Linux Cheat Sheet

linux-cheatsheet.md

Content

• Basics
  • File Hierarchy Standard (FHS)
  • Commands
  • Globs (Wildcards)
  • Regex
  • Stream redirection
• Disk and File System Management
  • General Disk Manipulation (non-LVM)
  • LVM
• Automatic mounting (/etc/fstab)
• Text Readers & Editors
  • Less
  • VI
• User and Group Management
• File System Permissions
• SSH
• IPv6 (disable)
• Cronjobs
• Package Management
  • RPM
  • YUM
• Services (systemd)
• NFS
• Samba
• FTP
• MySQL Server
• Other

Basics

File Hierarchy Standard (FHS)

Path	Content
/bin	Binaries (User)
/boot	Static boot loader files
/etc	Host specific configs
/lib	Shared libraries and kernel modules
/sbin	Binaries (System/root)
/var	Varying files (e.g. Logs)
/usr	3rd party software
/proc	Pseudo file system
/sys	Pseudo file system
/mnt	Mountpoint for internal drives
/media	Mountpoint for external drives
/home	User homes
/run	PID files of running processes

Commands

File System Commands

Command	Param	Description
cd	-	Navigate to last dir
	~	Navigate to home
	~username	Navigate to home of specified user
pwd		Print working dir
ls		Print dir content
	-l	Format as list
	-a	Show hidden items (-A without . and ..)
	-r	Invert order
	-R	Recurse
	-S	Sort by size
	-t	Sort by date modified
mkdir	-p	Create dir with parents
cp	-r	Copy dir
rmdir	-p	Remove dir and empty parents
rm	-rf	Remove dir recursively, -f without confirmation
mv		Move recursively
find	-iname pattern	Search dir/file case-insensitive
	-mmin n	Last modified n minutes ago
	-mtime n	Last modified n days ago
	-regex pattern	Path matches pattern
	-size n[kMG]	By file size (-n less than; +n greater than)
	! searchparams	Invert search

File Manipulation

Command	Param	Description
cat	file	Print content
tac	file	Print content inverted
sort	file	Print sorted
	file -r -u	Print sorted descending without dublicates
wc	file	Count Lines, Words, Chars (Bytes)
head	`-n10 file	tail -n5`
tail	-f file	Print new lines automatically
cut	-f -4,7-10,12,15- file	Print selected fields (tab delimited)
	-c -4,7-10,12,15- file	Print selected characters positions
	-f 2,4 -d, --output-delimiter=$'\t' file	Change delimiter (but use tab for output)
uniq	file	Hide consecutive identical lines
	file -c	Show consecutive identical line count
	file -u	Hide consecutive identical lines
file	file	Get file type

Archiving

Command	Param	Beschreibung
tar	cfv archiv.tar file1 file2	Archiv erstellen / Inhalt hinzufügen oder überschreiben
	tfv archiv.tar	Inhalt anzeigen
	xf archiv.tar [-C ~/extracted]	Archiv (nach ~/extracted) entpacken (und dekomprimieren)
	cfvj archiv.tar.bz2 file	bzip2 komprimiertes Archiv erstellen
	cfvz archiv.tar.gz file	gzip komprimiertes Archiv erstellen
	cfa archiv.tar.[komp] file	komprimiertes Archiv erstellen (auto Typ anhand Name)
bzip2 / gzip	file1 file2	Dateien (einzeln) komprimieren
	-d file1 file2	Dateien dekomprimieren

Other

Command	Param	Beschreibung
<command>	--help	Help of current command (not standardized)
	-h
	-?
man	<command>	Manual page of command
	-k keyword	Search command by keyword (oder apropos)
alias		Show aliases
	name='befehl'	Create alias

Globs (Wildcards)

The dot . in front of hidden items is ignored by glob patterns!

Character	Description
?	Any single character
*	Any characters
[ac-e]	1 character in enum
[!ac-e]	1 character not in enum

Regex

Bash itself does not know regex. Use programs like grep, sed, awk.

Steuerzeichen

Character	Description
.	Any single character
[ac-e]	1 character in enum
[^ac-e]	1 character not in enum
^	Start of string
$	End of string
\d	Digit
\D	Not a digit
\s	Whitespace
\S	Not a Whitespace
\<	Start of word
\>	End of word
pattern?	Quantifier 0 or 1
pattern*	Quantifier 0..n
pattern+	Quantifier 1..n
pattern{x}	Quantifier exactly x
pattern{x,}	Quantifier x..n
pattern{x,y}	Quantifier x..y
pattern{,y}	Quantifier 0..y

Grep

Command	Param	Description
grep	pattern file	Extended Regex
	-E pattern file	Extended Regex
	-v pattern file	Invert match
	-w pattern file	Word match
	-i pattern file	Ignore case

Stream redirection

• > overwrite
• >> append

Character	Description
> file or 1> file	STDOUT to file
< file	Datei to STDIN
2> file	STDERR to file
2>&1	STDERR to same target as STDOUT
> file 2>&1	STDOUT and STDERR to file

Disk and File System Management

General Disk Manipulation (non-LVM)

Creating physical partitions is not required! You can create PVs directly!

Command	Description
fdisk -l	List physical disks and partitions
fdisk /dev/sdb n	Create new partition
fdisk /dev/sdb t 8e	Change partition type to Linux LVM
mkfs.xfs /dev/myVG/myVol	Format LV with XFS
mkfs.ext4 -f /dev/myVG/myVol	Format LV with EXT4 (overwrite)
blkid /dev/myVG/myVol	Show UUID and formatting of volume
mount	Show what is mounted where
mount -t ext4 /dev/myVG/myVol /mountpoint	Mount LV to /mountpoint
umount /dev/myVG/myVol	Unmount LV from /mountpoint
umount /mountpoint	Unmount LV from /mountpoint
mount -a	Mount as configured in /etc/fstab
df	Show disk usage
xfs_growfs /dev/myVG/myVol	Resize xfs filesystem
resize2fs /dev/myVG/myVol	Resize ext3/4 filesystem

LVM

Item	Description
Physical Volume	Representation of physical volume
Volume Group	Group of physical volumes
Logical Volume	Volumes placed on volume group

Physical Volume Commands

Command	Description
pvs	Show PVs
pvdisplay	Show detailed PV information
pvcreate /dev/sdb1	Initialize physical volume for use with LVM
pvs -o+pv_used	Show usage of PVs
pvmove /dev/sdb1	Move contents of PV to other PVs of VG
pvremove /dev/sdb1	Remove PV

Volume Group Commands

Command	Description
vgs	Show VGs
vgdisplay	Show detailed VG information
vgcreate myVG /dev/sda1	Create new VG
vgextend myVG /dev/sdb1	Add PV to VG
vgreduce myVG /dev/sdb1	Remove PV from VG (pvmove first!)
vgremove myVG	Remove VG

Logical Volume Commands

Command	Description
lvs	Show LVs
lvdisplay	Show detailed LV information
lvcreate -n myVol -L 3G myVG	Create LV
lvextend -L +10G /dev/myVG/myVol -r	Expand LV by 10GB and resize filesystem
lvextend -L 10G /dev/myVG/myVol --resizefs	Expand LV to 10GB and resize filesystem
lvextend /dev/myVG/myVol /dev/sdb1	Expand LV by PV's size
lvextend -l +100%FREE /dev/myVG/myVol	Expand LV to 100% available VG size
lvreduce -L -10G /dev/myVG/myVol	Reduce LV by 10GB
lvremove /dev/myVG/myVol	Remove logical volume

Automatic mounting (/etc/fstab)

Example:

file system	mount point	type	option	dump	pass
/dev/sdb1	/myDir1	ext4	defaults	0	0
/dev/mapper/myVG-myVol	/myDir2	xfs	defaults	0	0
UUID=a924e3de-0414-4228-86bf-b65a4c8ff8ab	/myDir3	xfs	defaults	0	0
/dev/mapper/myVG-swap	swap	swap	defaults	0	0
192.168.1.1:/shared	/targetdir	nfs	async	0	0
//server/share	/targetdir	cifs	credentials=/root/cifs/.username	0	0

Column dump configures file system for backup with dump (program).

Column pass configures order of file system check during boot. 0 means no check.

Text Readers & Editors

Less

Command	Description
q	Quit
R	Refresh content
F	Auto scroll
g number	Go to line
m lowercaseLetter	Mark line
' lowercaseLetter	Go to mark
/pattern	Search forward
?pattern	Search backward
n	Next search result
N	Last search result
ESC u	Remove highlighting from search

VI

Editing

To leave editing mode press ESC.

Command	Description
i	insert before cursor
a	insert after cursor
A	insert at end of line
o	new line below
O	new line above
u	undo
.	repeat last command
yy	copy line
5yy	copy 5 lines
p	paste below
P	paste above
x	delete character
5x	delete 5 characters
dd	delete line
5dd	delete 5 lines
:10,20d	delete lines 10-20
d0	delete to line begin
d$	delete to line end

Navigation

Navigate as usual with arrow keys, home, end, pg up, pg dn.

Command	Description
5G	go to line 5
H	go to top of screen
M	go to middle of screen
L	go to end of screen
5w	move over 7 words
5b	move back 5 words

Other

Command	Description
/foo	search forward
?foo	search backwards
n	repeat search
:w	save
:q	close
:wq	save and close
:q!	close without saving
:!command	run bash command
:r foo	read file foo into this file

User and Group Management

UID

UID	Type
<1000	system account
>1000	user account

User Database

User info without passwords is stored in /etc/passwd.

username	PW	UID	GID	Kommentar	HOME	SHELL
hfict	x	1000	1000		/home/hfict	/bin/bash

Group Database

Group info with secondary group members are stored in /etc/group. Primary group members are identified by GID in user database.

groupname	PW	GID	Users
wheel	x	10	hfict,user2

Password Database

Hashed user passwords are stored in /etc/shadow. Password encryption is configured in /etc/login.defs.

username	PW	Last PW change	Minimum	Maximum	Warn	Inactive	Expire
hfict	[hash]	17803	0	99999	7

PW:

• [hash] Verschlüsseltest Passwort
• ![hash] Account Gesperrt
• !! oder * Account gesperrt, kein Passwort gesetzt

Commands

Command	Param	Beschreibung
id	username	ID und Gruppen eines Benutzers anzeigen
who		Angemeldete Benutzer anzeigen
last		Letzte Logins anzeigen
lastb		Letzte fehlgeschlagene Logins anzeigen
sudo	-u user command	Befehl mit Rechten von user ausführen (Default ist root)
	-i oder su -	Shell mit root Rechten
su		Shell als root (non-login shell)
	-	Shell als root (login shell)
	- user	Shell als user
useradd	-u 2101 -g primarygroup -c comment username	Benutzer anlegen (ohne -g wird neue Gruppe erstellt)
usermod	-G group1,group2	Sekundärgruppen definieren (überschreiben)
	-aG group,group2	Sekundärgruppen hinzufügen
	-l username	Benutzernamen ändern
	-L	Lock Account
	-U	Unlock Account
	-s shellpath	Shell ändern
userdel	-r username	Benutzer inkl. Home und Mail-Spool löschen
passwd	username	Passwort ändern (interaktiv)
groupadd	groupname	Gruppe anlegen (Optional mit -g GID setzen)
groupdel	groupname	Gruppe löschen

File System Permissions

Permissions can be set on:

• User (owner)
• Group (owner)
• Others

Only root can change User. User can change Group.

Basic permissions (Add binary flags to combine):

Char	Binary Flag	Permission
r	4	read
w	2	write
x	1	execute

Advanced permissions (place in front of basic permissions: chmod 1777 shared).:

Char	Binary Flag	Name	Description
t / T	1	Sticky Bit	Others can't delete content (only applicable for directories)
s / S	2	SGID-Bit	File: run with permissions of Group Dir: New elements inherit Group
s / S	4	SUID-Bit	File is run with permissions of User (only applicable for files)

Advanced permissions replace the x when using ls -l. Lower case if x is set, upper case if x is not set.

Read permission on a directory only allows to see the directory itself but not it's contents. Use execute permission to show contents.

Commands

Command	Param	Beschreibung
chmod	-R [uog] dirname	Set permissions recursively using binary flags
	+[suog] filename	Add permissions using binary flags
	-[suog] filename	Remove permissions using binary flags
	u+x filename	Add execute permission for User
	g+wx filename	Add write and execute permissions for Group
	o-r filename	Remove read permission for Others
chown	-R user:group filename	Change owner (User & Group) recursively
	user filename	Change owner (User)
	:group filename	Change owner (Group)
chgroup	group filename	Change owner (Group)

SSH

Configuration is done in /etc/ssh/sshd_config.

Reload SSH service with systemctl reload sshd to apply changes!

DenyUsers, AllowUsers, DenyGroups, AllowGroups override each other and are applied in the order listed above.

Config	Option	Description
PermitRootLogin	no	Deny root to login via SSH
	yes	Allow root to login via SSH
	without-password	Allow only with private/public key auth
AllowUsers	user1 user2	Allow only user1 and user2
DenyUsers	user1 user2	Allow all users but user1 and user2
AllowGroups	group1 group2	Allow only users from specified groups
DenyGroups	group1 group2	Allow all users but those in specified groups

IPv6 (disable)

Change the grub config /etc/default/grub to disalbe IPv6.

1. Add ipv6.disable=1 to the GRUB_CMDLINE_LINUX property. e.g. GRUB_CMDLINE_LINUX="ipv6.disalbe=1 crashkernel=auto rd.lvm.lv=centos/root rd.lvm.lv=centos/swap biosdevname=0 net.ifnames=0 rhgb quiet"
2. Recreate grub config with grub2-mkconfig -o /boot/grub2/grub.cfg
3. Reboot the system with init 6 or shutdown -r now

Cronjobs

Crontab

Cronjobs are configured in crontab files. Do not edit these files directly. Use crontab -e instead. This runs all required actions to activate a cronjob after saving the edited crontab. The locations are as follows:

• /var/spool/cron/username user specific
• /etc/crontab system wide crontab

The format of the files is (user specific crontabs do not have the column user-name):

Example of job definition:
.---------------- minute (0 - 59 | */5 [every 5 minutes])
|  .------------- hour (0 - 23)
|  |  .---------- day of month (1 - 31)
|  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
|  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
|  |  |  |  |
*  *  *  *  * user-name  command to be executed

Command	Description
rpm -q cronie	Check if package is installed
systemctl status crond.service	Check if service is running
crontab -l	List current users crontab
crontab -e	Edit current users crontab
crontab -e -u username	Edit specific users crontab
crontab -r	Remove current users crontab

Script folders

Scripts in one of the following directories will be executed at the intervall specified by the directory's name:

• /etc/cron.hourly
• /etc/cron.daily
• /etc/cron.weekly
• /etc/cron.monthly

Allow / Deny usage

Add user names one per line to the following files:

• /etc/cron.allow Whitelist
• /etc/cron.deny Blacklist

If none of the files exists, all users are allowed.

Logs and Results

Execution of cronjobs is logged in /var/log/cron. Results are sent to the users mail /var/spool/mail/username.

Package Management

RPM

Command	Description
`rpm -i rpmfile	rpmurl`
rpm -e packagename	Uninstall package
rpm -q packagename	Check if package is installed
rpm -ql packagename	List files in a package
rpm -qa	List all installed packages
rpm -qf /path/to/file	Get package that installed the file
rpm -qf $(which <exe>)	Get package that installed the executable
rpm -V packagename	Validate installed package

YUM

YUM is configured in /etc/yum.conf

Repos are configured in /etc/yum.repos.d/

Log is in /var/log/yum.log

Command	Description
yum install packagename [-y]	Install package (-y no confirm message)
yum remove packagename	Uninstall package
yum update	Update all installed packages
yum update packagename	Update specific package
yum update pattern*	Update packages using wildcard
yum info packagename	Get detailed info about package
yum list packagename	List installed and available packages
yum search searchstring	search for a package (name & summary)
yum search all searchstring	search for a package (all infos)
yum deplist packagename	List dependencies of a package
yum reinstall packagename	Reinstall (corrupted) package
yumdownloader --resolve packagename	Download rpm package with dependencies

Services (systemd)

Command	Description
systemctl list-dependencies	Show Services
systemctl start servicename	Start service
systemctl enable servicename	Start service with system
systemctl status servicename	Show service status

NFS

Installation (Client & Server)

yum install nfs-utils

Server Configuration is done in /etc/exports.

systemctl start nfs-server.service  # Start service
systemctl enable nfs-server.service # Start service at system start
vi /etc/exports                     # Change config
exportfs -fa                        # Reload config (server)

In /etc/exports create a new line for every directory you want to share in the following format:

<dir> <destination1>(<options>) [<destination2>(<options>)]

• <dir> Directory to export
• <destinationx> Client to allow access (by FQDN, hostname, ip, ip networks 192.168.0.0/28)
• <options> Comma delimited configuration options
  • rw Read/Write access
  • ro default Read-only access
  • async Aynchronous write (faster but can corrupt files)
  • root_squash default Anfragen von root werden anonymous zugewiesen
  • no_root_squash Anfragen von root werden root zugewiesen
  • all_squash Alle Anfragen werden anonymous zugewiesen

Examples:

/               master(rw) trusty(rw,no_root_squash)
/projects       proj*.local.domain(rw)
/usr            *.local.domain(ro)
/home/joe       pc001(rw,all_squash,anonuid=150,anongid=100)
/pub            *(ro,insecure,all_squash)
/foo            2001:db8:9:e54::/64(rw) 192.0.2.0/24(rw)
/build          buildhost[0-9].local.domain(rw)

Client

showmount -e 192.168.1.1                    # Show NFS shares  
mkdir /targetdir                            # Create mountpoint
mount -t nfs 192.168.1.1:/shared /targetdir # Mount manually   
umount /targetdir                           # Unmount manually 
vi /etc/fstab                               # Mount on system start
mount -a                                    # Test fstab
init 0                                      # Auto mount

Example of /etc/fstab:

192.168.1.1:/shared  /targetdir nfs  async  0 0

Samba

Server

Configuration is done in /etc/samba/smb.conf.

Local accounts must be registered with Samba using smbpasswd. Enter the same password as for the local account and add unix password sync = yes to /etc/samba/smb.conf. Then it should be synced automatically when you change it with passwd.

yum install samba samba-client samba-common # Samba installation
systemctl start smb                         # Start service
systemctl enable smb                        # Start service on system start
vi /etc/samba/smb.conf                      # Change config
testparm                                    # Check if config is ok
systemctl restart smb                       # Reload config
smbpasswd -a username                       # Register user with samba

Example of /etc/samba/smb.conf:

[global]
  workgroup = WORKGROUP             # workgroup of smb server
  netbios name = centos             # name of smb server
  security = user                   # security mode (user / ads / domain)
  unix password sync = yes          # sync unix password with smb password
  invalid user = root bin daemon    # deny access globally (config in share section overrides this)

[Transfer]                        # name of share
  path = /transfer                  # path of directory to be shared
  comment = File transfer           # description
  read only = no                    # access permissions
  guest ok = no                     # allow guest access (no password)
  browsable = yes                   # visible or hidden?
  valid users = username @groupname # allow user/group to access the share

The special [homes] section automatically shares the users home directory under the following conditions:

• A users tries to access a share with the name of his user account
• There is no explicit section for this share in smb.conf
• The user account exists in /etc/passwd

If the path is not defined in the section, the home path of the user in /etc/passwd is used. The browsable options has a different meaning in this section and specifies wheter the personal share should be listed. The usual browsable option is inherited from the [global] section. You can use variables to change the path for example:

• %U current user name
• %H current users home directory

Example:

[homes]
  browsable = no
  writable = yes
  path = %H/smb

Client

yum install samba-client cifs-utils                       # Install client and tools
smbclient -L //server                                     # Show SMB shares
mkdir /targetdir                                          # Create mountpoint
mount.cifs -o username=jenkins //server/james /mountpoint # Mount manually
umount.cifs /mountpoint                                   # Unmount manually
vi /etc/fstab                                             # Mount on system start
vi /root/cifs/.username                                   # Create credential file
mount -a                                                  # Test fstab
init 0                                                    # Auto mount

Example for /etc/fstab:

//server/share  /targetdir cifs  credentials=/root/cifs/.username  0 0

Example for /root/cifs/.username:

username=username
password=password

FTP

Configuration is done in /etc/vsftpd/vsftpd.conf

Example:

anonymous_enable=NO
local_enable=YES
write_enable=YES
local_umask=022
dirmessage_enable=YES
xferlog_enable=YES
connect_from_port_20=YES
xferlog_std_format=YES
listen=NO
listen_ipv6=YES
pam_service_name=vsftpd
tcp_wrappers=YES
use_localtime=YES
dirlist_enable=YES
pasv_enable=YES
pasv_min_port=41361
pasv_max_port=65534
pasv_address=192.168.1.10

yum install vsftpd ftp    # Install server and client
systemctl start vsftpd    # Start service
systemctl enable vsftpd   # Start service on system start
vi /etc/vsftp/vsftpd.conf # Change config
systemctl restart vsftpd  # Load modified config

MySQL Server

yum install mariadb-server # Install server
systemctl start mariadb    # Start service
systemctl enable mariadb   # Start service on system start
mysql_secure_installation  # Run security wizard
mysql -u root -p           # Connect to MySQL

Other

Stuff probably relevant for the exam.

Exam result file: /sbin/result

useradd username [-G wheel]             # add user and allow sudo
usermod -aG wheel username              # allow sudo for existing user
sudo -i                                 # enter interactive root session
yum install man-pages                   # install man pages
timedatectl set-timezone Europe/Zurich  # change time zone
timedatectl set-local-rtc 0             # set RTC to use UTC
yum check-update                        # check for updates
yum update                              # install all available updates
localectl                               # show locale configuration
localectl list-keymaps | grep ch        # show available keymaps
localectl set-keymap ch                 # set keymap to ch (doesn't affect ssh session)
localectl set-x11-keymap ch             # set keymap to ch (doesn't affect ssh session)

Print sorted descending without dublicates -> duplicate

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