0.0
2. On local host 10.1.1.1, type xhost +
3. On local host 10.1.1.1, type telnet 10.2.2.2, then run on remote host the graphical application e.g. xclock &
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119/167 SSH configuration
SSH configuration
/etc/ssh/sshd_config SSH server daemon configuration file
/etc/ssh/ssh_config SSH client global configuration file
/etc/ssh/ssh_host_key Host's private key (should be mode 0600)
/etc/ssh/ssh_host_key.pub Host's public key
/etc/ssh/shosts.equiv Names of trusted hosts for host-based authentication
/etc/ssh/ssh_known_hosts Database of host public keys that were previously accepted as legitimate
~/.ssh/ User's SSH directory (must be mode 0700)
~/.ssh/config SSH client user configuration file
~/.ssh/id_rsa
~/.ssh/id_dsa
User's RSA or DSA private key, as generated by ssh-keygen
~/.ssh/id_rsa.pub
~/.ssh/id_dsa.pub
User's RSA or DSA public key, as generated by ssh-keygen
~/.ssh/known_hosts Host public keys that were previously accepted as legitimate by the user
~/.ssh/authorized_keys
~/.ssh/authorized_keys2 (obsolete)
Trusted public keys; the corresponding private keys allow the user to
authenticate on this host
/etc/ssh/sshd_config SSH server configuration file
PermitRootLogin yes Control superuser login via SSH. Possible values are:
yes Superuser can login
no Superuser cannot login
without-password Superuser cannot login with password
forced-commands-only Superuser can only run commands in SSH command line
AllowUsers jdoe ksmith
DenyUsers jhacker List of users that can/cannot login via SSH, or * for everybody
AllowGroups geeks
DenyGroups * List of groups whose members can/cannot login via SSH, or * for all groups
PasswordAuthentication yes Permit authentication via login and password
PubKeyAuthentication yes Permit authentication via public key
HostbasedAuthentication yes Permit authentication based on trusted hosts
Protocol 1,2 Specify protocols supported by SSH. Value can be 1 or 2 or both
X11Forwarding yes Allow X11 Forwarding
/etc/ssh/ssh_config and ~/.ssh/config SSH client configuration file
Host * List of hosts to which the following directives will apply, or * for all hosts
StrictHostKeyChecking yes Ask before adding new host keys to the ~/.ssh/known_hosts file, and refuse to
connect if the key for a known host has changed. This prevents MITM attacks
GSSAPIAuthentication yes Support authentication using GSSAPI
ForwardX11Trusted yes Allow remote X11 clients to fully access the original X11 display
IdentityFile ~/.ssh/id_rsa User identity file for authentication. Default values are:
~/.ssh/identity for protocol version 1
~/.ssh/id_rsa and ~/.ssh/id_dsa for protocol version 2
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
120/167 OpenSSL
OpenSSL
openssl x509 -text -in certif.crt -noout Read a certificate
openssl req -text -in request.csr -noout Read a Certificate Signing Request
openssl req -new -key private.key -out request.csr Generate a Certificate Signing Request (in PEM
format) for the public key of a key pair
openssl req -new -nodes -keyout private.key \
-out request.csr -newkey rsa:2048
Create a 2048-bit RSA key pair and generate a
Certificate Signing Request for it
openssl req -x509 -newkey rsa:2048 -nodes \
-keyout private.key -out certif.crt -days validity
Generate a self-signed root certificate, and create a
new CA private key
openssl ca -config ca.conf -in request.csr \
-out certif.crt -days validity -verbose
Generate a self-signed certificate
openssl ca -config ca.conf -gencrl -revoke certif.crt \
-crl_reason why
Revoke a certificate
openssl ca -config ca.conf -gencrl -out crlist.crl Generate a Certificate Revocation List containing all
revoked certificates so far
openssl x509 -in certif.pem -outform DER \
-out certif.der
Convert a certificate from PEM to DER
openssl pkcs12 -export -in certif.pem \
-inkey private.key -out certif.pfx -name friendlyname
Convert a certificate from PEM to PKCS#12 including
the private key
openssl pkcs12 -in certif.p12 -out certif.pem \
-clcerts -nokeys
Convert a certificate from PKCS#12 to PEM
openssl pkcs12 -in certif.p12 -out private.key \
-nocerts -nodes
Extract the private key from a PKCS#12 certificate
cat certif.crt private.key > certif.pem Create a PEM certificate from CRT and private key
openssl dgst -hashfunction -out file.hash file Generate the digest of a file
openssl dgst -hashfunction file | cmp -b file.hash Verify the digest of a file (no output means that
digest verification is successful)
openssl dgst -hashfunction -sign private.key \
-out file.sig file
Generate the signature of a file
openssl dgst -hashfunction -verify public.key \
-signature file.sig file
Verify the signature of a file
openssl enc -e -cipher -in file -out file.enc -salt Encrypt a file
openssl enc -d -cipher -in file.enc -out file Decrypt a file
openssl genpkey -algorithm RSA -cipher 3des \
-pkeyopt rsa_keygen_bits:2048 -out keypair.pem
Generate a 2048-bit RSA key pair protected by
TripleDES passphrase
openssl pkey -text -in private.key -noout Examine a private key
openssl pkey -in old.key -out new.key -cipher Change the passphrase of a private key
openssl pkey -in old.key -out new.key Remove the passphrase from a private key
1. openssl s_client -connect www.site.com:443 > tmpfile
2.
3. openssl x509 -in tmpfile -text
Retrieve and inspect a SSL certificate from a website
openssl list-message-digest-commands List all available hash functions
openssl list-cipher-commands List all available ciphers
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
CTRL C
121/167 CA.pl
CA.pl
CA.pl -newca Create a Certification Authority hierarchy
CA.pl -newreq Generate a Certificate Signing Request
CA.pl -signreq Sign a Certificate Signing Request
CA.pl -pkcs12 "Certificate name"Generate a PKCS#12 certificate from a Certificate Signing Request
CA.pl -newcert Generate a self-signed certificate
CA.pl -newreq-nodes Generate a Certificate Signing Request, with unencrypted private key
(for use in servers, because the private key must be accessed in non-
interactive mode, without typing a passphrase)
CA.pl -verify Verify a certificate against the Certification Authority certificate for "demoCA"
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
122/167 GnuPG
GnuPG
gpg --gen-key Generate a key pair
gpg --import alice.asc Import Alice's public key alice.asc into your keyring
gpg --list-keys List the keys contained into your keyring
gpg --list-secret-keys List your private keys contained into your keyring
gpg --list-public-keys List the public keys contained into your keyring
gpg --export -o keyring.gpg Export your whole keyring to a file keyring.gpg
gpg --export-secret-key -a "You" -o private.key Export your private key to a file private.key
gpg --export-public-key -a "Alice" -o alice.pub Export Alice's public key to a file alice.pub
gpg --edit-key "Alice" Sign Alice's public key
gpg -e -u "You" -r "Alice" file Sign file (with your private key) and encrypt it to Alice
(with Alice's public key)
gpg -d file.gpg -o file Decrypt file.gpg (with your own private key) and save the
decrypted file to file
md5sum
sha1sum
sha224sum
sha256sum
sha384sum
sha512sum
shasum
Print or check the digest of a file generated by a specific hashing algorithm
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
123/167 OpenVPN
OpenVPN
openvpn --genkey --secret keyfile Generate a shared secret keyfile for OpenVPN authentication.
The keyfile must be copied on both server and client
openvpn server.conf Start the VPN on the server side. The encrypted VPN tunnel uses UDP port 1194
openvpn client.conf Start the VPN on the client side
/etc/openvpn/server.conf Server-side configuration file:
dev tun
ifconfig server_IP client_IP
keepalive 10 60
ping-timer-rem
persist-tun
persist-key
secret keyfile
/etc/openvpn/client.conf Client-side configuration file:
remote server_public_IP
dev tun
ifconfig client_IP server_IP
keepalive 10 60
ping-timer-rem
persist-tun
persist-key
secret keyfile
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
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Key bindings - terminal
Key Alternate key Function
Move cursor forward one char
Move cursor backward one char
Move cursor to beginning of line
Move cursor to end of line
Delete char to the left of cursor
Delete word to the left of cursor
Delete all chars to the left of cursor
Delete all chars to the right of cursor
Swap current char with previous char
Swap current word with previous word
Scroll up the screen buffer
Scroll down the screen buffer
Clear screen (same as clear)
Previous command in history
Next command in history
Reverse history search
Autocomplete commands, filenames, and directory names
Autocomplete filenames and directory names only
Expand the Bash alias currently entered on the command line
Line feed
Carriage return
Pause transfer to terminal
Forward history search (if XON/XOFF flow control is disabled)
Resume transfer to terminal
Send a SIGTSTP to put the current job in background
Send a SIGINT to stop the current process
Send a EOF to current process (if it's a shell, same as logout)
Send a SIGINT to reboot the machine (same as shutdown -r now);
specified in /etc/inittab and /etc/init/control-alt-delete
Switch between text consoles (same as chvt n)
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
CTRL F
CTRL B
CTRL A HOME
CTRL E END
CTRL H BACKSPACE
CTRL W
CTRL U
CTRL K
CTRL T
ESC T
SHIFT PAGE UP
SHIFT PAGE DOWN
CTRL L
CTRL P
CTRL N
CTRL R
TAB
CTRL J RETURN
CTRL M
CTRL S
CTRL Q
CTRL Z
CTRL C
CTRL D
CTRL ALT DEL
CTRL ALT F1 ... F6
↑
↓
←
→
CTRL EALT
/ALT
125/167 Key bindings - X
Key bindings - X
Key Alternate key Function
Switch between X Window consoles
Increase X Window screen resolution
Decrease X Window screen resolution
Switch between X Window tasks
Switch to next workspace
Switch to previous workspace
Reboot the X Window server
GNOME
Switch between windows in the current workspace
Show activities overview
Lock screen
Show tray messages
Maximize current window
Restore normal size of current window
Maximize current window to left half screen
Maximize current window to right half screen
Run command
Increase terminal font size
Decrease terminal font size
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
CTRL ALT F7 ... F11
CTRL ALT +
CTRL ALT -
CTRL TAB
CTRL ALT BACKSPACE
CTRL ALT →
CTRL ALT ←
CTRL ALT ↓
CTRL ALT ↑
ALT TAB
SUPER L
SUPER M
SUPER
ALT F2
CTRL +
CTRL -
SUPER ↑
SUPER ↓
SUPER ←
SUPER →
126/167 udev
udev
The Hardware Abstraction Layer (HAL) manages device files and provides plug-and-play facilities. The HAL daemon hald
maintains a persistent database of devices.
udev is the device manager for the Linux kernel. It dynamically generates the device nodes in /dev/ for devices present on
the system; it also provides persistent naming for storage devices in /dev/disk.
When a device is added, removed, or changes state, the kernel sends an uevent received by the udevd daemon which will
pass the uevent through a set of rules stored in /etc/udev/rules.d/*.rules and /lib/udev/rules.d/*.rules.
udevadm monitor
udevmonitor
Show all kernel uevents and udev messages
udevadm info --attribute-walk --name=/dev/sda Print all attributes of device /dev/sda in udev rules key format
cat /sys/block/sda/size Print the size attribute of disk sda in 512-byte blocks.
This information is retrieved from sysfs
udevadm test /dev/sdb Simulate a udev event run for the device and print debug output
gnome-device-manager Browser for the HAL device manager
/etc/udev/rules.d/*.rules and /lib/udev/rules.d/*.rules udev rules
KERNEL=="hda", NAME="mydisk" Match a device which was named by the
kernel as hda; name the device node as
"mydisk". The device node will be therefore
/dev/mydisk
KERNEL=="hdb", DRIVER=="ide-disk", SYMLINK+="mydisk myhd" Match a device with kernel name and driver
as specified; name the device node with the
default name and create two symbolic links
/dev/mydisk and /dev/myhd pointing to
/dev/hdb
KERNEL=="fd[0-9]*", NAME="floppy/%n", SYMLINK+="%k" Match all floppy disk drives (i.e. fdn); place
device node in /dev/floppy/n and create a
symlink /dev/fdn to it
SUBSYSTEM=="block", ATTR{size}=="41943040", SYMLINK+="mydisk" Match a block device with a size attribute of
41943040; create a symlink /dev/mydisk
KERNEL=="fd[0-9]*", OWNER="jdoe" Match all floppy disk drives; give ownership
of the device file to user jdoe
KERNEL=="sda", PROGRAM="/bin/mydevicenamer %k", SYMLINK+="%c" Match a device named by the kernel as sda;
to name the device, use the defined
program which takes on stdin the kernel
name and output on stdout e.g. name1
name2. Create symlinks /dev/name1 and
/dev/name2 pointing to /dev/sda
KERNEL=="sda", ACTION=="add", RUN+="/bin/myprogram" Match a device named by the kernel as sda;
run the defined program when the device is
connected
KERNEL=="sda", ACTION=="remove", RUN+="/bin/myprogram" Match a device named by the kernel as sda;
run the defined program when the device is
disconnected
%n = kernel number (e.g. = 3 for fd3)
%k = kernel name (e.g. = fd3 for fd3)
%c = device name as output from program
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127/167 Kernel
Kernel
A kernel version number has the form major.minor.patchlevel.
Kernel images are usually gzip-compressed and can be of two types: zImage (max 520 Kb) and bzImage (no size limit).
Kernel modules can be loaded dynamically into the kernel to provide additional functionalities on demand, instead of being
included when the kernel is compiled; this reduces memory footprint.
kerneld (daemon) and kmod (kernel thread) facilitate the dynamic loading of kernel modules.
/lib/modules/X.Y.Z/*.ko Kernel modules for kernel version X.Y.Z
/lib/modules/X.Y.Z/modules.dep Modules dependencies.
This file needs to be recreated (via the command depmod -a)
after a reboot or a change in module dependencies
/etc/modules.conf
/etc/conf.modules (deprecated)
Modules configuration file
/usr/src/linux/ Contains the kernel source code to be compiled
/usr/src/linux/.config Kernel configuration file
freeramdisk Free the memory used for the initrd image. This command
must be run directly after unmounting /initrd
mkinitrd initrd_image kernel_version (Red Hat) Create a initrd image file
mkinitramfs (Debian) Create a initrd image file according to the configuration file
/etc/initramfs-tools/initramfs.conf
dracut Create initial ramdisk images for preloading modules
dbus-monitor Monitor messages going through a D-Bus message bus
dbus-monitor --session Monitor session messages (default)
dbus-monitor --system Monitor system messages
The runtime loader ld.so loads the required shared libraries of the program into RAM, searching in this order:
1. LD_LIBRARY_PATH Environment variable specifying the list of dirs where libraries should be searched for first
2. /etc/ld.so.cache Cache file
3. /lib and /usr/lib Default locations for shared libraries
/etc/ld.so.conf Configuration file used to specify other shared library locations
(other than the default ones /lib and /usr/lib)
ldconfig Create a cache file /etc/ld.so.cache of all available
dynamically linked libraries.
To be run when the system complains about missing libraries
ldd program_or_lib Print library dependencies
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128/167 Kernel management
Kernel management
lspci List PCI devices
lspci -d 8086: List all Intel hardware present. PCI IDs are stored in:
/usr/share/misc/pci.ids (Debian)
/usr/share/hwdata/pci.ids (Red Hat)
lsusb List USB devices
lsusb -d 8086: List all Intel USB devices present. USB IDs are stored in:
/var/lib/usbutils/usb.ids (Debian)
/usr/share/hwdata/usb.ids (Red Hat)
lsdev List information about the system's hardware
lshw List system hardware
lscpu List information about the CPU architecture
uname -s Print the kernel name
uname -n Print the network node hostname
uname -r Print the kernel release number X.Y.Z
uname -v Print the kernel version number
uname -m Print the machine hardware name
uname -p Print the processor type
uname -i Print the hardware platform
uname -o Print the operating system
uname -a Print all the above information, in that order
evtest Monitor and query input device events in /dev/input/eventn
dmesg Print the messages of the kernel ring buffer
dmesg -n 1 Set the logging level to 1 (= only panic messages)
journalctl Display the Systemd journal, which contains the kernel logs
journalctl -n nDisplay the most recent n log lines (default is 10)
journalctl --since "1 hour ago" Display events happened in the last hour
journalctl -x Display events, adding explanations from the message catalog
journalctl -f Display the journal in real-time
journalctl -u crond.service
journalctl _SYSTEMD_UNIT=crond.service
Display the log entries created by the cron service
mkdir -p /var/log/journal/ && \
systemctl restart systemd-journald
Enable persistent storage of logs in /var/log/journal/
(by default, journalctl stores the logfiles in RAM only)
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
129/167 Kernel compile and patching
Kernel compile and patching
Kernel compile
Download Download kernel source code linux-X.Y.Z.tar.bz2 from http://www.kernel.org
to the base of the kernel source tree /usr/src/linux
Clean
make clean Delete most generated files
make mrproper Delete all generated files and kernel configuration
make distclean Delete temporary files, patch leftover files, and similar
Configure
make config Terminal-based (options must be set in sequence)
make menuconfig ncurses UI
make xconfig
make gconfig GUI
make oldconfig Create a new config file, based on the options in the old config
file and in the source code
Components (e.g. device drivers) can be either:
- not compiled
- compiled into the kernel binary, for support of devices always used on the system or necessary
for the system to boot
- compiled as a kernel module, for optional devices
The configuration command creates a /usr/src/linux/.config config file containing
instructions for the compile
Build
make bzImage Compile the kernel
make modules Compile the kernel modules
make all Compile kernel and kernel modules
make -j2 all will speed up compilation by allocating 2 simultaneous compile jobs
Modules install make modules_install Install the previously built modules present in
/lib/modules/X.Y.Z
Kernel install
make install Install the kernel automatically
To install the kernel by hand:
Copy the new compiled kernel and other files into the boot partition
cp /usr/src/linux/arch/boot/bzImage /boot/vmlinuz-X.Y.Z (kernel)
cp /usr/src/linux/arch/boot/System.map-X.Y.Z /boot
cp /usr/src/linux/arch/boot/config-X.Y.Z /boot (config options used for this compile)
Create an entry in GRUB to boot on the new kernel
Package
Optionally, the kernel can be packaged for install on other machines
make rpm-pkg Build source and binary RPM packages
make binrpm-pkg Build binary RPM package
make deb-pkg Builds binary DEB package
Kernel patching
Download Download and decompress the patch to /usr/src
Patch
patch -p1 < file.patch Apply the patch
patch -Rp1 < file.patch Remove (reverse) a patch. Alternatively, you can apply the
patch again to reverse it
Build Build the patched kernel as explained previously
Install Install the patched kernel as explained previously
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
130/167 Kernel modules
Kernel modules
Kernel modules allow the kernel to access functions (symbols) for kernel services e.g. hardware drivers, network stack, or
filesystem abstraction.
lsmod List the modules that are currently loaded into the kernel
insmod module Insert a module into the kernel. If the module requires another module or if it
does not detect compatible hardware, insertion will fail
rmmod module Remove a module from the kernel. If the module is in use by another module, it
is necessary to remove the latter first
modinfo module Display the list of parameters accepted by the module
depmod -a Probe all modules in the kernel modules directory and generate the file that lists
their dependencies
It is recommended to use modprobe instead of insmod and rmmod, because it automatically handles prerequisites when
inserting modules, is more specific about errors, and accepts just the module name instead of requiring the full pathname.
modprobe module option=value Insert a module into the running kernel, with the specified parameters.
Prerequisite modules will be inserted automatically
modprobe -a Insert all modules
modprobe -t directory Attempt to load all modules contained in the directory until a module succeeds.
This action probes the hardware by successive module-insertion attempts for a
single type of hardware, e.g. a network adapter
modprobe -r module Remove a module
modprobe -c module Display module configuration
modprobe -l List loaded modules
Configuration of device drivers
Device drivers support the kernel with instructions on how to use that device.
Device driver compiled
into the kernel
Configure the device driver by passing a kernel parameter in the GRUB menu:
kernel /vmlinuz ro root=/dev/vg0/root vga=0x33c
Device driver provided
as a kernel module
Edit module configuration in /etc/modprobe.conf or /etc/modprobe.d/ (Red Hat):
alias eth0 3c59x Specify that eth0 uses the 3c59x.ko driver module
options 3c509 irq=10,11 Assign IRQ 10 and 11 to 3c509 devices
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
131/167 /proc
/proc
/proc is a pseudo filesystem that gives access to process data held in the kernel.
File Information stored (can be viewed via cat) Equivalent command
/proc/bus Buses (e.g. PCI, USB, PC Card)
/proc/cpuinfo CPUs information
/proc/devices Drivers currently loaded
/proc/dma DMA channels in use
/proc/filesystems Filesystems supported by the system
/proc/interrupts Current IRQs (Interrupt Requests) procinfo
/proc/ioports I/O addresses in use
/proc/loadavg System load averages uptime
/proc/mdstat Information about RAID arrays and devices
/proc/meminfo Total and free memory free
/proc/modules Kernel modules currently loaded lsmod
/proc/mounts Mounted partitions mount
/proc/net/dev Network interface statistics
/proc/partitions Drive partition information
/proc/swaps Size of total and used swap areas swapon -s
/proc/sys/ sysfs: exposes tunable kernel parameters
/proc/sys/kernel/ Kernel information and parameters
/proc/sys/net/ Network information and parameters
/proc/uptime Time elapsed since boot uptime
/proc/version Linux version uname -a
/proc/n/Information about process with PID nps n
/proc/n/cmdline Command by which the process was launched
/proc/n/cwd Symlink to process' working directory
/proc/n/environ Values of environment variables of process
/proc/n/exe Symlink to process' executable
/proc/n/fd Files currently opened by the process lsof -p n
/proc/n/root Symlink to process' filesystem root
/proc/n/status Status of process
/proc/sys is the only writable branch of /proc and can be used to tune kernel parameters on-the-fly.
All changes are lost after system shutdown, unless applied via sysctl -p.
sysctl fs.file-max
cat /proc/sys/fs/file-max
Get the maximum allowed number of open files
sysctl -w "fs.file-max=100000"
echo "100000" > /proc/sys/fs/file-max
Set the maximum allowed number of open files to 100000
sysctl -a List all available kernel tuning options
sysctl -p Apply all tuning settings listed in /etc/sysctl.conf.
This command is usually run at boot by the system initialization script,
to make permanent changes to kernel parameters
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132/167 System recovery
System recovery
If the kernel has been booted in emergency mode and init has not been run, some initial configuration is necessary e.g.
mount /proc
mount -o remount,rw /
mount -a
If mounting the filesystems fails:
mknod /dev/sda
mknod /dev/sda1
fdisk -l /dev/sda
fsck -y /dev/sda1
mount -t ext3 /dev/sda1 /mnt/sysimage
chroot /mnt/sysimage
To install a package using an alternative root directory (useful if the system has been booted from a removable media):
rpm -U --root /mnt/sysimage package.rpm
To install GRUB on the specified directory (which must contain /boot/grub/):
grub-install –-root-directory=/mnt/sysimage /dev/sda
Alternative method:
chroot /mnt/sysimage
grub-install /dev/sda
Run sync and unmount all filesystems before exiting the shell, to ensure that all changes have been written on disk.
How to reset the root password (RHEL 7)
1. Power up the system and, on the GRUB 2 boot screen, press to edit the current entry.
2. Edit the kernel line that mentions linux16, removing the rhgb and quiet parameters and adding rd.break at the end.
3. Press ; the system will boot on the initramfs switch_root prompt.
4. Remount the filesystem as writable: mount -o remount,rw /sysroot
5. Change the filesystem root: chroot /sysroot
6. Modify the root password: passwd root
7. Force SELinux to relabel context on next boot: touch /.autorelabel
8. Remount the filesystem as readonly (not strictly necessary): mount -o remount,ro /sysroot
9. Exit the chroot environment: exit
10. Resume system boot: exit
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
CTRL X
E
133/167 DNS
DNS
DNS implementations
BIND Berkeley Internet Name Domain system, is the standard DNS server for UNIX
dnsmasq Lightweight DNS, DHCP and TFTP server for a small network
djbdns Security-hardened DNS server that also includes DNS debugging tools
PowerDNS Alternative open-source DNS server
named BIND Name Daemon
ndc Name Daemon Controller for BIND 8
rndc Remote Name Daemon Controller for BIND 9, uses a shared key to communicate securely with named
dnswalk example.org. DNS debugger
rndc reconfig Reload BIND configuration and new zones
rndc reload example.org Reload the zone example.org
rndc freeze example.org Suspend updates for the zone example.org
rndc thaw example.org Resume updates for the zone example.org
rndc tsig-list List all currently active TSIG keys
DNSSEC was designed to secure the DNS tree and hence prevent cache poisoning.
The TSIG (Transaction SIGnature) standard, that authenticates communications between two trusted systems, is used to
sign zone transfers and DDNS (Dynamic DNS) updates.
dnssec-keygen -a dsa -b 1024 \
-n HOST dns1.example.org
Generate a TSIG key with DNSSEC algorithm nnn and key fingerprint fffff.
This will create two key files
Kdns1.example.org.+nnn+fffff.key
Kdns1.example.org.+nnn+fffff.private
which contain a key number that has to be inserted both in /etc/named.conf and
/etc/rndc.conf
rndc-confgen -a Generate a /etc/rndc.key key file:
key "rndc-key" {
algorithm hmac-md5;
secret "vyZqL3tPHsqnA57e4LT0Ek==";
};
options {
default-key "rndc-key";
default-server 127.0.0.1;
default-port 953;
};
This file is automatically read both by named and rndc
dnssec-signzone example.org Sign the zone example.org
named -u named -g named Run BIND as user/group named (both must be created if needed) instead of root
named -t /var/cache/bind Run BIND in a chroot jail /var/cache/bind
(actually is the chroot command that starts the named server)
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134/167 DNS configuration
DNS configuration
/etc/named.conf DNS server configuration file
controls {
inet 127.0.0.1 allow {localhost;} keys {rndckey;};
};
key "rndc-key" { // TSIG key
algorithm dsa;
secret "HYZur46fftdUQ43BJKI093t4t78lkp";
};
acl "mynetwork" {10.7.0.0/24;}; // Alias definition
// Built-in ACLs: any, none, localhost, localnets
options {
directory "/var/named"; // Working directory
version "0.0"; // Hide version number by replacing it with 0.0
listen-on port 53 {10.7.0.1; 127.0.0.1;}; // Port and own IP addresses to listen on
blackhole {172.17.17.0/24;}; // IPs whose packets are to be ignored
allow-query {mynetwork;}; // IPs allowed to do iterative queries
allow-query-on {any;}; // Local IPs that can accept iterative queries
allow-query-cache {any;}; // IPs that can get an answer from cache
allow-recursion {mynetwork;}; // IPs to accept recursive queries from (typically
// own network's IPs). The DNS server does the full
// resolution process on behalf of these client IPs,
// and returns a referral for the other IPs
allow-recursion-on {mynetwork;}; // Local IPs that can accept recursive queries
allow-transfer {10.7.0.254;}; // Zone transfer is restricted to these IPs (slaves);
// on slave servers, this option should be disabled
allow-update {any;}; // IPs to accept DDNS updates from
recursive-clients 1000; // Max number of simultaneous recursive lookups
dnssec-enable yes; // Enable DNSSEC
dialup no; // Not a dialup connection: external zone maintenance
// (e.g. sending heartbeat packets, external zone transfers)
// is then permitted
forward first; // Site-wide cache: bypass the normal resolution
forwarders {10.7.0.252; 10.7.0.253;}; // method by querying first these central DNS
// servers if they are available
};
// Define the root name servers
zone "." {
type hint;
file "root.cache";
}
// Configure system to act as a master server for the example.org domain
zone "example.org" IN {
type master;
file "master/example.org.zone"; // Zone file for the example.org domain
};
zone "240.123.224.in-addr.arpa" IN { // Configure reverse lookup zone (for 224.123.240.0/24)
type master;
file "slave/example.org.revzone";
};
// Configure system to act as a slave server for the example2.org domain
zone "example2.org" IN {
type slave;
file "slave/example2.org.zone"; // Slave: do not edit this zone file!
masters {10.7.0.254;};
};
zone "0.7.10.in-addr.arpa" IN { // Configure reverse lookup zone (for 10.7.0.0/24)
type slave;
file "slave/10.7.0.revzone";
masters {10.7.0.254;};
};
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135/167 DNS zone file
DNS zone file
/var/named/master/example.org.zone DNS zone file for the example.org zone
$TTL 86400 ; TTL (1 day)
$ORIGIN example.org.
example.org IN SOA dns1.example.org. help.example.org. ( ; Master DNS server is dns1.example.org
2014052300 ; serial ; For problems contact help@example.org
28800 ; refresh (8 hours)
7200 ; retry (2 hours)
604800 ; expire (1 week)
600 ) ; negative TTL (10 mins)
IN NS dns1.example.org.
IN NS dns2.example.org.
IN MX 10 mail1.example.org.
IN MX 20 mail2.example.org.
dns1 IN A 224.123.240.3
dns2 IN A 224.123.240.4
mail1 IN A 224.123.240.73
mail2 IN A 224.123.240.77
foo IN A 224.123.240.12
bar IN A 224.123.240.13
www IN A 224.123.240.19
baz IN CNAME bar
subdomain IN NS ns1.subdomain.example.org. ; Glue records
IN NS ns2.subdomain.example.org.
ns1.subdomain.example.org. IN A 224.123.240.201
ns2.subdomain.example.org. IN A 224.123.240.202
/var/named/master/example.org.revzone DNS reverse zone file for the example.org zone
$TTL 86400 ; TTL (1 day)
example.org IN SOA dns1.example.org. help.example.org. (
2014052300 ; serial
28800 ; refresh (8 hours)
7200 ; retry (2 hours)
604800 ; expire (1 week)
600 ) ; negative TTL (10 mins)
12.240.123.224.in-addr.arpa IN PTR foo
13.240.123.224.in-addr.arpa IN PTR bar
19.240.123.224.in-addr.arpa IN PTR www
Resource Records
$TTL How long to cache a positive response
$ORIGIN Suffix appended to all names not ending with a dot.
Useful when defining multiple subdomains inside the same zone
SOA Start Of Authority for the example.org zone
serial Serial number. Must be increased after each edit of the zone file
refresh How frequently a slave server refreshes its copy of zone data from the master
retry How frequently a slave server retries connecting to the master
expire How long a slave server relies on its copy of zone data. After this time period expires,
the slave server is not authoritative anymore for the zone unless it can contact a master
negative TTL How long to cache a non-existent answer
AAddress: maps names to IP addresses. Used for DNS lookups.
PTR Pointer: maps IP addresses to names. Used for reverse DNS lookups.
Each A record must have a matching PTR record
CNAME Canonical Name: specifies an alias for a host with an A record (even in a different zone).
Discouraged as it causes multiple lookups; it is better to use multiple A records instead
NS Name Service: specifies the authoritative name servers for the zone
MX Mailserver: specifies address and priority of the servers able to handle mail for the zone
Glue Records are not really part of the zone; they delegate authority for other zones, usually subdomains
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136/167 Apache
Apache
Apache is an open source and widespread HTTP server, originally based on the NCSA HTTPd server.
apachectl (Red Hat)
httpd (Red Hat)
apache2ctl (Debian)
Manage the Apache webserver
apachectl start Start the Apache webserver daemon
apachectl status Display a brief status report
apachectl fullstatus Display a detailed status report
apachectl graceful Gracefully restart Apache; currently open connections are not aborted
apachectl graceful-stop Gracefully stop Apache; currently open connections are not aborted
apachectl configtest
apachectl -t
Test the configuration file, reporting any syntax error
apachectl -M List all loaded and shared modules
/var/www/html Default document root directory
$HOME/public_html Default document root directory for users' websites
Web content must be readable by the user/group the Apache process runs as. For security reasons, it should be owned and
writable by the superuser or the webmaster user/group (usually www-data), not the Apache user/group.
/etc/httpd/conf/httpd.conf
/etc/httpd/conf.d/*.conf (Red Hat) Apache configuration files
/etc/apache2/httpd.conf (Debian and SUSE)
The Apache webserver contains a number of MPMs (Multi-Processing Modules) which can operate following two methods:
prefork MPM A number of child processes is spawned in advance, with each child serving one connection.
Highly reliable due to Linux memory protection that isolates each child process
worker MPM Multiple child processes spawn multiple threads, with each thread serving one connection.
More scalable but prone to deadlocks if third-party non-threadsafe modules are loaded
HTTPS
HTTPS (i.e. HTTP over SSL/TLS) allows securing communications between the webserver and the client by encrypting
all communications end-to-end between the two. A webserver using HTTPS hands over its public key to the client
when the client connects to the server via port 443. The server's public key is signed by a CA (Certification
Authority), whose validity is ensured by the root certificates stored into the client's browser.
The openssl command and its user-friendly CA.pl script are the tools of the OpenSSL crypto library that can be used
to accomplish all public key crypto operations e.g. generate key pairs, Certificate Signing Requests, and self-signed
certificates. Another user-friendly tool is genkey.
Virtual hosting with HTTPS requires assigning a unique IP address for each virtual host; this because the SSL
handshake (during which the server sends its certificate to the client's browser) takes place before the client sends
the Host: header (which tells to which virtual host the client wants to talk).
A workaround for this is SNI (Server Name Indication) that makes the browser send the hostname in the first
message of the SSL handshake. Another workaround is to have all multiple name-based virtual hosts use the same
SSL certificate with a wildcard domain e.g. *.example.org.
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137/167 Apache configuration
Apache configuration
httpd.conf Apache configuration file
Server configuration directives
ServerName www.mysite.org:80 Name and port (if omitted, uses default HTTP port 80) of server
ServerRoot /etc/httpd Root directory for configuration and log files
ServerAdmin webmaster@mysite.org Contact address that the server includes in any HTTP error
messages to the client. Can be an email address or an URL
StartServers 5 Number of servers to start initially
MinSpareServers 5
MaxSpareServers 10
Minimum and maximum number of idle child server processes
MaxClients 256 (before v2.3.13)
MaxRequestWorkers 256 (v2.3.13 and later)
Max number of simultaneous requests that will be served; clients
above this limit will get a HTTP error 503 - Service Unavailable.
Prefork MPM: max number of child processes launched to serve
requests.
Worker MPM: max total number of threads available to serve
requests
ServerLimit 256 Prefork MPM: max configured value for MaxRequestWorkers.
Worker MPM: in conjunction with ThreadLimit, max configured
value for MaxRequestWorkers
ThreadsPerChild 25 Worker MPM: number of threads created by each child process
ThreadLimit 64 Worker MPM: max configured value for ThreadsPerChild
LoadModule mime_module modules/mod_mime.so Load the module mime_module by linking in the object file or
library modules/mod_mime.so
Listen 10.17.1.1:80
Listen 10.17.1.5:8080
Make the server accept connections on the specified IP
addresses (optional) and ports
User nobody
Group nobody
User and group the Apache process runs as. For security
reasons, this should not be root
Main configuration directives
DocumentRoot /var/www/html Directory in filesystem that maps to the root of the website
Alias /image /mydir/pub/image Map the URL http://www.mysite.org/image/ to the directory
/mydir/pub/image in the filesystem. This allows Apache to
serve content placed outside of the document root
TypesConfig conf/mime.types Media types file. The path is relative to ServerRoot
AddType image/jpeg jpeg jpg jpe Map the specified filename extensions onto the specified content
type. These entries adds to or override the entries from the
media types file conf/mime.types
Redirect permanent /foo /bar Redirect to a URL on the same host. Status can be:
permanent return a HTTP status 301 - Moved Permanently
temp return a HTTP status 302 - Found
(i.e. the resource was temporarily moved)
seeother return a HTTP status 303 - See Other
gone return a HTTP status 410 - Gone
If status is omitted, default status temp is used
Redirect /foo http://www.example.com/foo Redirect to a URL on a different host
AccessFileName .htaccess Name of the distributed configuration file, which contains
directives that apply to the document directory it is in and to all
its subtrees
<Directory "/var/www/html/foobar">
AllowOverride AuthConfig Limit
</Directory>
Specify which global directives a .htaccess file can override:
AuthConfig Authorization directives for directory protection
FileInfo Document type and metadata
Indexes Directory indexing
Limit Host access control
Options Specific directory features
All All directives
None No directive
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138/167 Apache virtual hosts
Apache virtual hosts
httpd.conf Apache configuration file
Virtual hosts directives
NameVirtualHost * Specify which IP address will serve virtual hosting. The
argument can be an IP address, an address:port pair, or * for all
IP addresses of the server. The argument will be repeated in the
relevant <VirtualHost> directive
<VirtualHost *:80>
ServerName www.mysite.org
ServerAlias mysite.org *.mysite.org
DocumentRoot /var/www/vhosts/mysite
</VirtualHost>
The first listed virtual host is also the default virtual host.
It inherits those main settings that does not override.
This virtual host answers to http://www.mysite.org , and also
redirects there all HTTP requests on the domain mysite.org
<VirtualHost *:80>
ServerAdmin webmaster@www.mysite2.org
ServerName www.mysite2.org
DocumentRoot /var/www/vhosts/mysite2
ErrorLog /var/www/logs/mysite2
</VirtualHost>
Name-based virtual host http://www.mysite2.org .
Multiple name-based virtual hosts can share the same IP
address; DNS must be configured accordingly to map each name
to the correct IP address. Cannot be used with HTTPS
<VirtualHost *:8080>
ServerName www.mysite3.org
DocumentRoot /var/www/vhosts/mysite3
</VirtualHost>
Port-based virtual host answering to connections on port 8080.
In this case the config file must contain a Listen 8080 directive
<VirtualHost 10.17.1.5:80>
ServerName www.mysite4.org
DocumentRoot /var/www/vhosts/mysite4
</VirtualHost>
IP-based virtual host answering to http://10.17.1.5
Logging directives
LogFormat "%h %l %u %t \"%r\" %>s %b" Specify the format of a log
LogFormat "%h %l %u %t \"%r\" %>s %b" common Specify a nickname (here, "common") for a log format.
This one is the CLF (Common Log Format) defined as such:
%h IP address of the client host
%l Identity of client as determined by identd
%u User ID of client making the request
%t Timestamp the server completed the request
%r Request as done by the user
%s Status code sent by the server to the client
%b Size of the object returned, in bytes
CustomLog /var/log/httpd/access_log common Set up a log filename, with the format or (as in this case)
the nickname specified
TransferLog /var/log/httpd/access_log Set up a log filename, with format determined by the most
recent LogFormat directive which did not define a nickname
TransferLog "|rotatelogs access_log 86400" Set log rotation every 24 hours
HostnameLookups Off Disable DNS hostname lookup to save network traffic.
Hostnames can be resolved later by processing the log file:
logresolve <access_log >accessdns_log
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139/167 Apache directory protection
Apache directory protection
httpd.conf Apache configuration file
Limited scope directives
<Directory "/var/www/html/foobar">
[list of directives]
</Directory>
Limit the scope of the specified directives to the directory
/var/www/html/foobar and its subdirectories
<Location /foobar>
[list of directives]
</Location>
Limit the scope of the specified directive to the URL
http://www.mysite.org/foobar/ and its subdirectories
Directory protection directives
<Directory "/var/www/html/protected">
AuthName "Protected zone" Name of the realm. The client will be shown the realm name
and prompted to enter a user and password
AuthType Basic Type of user authentication: Basic, Digest, Form, or None
AuthUserFile "/var/www/.htpasswd" User database file. Each line has the format
user:encryptedpassword
To add a user to the database file:
htpasswd /var/www/.htpasswd user
(will prompt for password)
AuthGroupFile "/var/www/.htgroup" Group database file. Each line specifies a group followed by the
usernames of all its members:
group: user1 user2 user3
Require valid-user Control who can access the protected resource.
valid-user any user in the user database file
user user only the specified user
group group only the members of the specified group
Allow from 10.13.13.0/24 Control which host can access the protected resource
Satisfy Any Set the access policy concerning user and host control.
All both Require and Allow criteria must be satisfied
Any any of Require or Allow criteria must be satisfied
Order Allow,Deny Control the evaluation order of Allow and Deny directives.
Allow,Deny First, all Allow directives are evaluated; at
least one must match, or the request is
rejected. Next, all Deny directives are
evaluated; if any matches, the request is
rejected. Last, any requests which do not
match an Allow or a Deny directive are
denied
Deny,Allow First, all Deny directives are evaluated; if
any match, the request is denied unless it
also matches an Allow directive. Any
requests which do not match any Allow or
Deny directives are permitted
</Directory>
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140/167 Apache SSL/TLS
Apache SSL/TLS
httpd.conf Apache configuration file
SSL/TLS directives (mod_ssl module)
SSLCertificateFile \
/etc/httpd/conf/ssl.crt/server.crt
SSL server certificate
SSLCertificateKeyFile \
/etc/httpd/conf/ssl.key/server.key
SSL server private key (for security reasons, this file must
be mode 600 and owned by root)
SSLCACertificatePath \
/usr/local/apache2/conf/ssl.crt/
Directory containing the certificates of CAs. Files in this
directory are PEM-encoded and accessed via symlinks to
hash filenames
SSLCACertificateFile \
/usr/local/apache2/conf/ssl.crt/ca-bundle.crt
Certificates of CAs. Certificates are PEM-encoded and
concatenated in a single bundle file in order of preference
SSLCertificateChainFile \
/usr/local/apache2/conf/ssl.crt/ca.crt
Certificate chain of the CAs. Certificates are PEM-encoded
and concatenated from the issuing CA certificate of the
server certificate to the root CA certificate. Optional
SSLEngine on Enable the SSL/TLS Protocol Engine
SSLProtocol +SSLv3 +TLSv1.2 SSL protocol flavors that the client can use to connect to
server. Possible values are:
SSLv2 (deprecated)
SSLv3
TLSv1
TLSv1.1
TLSv1.2
All (all the above protocols)
SSLCipherSuite \
ALL:!aDH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP
Cipher suite available for the SSL handshake (key
exchange algorithms, authentication algorithms,
cipher/encryption algorithms, MAC digest algorithms)
ServerTokens Full Server response header field to send back to client.
Possible values are:
Prod send Server: Apache
Major send Server: Apache/2
Minor send Server: Apache/2.4
Minimal send Server: Apache/2.4.2
OS send Server: Apache/2.4.2 (Unix)
Full send Server: Apache/2.4.2 (Unix)
PHP/4.2.2 MyMod/1.2
If not specified, sends full header
ServerSignature Off Trailing footer line on server-generated documents.
Possible values are:
Off no footer line (default)
On server version number and ServerName
EMail as above, plus a mailto link to ServerAdmin
SSLVerifyClient none Certificate verification level for client authentication.
Possible values are:
none no client certificate is required
require the client needs to present a valid
certificate
optional the client may present a valid
certificate (this option is unused
as it doesn't work on all browsers)
optional_no_ca the client may present a valid
certificate but it doesn't need to
be successfully verifiable (this
option is practically useless and is
used only for SSL testing)
TraceEnable on Enable TRACE requests
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141/167 Apache proxy
Apache proxy
A forward proxy provides proxy services, typically web content caching and/or filtering, for clients located in a LAN.
All outgoing requests from the clients, and the responses from the Internet, pass through the proxy.
The clients must be manually configured to use the proxy.
httpd.conf Apache configuration file
Forward proxy
ProxyRequests On Enable forward proxy requests
ProxyVia On Add a Via: HTTP header line to every request and reply
<Proxy "*">
Require ip 10.1.1
</Proxy>
Serve only proxy requests coming from 10.1.1.0/24
A reverse proxy aka gateway allows to expose a single entry point for one or more webservers in a LAN. This
improves security and simplifies management, as features (e.g. load balancing, firewalling, automatic redirection from
HTTP to HTTPS, redirection on default ports) can be configured centrally.
It is necessary to create a DNS A record that maps site.example.com to the public IP address of the proxy.
httpd.conf Apache configuration file
Reverse proxy
<VirtualHost *:80> Virtual host for HTTP
ServerName site.example.com Define website name
ProxyPass / http://10.2.2.73:8080/
ProxyPassReverse / http://10.2.2.73:8080/
Enable reverse proxying for server 10.2.2.73
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}
Redirect all HTTP requests to HTTPS
</VirtualHost>
<VirtualHost *:443> Virtual host for HTTPS
ServerName site.example.com Define website name
ServerSignature On Set a footer line under server-generated pages
<Proxy *>
Order deny,allow
Allow from all
</Proxy>
Serve all proxy requests
SSLEngine on
SSLProtocol ALL -SSLv2 -SSLv3
SSLHonorCipherOrder on
SSLCipherSuite DEFAULT
SSLCertificateFile /etc/httpd/ssl/site.crt
SSLCertificateKeyFile /etc/httpd/ssl/site.key
SSLCACertificateFile /etc/httpd/ssl/site.ca.crt
Enable and configure SSL
ProxyPass / http://10.2.2.73:8080/
ProxyPassReverse / http://10.2.2.73:8080/
Enable reverse proxying for server 10.2.2.73
</VirtualHost>
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webserver
10.2.2.73:8080 Apache
reverse proxy Internet
http://site.example.com
https://site.example.com
Internet
webserver
10.2.2.73:8080 Apache
forward proxy
LAN
10.1.1.0/24
142/167 Tomcat
Tomcat
Tomcat is an open source Java Servlet Container implementing several Java EE specifications, and was originally part of the
Jakarta Project. It is composed of:
- Catalina, the core component and servlet container implementation;
- Coyote, an HTTP connector component, providing a pure Java webserver environment to run Java code;
- Jasper, a JSP (Java Server Pages) engine, which parses JSP files and compiles them into Java servlets.
$JAVA_HOME Root of the Java installation e.g.
/usr/lib/jvm/java-1.8.0-openjdk.x86_64/
$CATALINA_HOME Root of the Tomcat installation e.g. /usr/share/tomcat7.
Tomcat may also be configured for multiple instances by defining the
variable $CATALINA_BASE for each instance. If a single instance of
Tomcat is running, $CATALINA_BASE is the same as $CATALINA_HOME
Global files
$CATALINA_BASE/conf/server.xml Tomcat main configuration file
$CATALINA_BASE/conf/web.xml Options and values applied to all web applications running on a specific
Tomcat instance. These can be overridden by the application-specific
servlet configuration defined in
$CATALINA_BASE/webapps/appname/WEB-INF/web.xml
$CATALINA_BASE/conf/context.xml Context applied to all web applications running on a specific Tomcat
instance
$CATALINA_BASE/conf/tomcat-users.xml Users, passwords, and roles applied to a specific Tomcat instance
$CATALINA_BASE/conf/catalina.policy Tomcat's core security policy for the Catalina class
$CATALINA_BASE/conf/catalina.properties Java properties file for the Catalina class
$CATALINA_BASE/conf/logging.properties Java properties file for Catalina's built-in logging functions
$CATALINA_BASE/lib/ JAR files accessible by both web applications and internal Tomcat code
$JAVA_HOME/jre/lib/security/keystore.jks Java keystore
Application-specific files
$CATALINA_BASE/webapps/appname/WEB-INF/ HTML, JSP, and other files to serve to the client browser
$CATALINA_BASE/webapps/appname/WEB-INF/web.xml Description of servlets and other components of the
application, and initialization parameters
$CATALINA_BASE/webapps/appname/WEB-INF/classes/ Java class files that aren't in JAR format. The directory
hierarchy from here reflects the class hierarchy
$CATALINA_BASE/webapps/appname/WEB-INF/lib/ Other JAR files (e.g. third-party libraries, JDBC drivers)
required by the application
java -X Display all available -X options (nonstandard HotSpot JVM options)
java -XshowSettings:properties -version Print Java runtime settings
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143/167 Samba server
Samba server
Samba is a free-software, cross-platform implementation of SMB/CIFS.
SMB (Server Message Block) is Microsoft's proprietary protocol for file and printer sharing, while CIFS (Common Internet File
System) is the public version of SMB.
WINS (Windows Internet Name Service) is a name service used to translate NetBIOS names to IP addresses.
Commonly used ports in Samba
TCP/UDP 137 netbios-ns NetBIOS name service requests and responses
TCP/UDP 138 netbios-dgm NetBIOS datagram services e.g. server announcements
TCP/UDP 139 netbios-ssn NetBIOS session service e.g. file and printer sharing
TCP 445 microsoft-ds Active Directory; registration and translation of NetBIOS names, network browsing
TCP 389 LDAP
TCP 901 SWAT service
The full list of used ports can be found via the command grep -i netbios /etc/services
smbd Server Message Block daemon. Provides SMB file and printer sharing, browser services, user authentication,
and resource lock. An extra copy of this daemon runs for each client connected to the server
nmbd NetBIOS Name Service daemon. Handles NetBIOS name lookups, WINS requests, list browsing and elections.
An extra copy of this daemon runs if Samba functions as a WINS server.
Another extra copy of this daemon runs if DNS is used to translate NetBIOS names
/etc/smb/
/etc/samba/ (RHEL 7)
Samba directory
/etc/samba/lmhosts Samba NetBIOS hosts file
/etc/samba/netlogon User logon directory
smbd -V
smbclient -V
Show the version of the Samba server
testparm Check the Samba configuration file and report any error
smbpasswd jdoe Change the Samba password of user jdoe
smbpasswd -a ksmith Create a new Samba user ksmith and set his password
nmblookup smbserver Look up the NetBIOS name of a server and map it to an IP
address
nmblookup -U winsserver -R WORKGROUP#1B Query recursively a WINS server for the Domain Master
Browser for the specified workgroup
nmblookup -U winsserver -R WORKGROUP#1D Query recursively a WINS server for the Domain Controller
for the specified workgroup
net Tool for administration of Samba and remote CIFS servers
net rpc shutdown -r -S smbserver -U root%password Reboot a CIFS server
net rpc service list -S smbserver List available services on a CIFS server
net status sessions Show active Samba sessions
net status shares Show Samba shares
net rpc info Show information about the domain
net groupmap list Show group mappings between Samba and Windows
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144/167 Samba client
Samba client
mount.cifs
smbmount
Mount a Samba share on a Linux filesystem, using the CIFS
filesystem interface
mount //smbserver/share1 /mnt/share1 -t cifs \
-o username=jdoe
Mount a Samba share as user jdoe
smbstatus Display current information about shares, clients
connections, and locked files
smbclient //smbserver/share1 Access a Samba share on a server (with a FTP-like interface)
smbclient -L //smbserver -W WORKGROUP -U user List the Samba resources available on a server, belonging to
the specified workgroup and accessible to the specified user
cat msg.txt | smbclient -M client -U user Show a message popup on the client machine, using the
WinPopup protocol
Samba mount options
username=user Mount the share as user
password=password Specify the mount user's password
credentials=file Mount the share as the user defined in the credentials file which must
be formatted as such:
username=user
password=password
multiuser Mount the share in multiuser mode
sec=ntlmssp Set the security level to NTLMSSP.
This is required in RHEL 7 to enable multiuser mode
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Samba global configuration
/etc/samba/smb.conf Samba configuration
[global] Global server settings: defines parameters applicable for the whole
Samba server and sets the defaults that will be used for the
parameters not mentioned in other sections
workgroup = MYWORKGROUP Make Samba join the specified workgroup
server string = Linux Samba Server %L Describe server to the clients
hosts allow = 10.9.9.0/255.255.255.0 Allow only the specified machines to connect to the server
security = user Set up user-level authentication
encrypt passwords = yes Use encrypted passwords
smb passwd file = /etc/samba/smbpasswd Refer to the specified password file for user authentication.
A new user's password will need to be set both in Linux and Samba by
using these commands from shell prompt:
passwd newuser
smbpasswd newuser
unix password sync = yes When the password of a client user (e.g. under Windows) is changed,
change the Linux and Samba password too
username map = /etc/samba/smbusers Map each Samba server user name to client user name(s).
The file /etc/samba/smbusers is structured as follows:
root = Administrator Admin
jdoe = "John Doe"
kgreen = "Kim Green"
netbios name = Mysambabox
netbios aliases = Mysambabox1
Set NetBIOS name and alias
wins support = yes Make Samba play the role of a WINS server.
Note: There should be only one WINS server on a network
logon server = yes Enable logon support.
Logon script parameters will be defined in a [netlogon] section
log file = /var/log/samba/log.%m Use a separate logfile for each machine that connects
max log size = 1000 Maximum size of each logfile, in Kb
syslog only = no Whether to log only via Syslog
syslog = 0 Log everything to the logfiles /var/log/smb/log.smbd and /var/log/
smb/log.nmbd, and log a minimum amount of information to Syslog.
This parameter can be set to a higher value to have Syslog log more
information
panic action = \
/usr/share/samba/panic-action %d
Mail a backtrace to the sysadmin in case Samba crashes
[netlogon]
comment = Netlogon for Windows clients
Section defining a logon script
path = /home/netlogon
logon script = %U.bat
Specifies a per-user script e.g. /home/netlogon/jdoe.bat will be
called when user jdoe logs in.
It is also possible to specify a per-clientname script %m.bat, which will
be called when a specific machine logs in.
browseable = no
writeable = no
guest ok = no Guest access to the service (i.e. access without entering a password)
is disabled
[Canon LaserJet 3]
printer name = lp
comment = Canon LaserJet 3 main printer
path = /var/spool/lpd/samba
printable = yes
writeable = no
Section defining a printer accessible via the network
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146/167 Samba share configuration
Samba share configuration
/etc/samba/smb.conf Samba configuration
[public] Section defining a public share accessible on read/write by anyone
comment = Public Storage on %L Describe the public share to users
path = /home/samba Path of the public share on the server
browsable = yes Whether to show the public share when browsing
writeable = yes Whether to allow all users to write in this directory
[homes] Section enabling users that have an account and a home directory
on the Samba server to access it and modify its contents from a
Samba client.
The path variable is not set, by default is path=/home/%S
comment = %U's home directory on %L from %m Describe the share to the user
browseable = no Whether to show the homes share when browsing
writeable = yes Whether to allow the user to write in his home directory
[foobar] Section defining a specific share
path = /foobar Path of the share on the server
comment = Share Foobar on %L from %m Describe the share to users
browsable = yes Whether to show the share when browsing
writeable = yes Whether to allow the users to write in this share
valid users = jdoe, kgreen, +geeks Allow access only to users jdoe and kgreen, and local group geeks
invalid users = csmith Deny access to user csmith
read list = bcameron Allow read-only access to user bcameron
write list = fcastle Allow read-write access to user fcastle
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Samba access configuration
/etc/samba/smb.conf Samba configuration
User-level authentication
[global]
security = user Set up user-level authentication
guest account = nobody Map the guest account to the system user nobody (default)
map to guest = Never Specify how incoming requests are mapped to the guest account:
Bad User redirect from an invalid user to guest account on server
Bad Password redirect from an invalid password to guest account on server
Never reject unauthenticated users
Server-level authentication
[global]
security = server Set up server-level authentication
password server = srv1 srv2 Authenticate to server srv1, or to server srv2 if srv1 is unavailable
Domain-level authentication
[global]
security = ADS Set up domain-level authentication as an Active Directory member server
realm = KRB_REALM Join the specified realm.
Kerberos must be installed and an administrator account must be created:
net ads join -U Administrator%password
Share-level authentication
[global]
security = share Set up share-level authentication
[foobar]
path = /foobar
username = quux
only user = yes
Define a foobar share accessible to any user which can supply quux's password.
The user quux must be created on the system:
useradd -c "Foobar account" -d /tmp -m -s /sbin/nologin quux
and added to the Samba password file:
smbpasswd -a quux
Samba macros
%S Username The substitutes below apply only to the
configuration options that are used when
a connection has been established:
%U Session username (the username that the client requested,
not necessarily the same as the one he got)
%G Primary group of session username %S Name of the current service, if any
%h Samba server hostname %P Root directory of the current service, if any
%M Client hostname %u Username of the current service, if any
%L NetBIOS name of the server %g Primary group name of username
%m NetBIOS name of the client %H Home directory of username
%d Process ID of the current server process %N Name of the NIS home directory server as
obtained from the NIS auto.map entry.
Same as %L if Samba was not compiled with
the --with-automount option
%a Architecture of remote machine
%I IP address of client machine
%i Local IP address to which a client connected %p Path of service's home directory as obtained
from the NIS auto.map entry.
The NIS auto.map entry is split up as %N:%p
%T Current date and time
%D Domain or workgroup of the current user
%w Winbind separator
%$(var)Value of the environment variable var
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Samba setup
Samba setup
This procedure allows sharing on read-write the local directory /smbshare on server 10.1.1.1 to client 10.2.2.2.
Server setup:
1. Create the group for write access to the share groupadd -r geeks
2. Create the user and assign it to the group useradd -G geeks jdoe
3. Add the user to Samba.
You will be prompted to enter a password
smbpasswd -a jdoe
4. Assign correct ownership to the share chgrp geeks /smbshare
5. Set the SGID bit to the share chmod 2775 /smbshare
6. Set the correct SELinux label to the share semanage fcontext -a -t samba_share_t '/smbshare'
restorecon -FR /smbshare
7. Enable the SELinux boolean for write access to
the share
setsebool -P samba_export_all_rw=on
8. Add a section for the share on /etc/samba/smb.conf
[smbshare]
path = /smbshare
hosts allow = 10.2.2.2
write list = @geeks
9. Ensure that the smb and nmb services are running
Client setup:
1. Add an entry to /etc/fstab to mount the Samba share device automatically
//10.1.1.1/smbshare /mountpoint cifs username=jdoe,password=s3cr3t 0 0
Client multiuser setup:
1. Add an entry to /etc/fstab to mount the Samba share device automatically in multiuser mode
//10.1.1.1/smbshare /mountpoint cifs username=jdoe,password=s3cr3t,multiuser,sec=ntlmssp 0 0
2. Login as another user (there must be a matching
Samba user on the Samba server 10.1.1.1)
su - ksmith
3. Store the Samba username and password in the
kernel keyring for the current session
cifscreds add 10.1.1.1
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149/167 NFS
NFS
A Network File System (NFS) server makes filesystems available to remote clients for mounting.
The portmapper is needed by NFS to map incoming TCP/IP connections to the appropriate NFS RPC calls. Some Linux
distributions use rpcbind instead of the portmapper.
For security, the TCP Wrapper should be configured to limit access to the portmapper to NFS clients only:
file /etc/hosts.deny should contain portmap: ALL
file /etc/hosts.allow should contain portmap: IP_addresses_of_clients
NFS handles user permissions across systems by considering users with same UID and username as the same user.
Group permission is evaluated similarly, by GID and groupname.
rpc.nfsd
rpc.mountd
rpc.lockd
rpc.statd
NFS daemons
/etc/exports List of the filesystems to be exported (via the command exportfs)
/var/lib/nfs/xtab List of exported filesystems, maintained by exportfs
/proc/fs/nfs/exports Kernel export table (can be examined via the command cat)
exportfs -ra Export or reexport all directories.
When exporting, fills the kernel export table /proc/fs/nfs/exports.
When reexporting, removes those entries in /var/lib/nfs/xtab that are
deleted from /etc/exports (therefore synchronizing the two files), and
removes those entries from /proc/fs/nfs/exports that are no longer valid
exportfs -ua Unexport all directories.
Removes from /proc/fs/nfs/exports all those entries that are listed in
/var/lib/nfs/xtab, and clears the latter file
showmount Show the remote client hosts currently having active mounts
showmount --directories Show the directories currently mounted by a remote client host
showmount --exports Show the filesystems currently exported i.e. the active export list
showmount --all Show both remote client hosts and directories
showmount -e nfsserver Show the shares a NFS server has available for mounting
rpcinfo -p nfsserver Probe the portmapper on a NFS server and display the list of all registered
RPC services there
rpcinfo -t nfsserver nfs Test a NFS connection by sending a null pseudo request (using TCP)
rpcinfo -u nfsserver nfs Test a NFS connection by sending a null pseudo request (using UDP)
nfsstat Display NFS/RPC client/server statistics.
Options:
NFS RPC both
server -sn -sr -s
client -cn -cr -c
both -n -r -nr
mount -t nfs nfsserver:/share /usr Command to be run on a client to mount locally a remote NFS share.
NFS shares accessed frequently should be added to /etc/fstab e.g.
nfsserver:/share /usr nfs intr 0 0
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/etc/exports
/etc/exports
/export/ 10.3.3.3(rw)
/export2/ 10.4.4.0/24
/export3/ *(ro,sync)
/home/ftp/pub client1(rw) *.example.org(ro)
/home/crew @FOOWORKGROUP(rw) (ro)
filesystem Filesystem on the NFS server to be exported to clients
client
identity
Client systems permitted to access the exported directory. Can be specified by hostname, IP address,
wildcard, subnet, or @NIS workgroup.
Multiple client systems can be listed, and each one can have different options
client
options
ro Read-only access (default)
rw Read and write access. The client may choose to mount read-only anyway
sync Reply to requests only after the changes made by these requests have been committed
to stable storage
async Reply to requests without waiting that changes are committed to stable storage.
Improves performances but might cause loss or corruption of data if server crashes
root_squash Requests by user root on client will be done as user nobody on server (default)
no_root_squash Requests by user root on client will be done as same user root on server
all_squash Requests by a non-root user on client will be done as user nobody on server
no_all_squash Requests by a non-root user on client will be attempted as same user on server (default)
NFS mount options
rsize=nnn Size for read transfers (from server to client)
wsize=nnn Size for write transfers (from client to server)
nfsvers=n Use NFS version n for transport
retry=n Keep retrying a mount attempt for n minutes before giving up
timeo=n A mount attempt times out after n tenths of a second
intr User can interrupt a mount attempt
nointr User cannot interrupt a mount attempt (default)
hard The system will try a mount indefinitely (default)
soft The system will try a mount until an RPC timeout occurs
bg Try a mount in the foreground; all retries occur in the background
fg All mount attempts occur in the foreground (default)
tcp Connect using TCP
udp Connect using UDP
sec=krb5p Use Kerberos to encrypt all requests between client and server
v4.2 Enable NFS v4.2, which allows the server to export the SELinux context
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NFS setup
NFS setup
This procedure allows sharing on read-write the local directory /nfsshare on server 10.1.1.1 to client 10.2.2.2.
Server setup:
1. Ensure that the nfs-server service is running
2. Change ownership of the share chown nfsnobody /nfsshare
3. Add an entry for the share on /etc/exports
/nfsshare 10.2.2.2(rw)
4. Reload the exports file exportfs -r
Client setup:
1. Add an entry to /etc/fstab to mount the NFS share device automatically
10.1.1.1:/nfsshare /mountpoint nfs defaults 0 0
Secure NFS setup
This procedure allows sharing on read-write the local directory /nfsshare on server 10.1.1.1 to client 10.2.2.2, securely
with Kerberos enabled.
Server setup:
1. Install the appropriate server keytab on /etc/krb5.keytab
2. Ensure that the nfs-secure-server service is running
3. Change ownership of the share chown nfsnobody /nfsshare
4. Add an entry for the share on /etc/exports
/nfsshare 10.2.2.2(sec=krb5p,rw)
5. Reload the exports file exportfs -r
Client setup:
1. Install the appropriate client keytab on /etc/krb5.keytab
2. Ensure that the nfs-secure service is running
3. Add an entry to /etc/fstab to mount the NFS share device automatically
10.1.1.1:/nfsshare /mountpoint nfs defaults,sec=krb5p 0 0
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iSCSI
iSCSI (Internet Small Computer System Interface) is a network protocol that allows emulating a SCSI local storage device
over a TCP/IP network. By default it uses TCP port 3260.
An iSCSI server can use a local block device (physical or virtual disk, disk partition, or Logical Volume), a file, a physical
SCSI device, or a ramdisk as the underlying storage resource (backstore) and make it available by assigning it a LUN
(Logical Unit Number). An iSCSI server provides one or more targets, each of which presents one or more LUNs and is able
to accept connections from an iSCSI client (initiator).
Targets and initiators are called nodes and are identified by a unique IQN (iSCSI Qualified Name) e.g.
iqn.2017-11.org.example.subdomain:foo:bar . The IP address and port of a node is called a portal.
A target accepts connections from an initiator via a TPG (Target Portal Group) i.e. its IP address and port. A TPG may have
in place an ACL so to accept connections only from a specific initiator's IQN.
targetcli Target configurator (server side). Can be used as a command line tool or as an interactive shell.
Configuration is saved to /etc/target/saveconfig.json
iscsiadm Administration tool for iSCSI devices (client side)
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iSCSI setup
iSCSI setup
This procedure makes available the local disk /dev/sbd on server 10.1.1.1 to the client having IQN iqn.2017-
11.org.example:client .
Server (target) setup:
1. Ensure that the targetcli service is running
2. Enter the targetcli shell targetcli
3. Create a backstore cd /backstores/block
create mydisk /dev/sdb
4. Create a IQN for the target.
This automatically creates a TPG for the IQN
cd /iscsi
create iqn.2017-11.org.example:target
5. On the TPG, create an ACL to allow connections
from the initiator with a specific IQN
cd /iscsi/iqn.2017-11.org.example:target/tpg1/acls
create iqn.2017-11.org.example:client
6. On the TPG, create a LUN for the backstore cd /iscsi/iqn.2017-11.org.example:target/tpg1/luns
create /backstores/block/mydisk
7. On the TPG, create a portal listening from the
server's IP address
cd /iscsi/iqn.2017-11.org.example:target/tpg1/portals
delete 0.0.0.0 ip_port=3260
create 10.1.1.1
8. Verify the configuration ls /
o- / ........................................................................................ [...]
o- backstores ............................................................................. [...]
| o- block ................................................................. [Storage Objects: 1]
| | o- mydisk ........................................ [/dev/sdb (100.0MiB) write-thru activated]
| | o- alua .................................................................. [ALUA Groups: 1]
| | o- default_tg_pt_gp ...................................... [ALUA state: Active/optimized]
| o- fileio ................................................................ [Storage Objects: 0]
| o- pscsi ................................................................. [Storage Objects: 0]
| o- ramdisk ............................................................... [Storage Objects: 0]
o- iscsi ........................................................................... [Targets: 1]
| o- iqn.2017-11.org.example:target ................................................... [TPGs: 1]
| o- tpg1 .............................................................. [no-gen-acls, no-auth]
| o- acls ......................................................................... [ACLs: 1]
| | o- iqn.2017-11.org.example:client ...................................... [Mapped LUNs: 1]
| | o- mapped_lun0 ............................................... [lun0 block/mydisk (rw)]
| o- luns ......................................................................... [LUNs: 1]
| | o- lun0 .................................... [block/mydisk (/dev/sdb) (default_tg_pt_gp)]
| o- portals ................................................................... [Portals: 1]
| o- 10.1.1.1:3260 ................................................................... [OK]
o- loopback ........................................................................ [Targets: 0]
9. Exit the targetcli shell.
Configuration is automatically saved
exit
Client (initiator) setup:
1. Set the correct initiator IQN in the file /etc/iscsi/initiatorname.iscsi
InitiatorName=iqn.2017-11.org.example:client
2. Ensure that the iscsi service is running
3. Discover the iSCSI target(s) provided
by the portal. This echoes the target(s)
IQN found
iscsiadm -m discovery -t sendtargets -p 10.1.1.1
4. Login to the target IQN found iscsiadm -m node -T iqn.2017-11.org.example:target -p 10.1.1.1 -l
The iSCSI device is now locally available and can be formatted and mounted. Node records remain after logout or
reboot; the system will login again to the target IQN automatically
5. Add an entry to /etc/fstab to mount the iSCSI device automatically
UUID=nnnnnnnn-nnnn-nnnn-nnnn-nnnnnnnnnnnn /mountpoint fstype _netdev 0 0
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154/167 DHCP
DHCP
A DHCP (Dynamic Host Configuration Protocol) server listens for requests on UDP port 67 and answers to UDP port 68.
The assignment of an IP address to a host is done through a sequence of DHCP messages initiated by the client host:
DHCP Discover, DHCP Offer, DHCP Request, DHCP Acknowledgment.
Because DHCP Discover messages are broadcast and therefore not routed outside a LAN, a DHCP relay agent is necessary
for those clients situated outside the DHCP server's LAN. The DHCP relay agent listens to DHCP Discover messages and
relays them in unicast to the DHCP server.
/etc/dhcpd.conf Configuration file for the DHCP server
/etc/sysconfig/dhcrelay (SUSE) Configuration file for the DHCP relay agent
/var/lib/dhcpd/dhcpd.leases DHCP current leases
/etc/dhcpd.conf DHCP server configuration
option domain-name-servers 10.2.2.2;
option smtp-servers 10.3.3.3;
option pop-servers 10.4.4.4;
option time-servers 10.5.5.5;
option nntp-servers 10.6.6.6;
Global parameters for DNS, mail, NTP, and news servers
specification
shared-network geek-net { Definition of a network
default-lease-time 86400; Time, in seconds, that will be assigned to a lease if a client
does not ask for a specific expiration time
max-lease-time 172800; Maximum time, in seconds, that can be assigned to a
lease if a client asks for a specific expiration time
option routers 10.0.3.252;
option broadcast-address 10.0.3.255;
subnet 10.0.3.0 netmask 255.255.255.128 {
range 10.0.3.1 10.0.3.101;
}
subnet 10.0.3.128 netmask 255.255.255.128 {
range 10.0.3.129 10.0.3.229;
}
}
Definition of different subnets in the network, with
specification of different ranges of IP addresses that will be
leased to clients depending on the client's subnet
group { Definition of a group
option routers 10.0.17.252;
option broadcast-address 10.0.17.255;
netmask 255.255.255.0;
host linuxbox1 {
hardware ethernet AA:BB:CC:DD:EE:FF;
fixed-address 10.0.17.42;
option host-name "linuxbox1";
}
host linuxbox2 {
hardware ethernet 33:44:55:66:77:88;
fixed-address 10.0.17.66;
option host-name "linuxbox2";
}
}
Definition of different hosts to whom static IP addresses
will be assigned to, depending on their MAC address
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PAM
PAM (Pluggable Authentication Modules) is an abstraction layer that allows applications to use authentication methods while
being implementation-agnostic.
/etc/pam.d/service PAM configuration for service
/etc/pam.conf (obsolete) PAM configuration for all services
ldd /usr/sbin/service | grep libpam Check if service is enabled to use PAM
/etc/pam.d/service
auth requisite pam_securetty.so
auth required pam_nologin.so
auth required pam_env.so
auth required pam_unix.so nullok
account required pam_unix.so
session required pam_unix.so
session optional pam_lastlog.so
password required pam_unix.so nullok obscure min=4 max=8
type
auth Authentication module to verify user identity and group membership
account Authorization module to determine user's right to access a resource (other than his identity)
password Module to update an user's authentication credentials
session Module (run at end and beginning of an user session) to set up the user environment
control
optional Module is not critical to the success or failure of service
sufficient If this module successes, and no previous module has failed, module stack processing ends
successfully. If this module fails, it is non-fatal and processing of the stack continues
required If this module fails, processing of the stack continues until the end, and service fails
requisite If this module fails, service fails and control returns to the application that invoked service
include Include modules from another PAM service file
module
PAM module and its options, e.g.:
pam_unix.so Standard UNIX authentication module via /etc/passwd and /etc/shadow
pam_nis.so Module for authentication via NIS
pam_ldap.so Module for authentication via LDAP
pam_fshadow.so Module for authentication against an alternative shadow passwords file
pam_cracklib.so Module for password strength policies (e.g. length, case, max n of retries)
pam_limits.so Module for system policies and system resource usage limits
pam_listfile.so Module to deny or allow the service based on an arbitrary text file
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LDAP
LDAP (Lightweight Directory Access Protocol) is a simplified version of the X.500 standard and uses TCP port 389.
LDAP permits to organize hierarchically a database of entries, each one of which is identified by a unique DN (Distinguished
Name). Each DN has a set of attributes, each one of which has a value. An attribute may appear multiple times.
Most frequently used LDAP attributes
Attribute Example Meaning
dn dn: cn=John Doe,dc=example,dc=org Distinguished Name
(not an attribute; identifies the entry)
cn cn: John Doe Common Name
dc dc=example,dc=org Domain Component
givenName givenName: John Firstname
sn sn: Doe Surname
mail mail: jdoe@example.org Email address
telephoneNumber telephoneNumber: +1 505 1234 567 Telephone number
uid uid: jdoe User ID
c c: US Country code
l l: San Francisco Locality
st st: California State or province
street street: 42, Penguin Road Street
o o: The Example Foundation Organization
ou ou: IT Dept Organizational Unit
manager manager: cn=Kim Green,dc=example,dc=org Manager
ldapsearch -H ldap://ldapserver.example.org \
-s base -b "ou=people,dc=example,dc=com" \
"(sn=Doe)" cn sn telephoneNumber
Query the specified LDAP server for entries where
surname=Doe, and print common name, surname, and
telephone number of the resulting entries.
Output is shown in LDIF
ldappasswd -x -D "cn=Admin,dc=example,dc=org" \
-W -S "uid=jdoe,ou=IT Dept,dc=example,dc=org"
Authenticating as Admin, change the password of user jdoe in
the OU called IT Dept, on example.org
ldapmodify -b -r -f /tmp/mods.ldif Modify an entry according to the LDIF file /tmp/mods.ldif
ldapadd -h ldapserver.example.org \
-D "cn=Admin" -W -f /tmp/mods.ldif
Authenticating as Admin, add an entry by adding the content
of the LDIF file /tmp/mods.ldif to the directory.
This command actually invokes ldapmodify -a
ldapdelete -v "uid=jdoe,dc=example,dc=org" \
-D "cn=Admin,dc=example,dc=org" -W
Authenticating as Admin, delete the entry of user jdoe
LDIF (LDAP Data Interchange Format)
dn: cn=John Doe, dc=example, dc=org
changetype: modify
replace: mail
mail: johndoe@otherexample.com
-
add: jpegPhoto
jpegPhoto:< file://tmp/jdoe.jpg
-
delete: description
-
This LDIF file will change the email address
of jdoe, add a picture, and delete the
description attribute for the entry
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
157/167 OpenLDAP
OpenLDAP
slapd Standalone OpenLDAP daemon
/var/lib/ldap/ Files constituting the OpenLDAP database
/etc/openldap/slapd.conf
/usr/local/etc/openldap/slapd.conf
OpenLDAP configuration file
slapcat -l file.ldif Dump the contents of an OpenLDAP database to a LDIF file
slapadd -l file.ldif Import an OpenLDAP database from a LDIF file
slapindex Regenerate OpenLDAP's database indexes
yum install openldap openldap-clients \
authconfig sssd nss-pam-ldapd authconfig-gtk
Install the OpenLDAP client (on RHEL 7)
authconfig --enableldap --enableldapauth \ --
ldapserver=ldap://ldapserver \ --
ldapbasedn="dc=example,dc=org" \
--enablesssd --update
Set up the LDAP client to connect to a ldapserver.
This will update the configuration files /etc/sssd/sssd.conf and
/etc/openldap/ldap.conf
getent group groupname Get entries about groupname from NSS libraries
authconfig-gtk
system-config-authentication
OpenLDAP configuration GUI
sssd (the System Security Services Daemon) must be running to provide access to OpenLDAP as an authentication and
identity provider.
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
158/167 SELinux
SELinux
Security-Enhanced Linux (SELinux) is a Linux kernel security module that provides a mechanism for supporting access
control security policies.
SELinux implements a Mandatory Access Control framework that allows the definition of fine-grained permissions for how
subjects (i.e. processes) interact with objects (i.e. other processes, files, devices, ports, sockets); this improves security
with respect to the standard Discretionary Access Control, which defines accesses based on users and groups. The security
context of a file is stored in its extended attributes.
The decisions SELinux takes about allowing or disallowing access are stored in the AVC (Access Vector Cache).
setenforce 0
echo 0 > /selinux/enforce
Enter permissive mode
setenforce 1
echo 1 > /selinux/enforce
Enter enforcing mode
getenforce
cat /selinux/enforce
sestatus -v
Display current mode
SELinux mode can be configured permanently in /etc/selinux/config (symlinked in /etc/sysconfig/selinux):
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
# enforcing - SELinux security policy is enforced.
# permissive - SELinux prints warnings instead of enforcing.
# disabled - No SELinux policy is loaded.
SELINUX=enforcing
# SELINUXTYPE= can take one of these two values:
# targeted - Only targeted network daemons are protected.
# strict - Full SELinux protection.
SELINUXTYPE=targeted
chcon context file Change the security context of file to the specified context
chcon --reference=file0 file Change the security context of file to be the same as file0
restorecon -f file Restore the security context of file to the system default
ls -Z List files and their security context
ps -eZ List processes and their security context
semanage Manage SELinux policies
semanage fcontext -l List files and their assigned SELinux labels
semanage fcontext -a -t label file Assign the SELinux label to file.
You then need to apply the label via restorecon -f file
semanage port -l List port numbers and their assigned SELinux type definitions
semanage port -a -t portlabel -p tcp nAssign the SELinux portlabel to TCP port n
semanage port -a -t http_port_t -p tcp 8888 Allow a local webserver to serve content on port 8888
semanage port -d -t http_port_t -p tcp 8888 Remove the binding of http_port_t port label to TCP 8888
semanage port -m -t http_cache_port_t -p tcp 8888 Modify the port label bound to TCP 8888
getsebool boolean Get the value of a SELinux boolean
setsebool boolean=value Set the value of a SELinux boolean
tar --selinux [other args]
star -xattr -H=exustar [other args]
Create or extract archives that retain the security context of
the original files
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
159/167 AVC
AVC
/selinux/ Pseudo filesystem created by SELinux, containing commands used by
the kernel for its operations
/var/log/audit/audit.log Logfile containing AVC denials, if auditd is running
/var/log/messages Logfile containing AVC denials, if rsyslogd is running
sealert -a logfile Analyze a SELinux logfile and display SELinux policy violations
grep nnnnn.mmm:pp logfile | audit2why Diagnostic a specific AVC event entry from a SELinux logfile:
type=AVC msg=audit(nnnnn.mmm:pp): avc: denied (...)
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
160/167 KVM
KVM
KVM (Kernel-based Virtual Machine) is a virtualization infrastructure for the Linux kernel that allows it to function as an
hypervisor.
/etc/libvirt/qemu/ Directory containing the XML files that define VMs properties.
libvirtd must be restarted after modifying a XML file
/var/lib/libvirt/ Directory containing files related to the VMs
virt-manager KVM GUI
virt-install --prompt Interactive command-line program to create a VM
virt-install -n vmname -r 2048 \
--disk path=/var/lib/libvirt/images/vmname.img \
-l /root/vmstuff/inst/ \
-x "ks=/root/vmstuff/kickstart.cfg"
Create a VM with 2 Gb of RAM, specifying path of virtual disk,
location of installation files, and (as extra argument) the
Kickstart configuration to use
virt-clone --prompt Interactive command-line program to clone a VM.
A VM must be shut off or paused before it can be cloned
virt-clone -o vmname -n vmclonename Clone a VM
virsh Interface for VM management
virsh list --all List all VMs present on the system
virsh start vmname Start a VM
virsh destroy vmname Brutally shut down a VM
virsh shutdown vmname Gracefully shut down a VM
virsh autostart vmname Set a VM to be automatically started when the system boots.
Done by symlinking the VM to /etc/libvirt/qemu/autostart/
virsh autostart --disable vmname Disable the autostart of a VM at system boot
virsh edit vmname Edit the XML file defining a VM's properties
virt-what Detect whether the current machine is a VM
Kickstart
Kickstart is a method to perform automatic installation and configuration of RHEL machines.
This can be done by specifying inst.ks=hd:/dev/sda:/root/path/ksfile either as a boot option, or an option
to the kernel command in GRUB 2.
system-config-kickstart GUI tool to create a Kickstart file
ksvalidator ksfile Check the validity of a Kickstart file
/root/anaconda-ks.cfg Kickstart file describing the current system. This file was automatically
generated during the installation of the current system
ksverdiff -f RHEL6 -t RHEL7 Show the differences in the Kickstart syntax between RHEL 6 and RHEL 7
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
161/167 Git
Git
Git is an open source version control system with a small footprint and very high performances. A Git directory is a
complete repository with full history and version tracking abilities, independent from any remote repository.
git init Initialize the current directory as a repository
git clone repoaddress Clone a remote repository.
repoaddress can be a URL (SSH, HTTP, HTTPS, FTP, FTPS, Git) or a local path e.g.
ssh://user@example.com:8888/path/to/repo.git
git://example.com:9999/path/to/repo.git
/path/to/repo.git
git checkout branch Start working into an already existing branch
git checkout -B branch Create branch and start working into it
git pull Pull the changes from the remote repository branch to the local branch
git add file Add file to the content staged for the next commit (hence starting to track it)
git rm file Remove file from the content staged for the next commit
git status See the status (e.g. files changed but not yet staged) of the current branch
git commit -am "Message"Commit all staged files in the current branch
git push Push the local commits from the current branch to the remote repository
git push origin branch Push the local commits from branch to the remote repository
git merge branch Merge changes made on branch to the master branch
git diff checksum1 checksum2 Compare two commits
git log -Gword Show the commits whose added or deleted lines contain word
git branch Show local branches
git branch -r Show remote branches
git branch -a Show remote and local branches
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
162/167 Vagrant
Vagrant
Vagrant is an open source software that allows building and maintaining lightweight and portable virtual environments for
software development. It relies on an underlying virtualization solution e.g. VirtualBox.
vagrant -h Print the list of commands recognized by Vagrant
vagrant command -h Print help about the Vagrant command
vagrant init hashicorp/precise64 Initialize the current directory as a specific Vagrant environment (in this case,
Ubuntu 12.04 64-bit) by creating a Vagrantfile on it
vagrant up vmname Start a guest virtual machine and do a first provisioning according to the
Vagrantfile
vagrant provision vmname Provision a virtual machine
vagrant ssh vmname Connect via SSH to a virtual machine
vagrant halt vmname Shut down the virtual machine
vagrant destroy vmname Delete the virtual machine and free any resource allocated to it
vagrant status Print the status of the virtual machines currently managed by Vagrant
vagrant global-status Print the status of all Vagrant environments on the system. This command
reads cached data, hence completes quickly but can print outdated results;
use the --prune option to rebuild the cache and obtain correct results
The directory containing the Vagrantfile on the host can be accessed on the guest via /vagrant.
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
163/167 HTML 4.01 components
HTML 4.01 components
Tag Attributes
<h1>...<h6> Heading align=left|center|right|justify Heading alignment †
<br> Line break Line break and
carriage return
<hr> Horizontal line
align=left|center|right Line alignment †
noshade Solid rendering instead of 3D †
size=npixels Line height
width=npixels|percent%Line width
<p> Paragraph
<div> Section align=left|center|right|justify Paragraph or section alignment †
<span> Group Group of elements
<a> Anchor Hyperlink
charset=encoding Character encoding of target URL
coords=left,top,right,bottom|
cx,cy,radius|x1,y1,...,xn,yn
Coordinates of region; depends
on shape
href=url Target URL for the link
hreflang=language Language of document at the
target URL
name=section Name of anchor for document
bookmarking
rel|rev=alternate|stylesheet|
start|next|prev|contents|index|
glossary|copyright|chapter|
section|subsection|appendix|
help|bookmark
Relationship between this
document and the target URL
(rel) or vice versa (rev)
shape=rectangle|circle|polygon Shape of region
target=_blank|_parent|_self|_top Destination of target URL
type=mimetype MIME type of target URL
<dl>
Definition list
<dt>
Definition term
<dd>
Definition description
Description of a
definition term
<ol> Ordered list
compact=compact List must be more compact †
start=firstnumber Number to start the list on †
type=A|a|I|i|1 List numbers type †
<ul> Unordered list
compact=compact List must be more compact †
type=disc|square|circle List type †
<li> List item
type=disc|square|circle|A|a|I|i|1 List item type †
value=itemno List item value †
† = deprecated
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
164/167 HTML 4.01 text
HTML 4.01 text
Tag Attributes
<i> Italic
<b> Bold
<s>
<strike> Strike-through Strike-through text †
<u> Underlined Underlined text †
<big> Bigger
<small> Smaller
<sub> Subscript
<sup> Superscript
<tt> Teletype Monospaced text
<em> Emphasized
<strong> Strong
<del> Deleted
<ins> Inserted Deleted/inserted text
cite=url URL to document explaining
deletion/insertion
datetime=yyyy-mm-dd When the text was deleted/inserted
<pre> Preformatted width=ncharacters Max number of characters per line †
<code> Code Source code text
<samp> Sample Sample code text
<kbd> Keyboard Keyboard key
<var> Variable Variable name
<cite> Citation Citation block
<blockquote> Quotation
<q> Short quotation cite=url URL to document containing the quote
<address> Address Address block
<abbr> Abbreviation
<acronym> Acronym
<dfn> Definition Definition term
<font> Font Font †
color=rgb(r,g,b)|#rrggbb|color Text color
face=fontname Text font
size=[1...7]|[-6...+6] Text size
<bdo> Bidirectional override dir=ltr|rtl Direction of text: left-to-right or right-
to-left
<xmp> XMP Non-formatted text † ;
ignores other HTML tags
other tags Attributes common to
almost all other tags
class=class|style Class of the element
id=id Unique ID of the element
style=styledef Inline style definition
title=tooltip Text of the tooltip to display
dir=ltr|rtl Direction of text: left-to-right or right-
to-left
lang=language Language of the content
accesskey=character Keyboard shortcut for the element
tabindex=ntab N of tab for the element
† = deprecated
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
165/167 HTML 4.01 images
HTML 4.01 images
Tag Attributes
<img>
Image
align=top|bottom|left|middle|right Image alignment with respect to surrounding text †
alt=alternatetext Description of the image for text-only browsers
border=npixels Border width around the image †
height=npixels|percent%Image height
hspace=npixels Blank space on the left and right side of image †
ismap=url URL for server-side image map
longdesc=url URL containing a long description of the image
src=url URL of the image
usemap=url URL for client-side image map
vspace=npixels Blank space on top and bottom of image †
width=npixels|percent%Image width
<map>
Image map
id=id Unique ID for the map tag
name=name Unique name for the map tag
<area>
Area of
image map
alt=alternatetext Description of area for text-only browsers
coords=left,top,right,bottom|
cx,cy,radius|x1,y1,...,xn,yn Coordinates of clickable area; depends on shape
href=url Target URL of area
nohref=true|false Excludes or includes the area from image map
shape=rectangle|circle|polygon Shape of area
target=_blank|_parent|_self|_top Destination of target URL
† = deprecated
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
166/167 HTML 4.01 tables
HTML 4.01 tables
Tag Attributes
<table>
Table
align=left|center|right Table alignment †
bgcolor=rgb(r,g,b)|#rrggbb|color Table background color †
border=npixels Border width
cellpadding=npixels|percent%Space around the content of each cell
cellspacing=npixels|percent%Space between cells
frame=void|above|below|
lhs|rhs|hsides|vsides|box|border Visibility of sides of the table border
rules=none|groups|rows|cols|all Horizontal or vertical divider lines
summary=summary Summary of the table for text-only browsers
width=npixels|percent%Table width
<tr>
Table row
align=left|center|right|justify|char Horizontal text alignment
bgcolor=rgb(r,g,b)|#rrggbb|color Row background color †
char=character Character to align text on, if align=char
charoff=npixels|percent%Alignment offset to first character, if align=char
valign=top|middle|bottom|baseline Vertical text alignment
<td>
Table cell
<th>
Table header
abbr=content Abbreviated content in a cell
align=left|center|right|justify|char Horizontal text alignment
axis=category Cell name
bgcolor=rgb(r,g,b)|#rrggbb|color Cell background color †
char=character Character to align text on, if align=char
charoff=npixels|percent%Alignment offset to first character, if align=char
colspan=ncolumns Number of columns this cell spans on
headers=headerid Cell header information for text-only browsers
height=npixels Cell height †
nowrap Text in cell stays on a single line †
rowspan=nrows Number of rows this cell spans on
scope=col|colgroup|row|rowgroup Target for cell header information
valign=top|middle|bottom|baseline Vertical text alignment
width=npixels|percent% Cell width †
† = deprecated
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo
167/167 7-bit ASCII table
7-bit ASCII table
Dec Hex Char Dec Hex Char Dec Hex Char Dec Hex Char
0 0 NUL Null 32 20 space 64 40 @96 60 `
1 1 SOH Start of heading 33 21 !65 41 A97 61 a
2 2 STX Start of text 34 22 "66 42 B98 62 b
3 3 ETX End of text 35 23 #67 43 C99 63 c
4 4 EOT End of transmission 36 24 $68 44 D100 64 d
5 5 ENQ Enquiry 37 25 %69 45 E101 65 e
6 6 ACK Acknowledge 38 26 &70 46 F102 66 f
7 7 BEL Bell 39 27 '71 47 G103 67 g
8 8 BS Backspace 40 28 (72 48 H104 68 h
9 9 TAB Horizontal tab 41 29 )73 49 I105 69 i
10 A LF Line feed 42 2A *74 4A J106 6A j
11 B VT Vertical tab 43 2B +75 4B K107 6B k
12 C FF Form feed 44 2C ,76 4C L108 6C l
13 D CR Carriage return 45 2D -77 4D M109 6D m
14 E SO Shift out 46 2E .78 4E N110 6E n
15 F SI Shift in 47 2F /79 4F O111 6F o
16 10 DLE Data link escape 48 30 080 50 P112 70 p
17 11 DC1 Device control 1 49 31 181 51 Q113 71 q
18 12 DC2 Device control 2 50 32 282 52 R114 72 r
19 13 DC3 Device control 3 51 33 383 53 S115 73 s
20 14 DC4 Device control 4 52 34 484 54 T116 74 t
21 15 NAK Negative ACK 53 35 585 55 U117 75 u
22 16 SYN Synchronous idle 54 36 686 56 V118 76 v
23 17 ETB End of Tx block 55 37 787 57 W119 77 w
24 18 CAN Cancel 56 38 888 58 X120 78 x
25 19 EM End of medium 57 39 989 59 Y121 79 y
26 1A SUB Substitute 58 3A :90 5A Z122 7A z
27 1B ESC Escape 59 3B ;91 5B [123 7B {
28 1C FS File separator 60 3C <92 5C \124 7C |
29 1D GS Group separator 61 3D =93 5D ]125 7D }
30 1E RS Record separator 62 3E >94 5E ^126 7E ~
31 1F US Unit separator 63 3F ?95 5F _127 7F DEL Delete
Characters 0-31 and 127 are non-printable.
The ascii command and its manpage man ascii can be used to display an ASCII table.
Linux Quick Reference Guide 6th ed., Aug 2018 © Daniele Raffo www.crans.org/~raffo

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