Cheatsheet Linux and Bash

Summary: Linux Management & Bash hints, tips, oneliners and best practices.
Date: 8 December 2024

See release

cat /etc/os-release
cat /etc/SuSE-release

See kernel version

# Kernel version
uname -r
# Full version
uname -a

This is the runlevel config as defined on SuSE:

• 0 System halt
• 1 Single user mode
• 2 Local multiuser mode zonder netwerk
• 3 Full multiuser mode met netwerk
• 4 Niet gebruikt
• 5 Full multiser met netwerk en GUI
• 6 System Reboot

The default runlevel is set in /etc/inittab.

When the box is already running you can issue the command

"init x"

while replacing the x for the appropriate runlevel number to change the runlevel. If you want to change the runlevel while booting issue the same command as a boot parameter.

• .profile
  • Set variables (must be exported), aliases and configure the shell
  • ENV: special variable, used to set a file that will be executed on starting the shell (usually .bashrc en .kshrc )
    • Defines extra variabels and aliases.
• .dtprofile
  • Set GUI, DTSOURCEPROFILE must be TRUE to also load .profile
• .exrc
  • Loaded on starting vi
    • Allows for stuff like “set number” and “set showmode”
• .hushlogin
  • Add the empty .hushlogin in your HOME to discard the MOTD (Message Of The Day) when logging in.

Check who is logged in:

who

Check who you are with session information:

who am i

Check your own userid and group information:

whoami
id

Check the user information of a specific user:

id userid
finger userid

Check the last login of a user:

last
> Show
last
# Show the last 20 logins
last -20
# Show the last logins of a specific user
last userid

• Turn off syntax :syntax off
• Show line numbers :set number
• Show mode (input or command) :set showmode

• Insert text before the cursor i
• Append text after the cursor a
• Insert text at the beginning of the line I
• Append text at the end of the line A
• Replace character r
• Replace text R
• Delete character x
• Delete 5 characters 5x
• Delete current line dd
• Delete the next word dw
• Delete 10 lines 10dd
• Delete lines 2 to 4 2,4dd
• Delete all below lines, including current: dG
• Copy 10 lines 10yy
• Paste the copied lines p
• Undo last change u
• New line below the current line and insert o
• New line above the current line and insert O

• Search for a string downwards /"string"
• Search for a string upwards ?"string"
• Search and replace in the complete document :%s/stringold/stringnew/
• Search and replace in the complete document, confirm each change :%s/stringold/stringnew/gc

• Go to the next word w
• Go to the previous word b
• Go to the end of the line $
• Go to the beginning of the line 0
• Go to the last line of the file G
• Go to line 1 1G
• Go to line 206 206G

• Past text from windows clipboard through putty: SHIFT + INS

Create csr with new private key (preferred)

openssl req -nodes -newkey rsa:2048 -keyout *.shiftwiki.nl.key -out *.shiftwiki.nl.key.csr

Create csr with existing key

openssl req -new -key /data/keys/httpd-wild.shiftwiki.nl/oeserver.pem  -out *.shiftwiki.nl.key.csr

View certificate

openssl x509 -text -noout -in certificate.crt

View csr

openssl req -text -in request.csr

Truncate a logfile that has grown too big

sudo truncate -s 0 odoo.log

Show and follow a logfile

tail -f "logfile"

Show and follow a logfile with a specific number of lines

tail -f -n 100 "logfile"

Show and follow a logfile starting from a specific line

tail -f +100 "logfile"

Show and follow a logfile starting from a specific line counted from the end

tail -f -100 "logfile"

Show the first 8 lines from a file

head -8 "logfile"

Kernel log file: /var/log/messages

dmesg

See link between device and filesystem for AWS EBS disks. From the instance, go to Storage tab and see the device name, then use these commands:

[sjoerd@server ~]$  df -hT /dev/sdf
Filesystem     Type  Size  Used Avail Use% Mounted on
/dev/nvme1n1   xfs    20G   14G  6.8G  67% /var/lib/docker
[sjoerd@server ~]$  df -hT /dev/sdg
Filesystem     Type  Size  Used Avail Use% Mounted on
/dev/nvme2n1   xfs    20G   15G  5.8G  72% /data

Understanding UNIX / Linux filesystem Inodes The inode (index node) is a fundamental concept in the Linux filesystem. Each object in the filesystem is represented by an inode. But what are the objects? Let us try to understand it in simple words. Each and every file under Linux (and UNIX) has following attributes:

• File type (executable, block special etc)
• Permissions (read, write etc)
• Owner
• Group
• File Size
• File access, change and modification time (remember UNIX or Linux never stores file creation time, this is favorite question asked in UNIX/Linux sys admin job interview)
• File deletion time
• Number of links (soft/hard)
• Extended attribute such as append only or no one can delete file including root user (immutability)
• Access Control List (ACLs)

All the above information stored in an inode. In short the inode identifies the file and its attributes (as above). Each inode is identified by a unique inode number within the file system. Inode is also know as index number.

How do I see file inode number?

$ ls -i /etc/passwd

You can also use stat command to find out inode number and its attribute:

$ stat /etc/passwd

Find and remove file using find command (necessary with special characters), type the command as follows:

$ find . -inum 782263 -exec rm -i {} \;

Additional info: A directory is nothing more than a table of the underlying files and a pointer to the inode information of this file. To “jump” to this inode table you need execute rights on the direcory. So if you want to open a file you'll also need execute rights on the directory.

Find the MD5 hash of files on Linux by entering the command below and replace the “Your-File-Here” with the name of the file:

echo -n "Your-File-Here" | md5sum

Show the hostname

hostname
hostname -s

Use scp to securely copy files between hosts

scp "source" "target"
scp host:/home/file /targetfile
# Copy a directory
scp -r host:/home/dir /targetdir

Check all open ports

netstat -a
# -n only show port number
# -r Route tabel
# -p Including processes
# Search for open port 389 with the corresponding process
netstat -nap | grep :389
# Show the routing table
netstat -rn

Using top to get a list of running processes and their resource usage

top
# c Expand processes
# 1 separate all processes
# s set refresh rate
# M sort on memory usage
# P sort on processor usage

Using ps to get a list of running processes

# all running processes
ps -e
# all running processes with full info excluding kernel processes
ps -ef
# all running processes with full info
ps -Af
# all running processes for a specific user
ps -u "userid"
# Start a process in the background and keep it running after the session is closed
nohup command &

grep file without empty lines and comments

grep -v '^\s*$\|^\s*\#' postgresql.conf

grep options

# -i : case insensitive
# -v : invert match
# -w : match whole word
# -r : recursive
# -l : only filenames
# -n : show line numbers
# -c : count matches
# Search for test in all files in the current directory and subdirectories and only show the filenames
grep -rlw test .

Find the Linenumber of a Specific Match

grep -n <match> <file>

Find Specific String in Files - Recursive

grep -r '172.18' *

Find is always recursive

# find file in current and subdirectories
sudo find . -name main.cf
# find all files starting with a and execute ls -l on them
find . -name 'a*' -exec ls -l {} \;
# find all files starting with a and execute ls -l on them, but ask for confirmation
find . -name 'a*' -ok ls -l {} \;
# Set read permissions on other for all files in /var/news
find /var/news -type f -exec chmod o+r {} \;
# Rename all files in /tmp to filename.old
find /tmp -type f -exec mv {} {}.old \;
# Find only files older than 31 days and remove them
find /tmp/archive/. -type f -mtime +31 -exec rm {} \;

# Check disk usage
df -H
# Find largest directories
sudo du -Sh / | sort -rh | head -5
# Find largest files in /data
sudo find /data -type f -exec du -Sh {} + | sort -rh | head -n 10
# Find largest files in /var/lib/docker
sudo find /var/lib/docker -type f -exec du -Sh {} + | sort -rh | head -n 10
# Find largest files except in /data
sudo find / -type f -not -path "/data/*" -exec du -Sh {} + | sort -rh | head -n 10

display all crontabs for all users as root sudo su -

for user in $(cut -f1 -d: /etc/passwd); do echo $user; crontab -u $user -l; done

Note this is especially useful in pipelines

set

# -e : exit the script if any command fails
# -u : treat unset variables as errors and exit
# -o pipefail : ensure the entire pipeline fails if any command within it fails
# -x : print all the commands to the terminal
# -v : print all lines without variable resolution
# - For all, to disable use +, for example `set +v`
set -euo pipefail

Long listing of files

ls -l

List files ordered by change date

ls -t

Reverse file listing order

ls -r

List files including hidden files (files starting with a dot)

ls -a

Count files in a directory

# wc: word count; -l: count lines
ls -l | wc -l

Compare two files

diff file file2

Compare two files next to each other, only differences

diff -y --suppress-common-lines file1 file2

Show file type

file "filename"

Show file type, inode number and all three dates (modify, access, change)

stat "filename"

List files with a, b or c in the name

ls [abc]

List files without a, b or c in the name

ls [!abc]

List files that start with a, b, c or d

ls [a-d]*

List files that start with a or d

ls [ad]*

List files that start with a or d and have a or e as second letter

ls [ad][ae]*

List files that start with a, A, b or B

ls [a-bA-B]*

List files that start with a or b and have at least one more character

ls [ab]?*

List files that don't end with a letter

ls *[!a-zA-Z]

List disk usage

# show diskspace of all directories 1 level deep in human readable format
du --max-depth=1 -h -c
# do 'du' for every existing directory
## ls -1 lists all files and directories each on a separate line
## [[ -d ]] is true if listed variable is a directory
for i in `ls -1`; do if [[ -d $i ]]; then sudo du -sm $i; fi; done

for i in `ls -1 | grep was-`; do if [ -f $i/.run ]; then echo start slot $i; fi; done

• ls -1 lists all files and directories each on a separate line
• [ -f ] is true if listed file is a regular file

# Standard variables
# Prompt
PS1
# Home
HOME
# Path
PATH
# User
USER
# Add text directly after a variable (useful in scripts)
echo ${var}tekst
# Assign output of command to variable
x=$(command)
# Create a readonly variable
readonly var=waarde
# Remove / unset a variable
unset var
# Export a variable so it is also available in subshells
export var

.       Any single character
*       0,1 or more of any character
.*      0,1 or more of any character
[aA]    a or A
[a-f]   a to and including f
^a      Starts with a
a$      Ends with a
^a.*b$  Everything starting with a and ending with b
^$      Empty line

Combined with grep you can use regular expressions to filter out comments and empty lines out of config files. However, this should take you two regular expressions: grep -v '^&' file | grep -v '^#'. Using egrep allows you to do that in one go: egrep -v '^$|^#' file

# Run command in the background (output and errors still go to the screen)
& 'command'
# Input redirection
< inputfile
# Output redirection
> outputfile
# Error redirection
2> errorfile
# Combine output and errors for redirection to a command or a file
2>&1
# Output redirection append
>> outputfile
# reads the input, then writes the output of a program to standard output and simultaneously copies it into the specified file or files, with example: The output of command is written to file1 and also used as input for command2:
command | tee file1 | command2

Basic sed search and replace line:

sed "s/$FIND/$REPLACE/g" $inputfile > /tmp/outputfile

Code line to make email adresses unusable by adding the extension .local to the complete email address:

sed "s/^mail.*$/&\.local/g" inputfile > outputfile

* s : substitute

• ^mail : all lines that start with mail
• .*$ : and match everything after the initial match
• & : everything that has been matched
• \.local : Add .local (\ is for escaping the '.')
• g : global

There are three ways to run scripts:

• bash script
  • Script runs in subshell and read permissions are sufficient
• (./)script
  • Script runs in subshell and execute permissions are required
• . (./)script
  • Script runs in current shell and read permissions are sufficient (used to export variables)

• $$ = The PID number of the process executing the shell.
• $? = Exit status variable.
• $0 = The name of the command you used to call a program.
• $1 = The first argument on the command line.
• $2 = The second argument on the command line.
• $n = The nth argument on the command line.
• $* = All the arguments on the command line.
• $# = The number of command line arguments.
• $@ = All arguments

The “shift” command can be used to shift command line arguments to the left, ie $1 becomes the value of $2, $3 shifts into $2, etc. The command, “shift 2” will shift 2 places meaning the new value of $1 will be the old value of $3 and so forth.

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