Basics

This cheat sheet-style guide provides a quick reference to commands and practices commonly used when working Command Line (CLI). This cheat sheet mostly uses bash or zsh.

explainshell demystifies the complex CLI commands. cheat.sh and tldr.sh are also amazing resources for references.

Documentation

• man ‑ By default the system documentation/manuals are documented in the man pages

man <COMMAND>

• help ‑ Lists all the internal commands of shell

help

Bash structure

• ~/.bashrc ‑ configure the shell, define command aliases and set command shell options
• ~/.bash_profile ‑ initialization commands that set environment variables, a shell’s prompt
• ~/.bash_logout ‑ cleanup operations and other commands that you want the shell to execute whenever a user logs out of a shell
• ~/.bash_aliases ‑ if storing many aliases commands

Shell Script

set -x # Enable RAW i/o
set -v # Enable RAW i/o
set -e # abort on error(non‑zero exit code)
set -u # detect unset variables

variable=${VARIABLE_NAME:-default} # default value if variable empty
$(( (1+1)/5 )) # Arithmetic operation
${1..10} # Sequence
${var%suffix} # trim a suffix named 'suffix'
${var#prefix} # trim a prefix named 'prefix'
${foo,bar} # expand into multiple values

diff /etc/hosts <(ssh somehost cat /etc/hosts) # parenthesis to execute a sub‑process
{..} # Include code in between this block, as a good practice
cat <<EOF
touch somefile
echo foo > somefile
EOF

Bash Script strict mode

#!/bin/bash
set -euo pipefail
IFS=$'\n\t'

Redirection

File Descriptors ‑ every file has an associated number called FD. Default ones are:

• 0 ‑ STDIN
• 1 ‑ STDOUT
• 2 ‑ STDERR

Types of redirection

• > ‑ STDOUT redirection to file, overwrites at destination

ls -lrt > test.out

• >> ‑ STDOUT redirection to file, appends at destination

ls -lrt >> test.out

• < ‑ STDIN redirection from a file

wc < input_file.out

• File Descriptors based redirection

ls -lrt 2>&1

• | ‑ redirects STDOUT of one command into the STDIN of a second command

ls -lrt | wc

Navigation

• Ctrl + a ‑ Go to beginning
• Ctrl + e ‑ Go to end
• Alt + b ‑ Go back one word
• Alt + f ‑ Go forward one word
• Ctrl + w ‑ Delete a word backward
• Alt + d ‑ Delete next word
• Ctrl + u ‑ Delete to beginning of line OR delete line
• Ctrl + k ‑ Delete to end
• Ctrl + r ‑ Search backwards (recursive)
• Ctrl + s ‑ Search forwards
• Ctrl + l ‑ Clears the screen
• Alt + t ‑ Transpose the last 2 words/characters

Job Management

• & ‑ background process.
• Ctrl+z ‑ pauses the process temporarily, and places it in SUSPENDED mode.
• fg ‑ used to bring a background / paused process back to the foreground and continue its execution.
• bg ‑ resume any SUSPENDED jobs and run them in background
• jobs ‑ check the job status of all the jobs
• kill ‑ used for killing a process

Status: SUSPENDED, CONTINUED, RUNNING, STOPPED

sleep 100 &            # Run the command as a background process
jobs                   # view all the jobs
jobs -l                # view all the jobs along with their job ID & process PID
fg %<JOB_ID>           # Resume the job in foreground which has the JOB_ID
bg %<JOB_ID>           # Resume the job in the background which has the JOB_ID
# while a job is running:
Ctrl+z                 # Pauses executing of the current running job
Ctrl+c                 # terminates the current executing process

Other Linux Commands

ls – listing of files

# --color = enabling coloured output
# -l = long listing
# -h = human readable Listing of data
# -t = sort by time
# -R = Recursive listing of directories

# Must use aliases – `.bash_aliases`
alias ls='ls -A -F -B --human --color'
alias ll='ls -l'
alias la='ls -A'
alias lh='ls -h'

ls -lrt

less – terminal pager

# -N = Line number enabled
# +F = Real Time Monitoring

sudo dmesg | less +F
cat test.txt | less

head – starting content of the files

# -n = number of lines to display from the start
# -c = number of bytes to be displayed
head file.txt
head -n 10 file.txt
head -c 150 file.txt

tail – end of the content of files

# -f = real time updates as more content is appended to the end of file.
# -n = number of lines of tail to be displayed
tail file.txt
tail -f file.log

ln – soft/hard link

# -s = softlink/symlink
ln sourceFileForHardLink hardLinkFileNameWithPath
ln -s sourceFileForHardLink softLinkFileNameWithPath

chmod – change file permissions

• User‑Group‑Others (--- --- ---)
• Read‑Write‑Execute (rwx)

# -R = Recursive
# -V = Verbose
chmod +x file.txt
chmod 600 file.txt
chmod u-w file.txt
chmod u+x file.txt

chown – change the ownership of files

# -R = Recursive
# -V = Verbose
chown user:group directory

du – directory usage

# -h = human readable
du -h

df – disk filesystem

# -h = human readable
# -i = inode listing
# --total = total summary
# --output = specify the output strategy
df -h /proc
df -h --output=source,avail,pcent,target

inode – index number

• Data structures that keep track of all the files in the filesystem
• all unique inode in a given filesystem
• stores metadata (type, size, group, user, permission, access, change & modification time)

ps – lists all the processes which are running.

# -A or -e = list all the processes running for all users
# -T = processes running with the current terminal(tty)
# -f = full listing for the process
# -u = filter process for a specific user
# -L = List threads for a process PID
# -G = List process associated with a group
# -C = search process by name as PID is not known
# -o = specifies the format for the columns

ps -ef
ps aux
ps -L <PID>

nohup – run process in the background, even when session is logged out, “no hangup”: prevents from receiving SIGHUP signals

nohup ./example.sh > output.log 2>&1 &
nohup python example.py &

disown – delete/remove jobs so that it doesn’t send SIGHUP signal

# -a = removes all the jobs
# -h = marks job so that it DOESN’T send SIGHUP when shell receives a SIGHUP signal
# -r = deletes running job
disown -r
disown -h %2
disown -a

kill – kills the running process

# -l = lists all the SIG for kill
#   1 = HUP – reload a process
#   9 = KILL – Force kill a process
#   15 = TERM – Graceful kill of process
kill PID_NUMBER
kill -9 PID

fzf – CLI fuzzy finder

rg – ripgrep ‑ recursive grep

pandoc – Convert documents

pandoc README.md --from markdown --to docx -o tmp.docx

sort – alphabetical (lowercase < uppercase) / increasing sorting by default

# -r = reverse order
# -n = recognise numerical results
# -k = sort based on specific column
# -u = remove duplicate entries
# -c = check if the file is in sorted format
# -M = arrange based on months
# -s = stable sort

sort -nr numeric.txt
ls -l | sort -nk 5
sort -c test.txt

uniq – remove duplicate entries, must be in sorted format

# -c = count frequency
# -d = print only the lines which are duplicated
# -u = print all unique lines
# -i = ignore case
sort file2 | uniq -c
sort file2 | uniq -u
sort file2 | uniq -cd

cut – cut a field out

# -f = field to cut
# -d = delimiter to use for cutting
cut -d: -f1,6 /etc/passwd
ls -lrt | cut -d ' ' -f 1

wc – word count

# -l = number of lines
# -w = number of words
# -m = number of characters
wc -l

tr – translate character

# -C = Complement character
# -d = delete character
tr A‑Z a‑z
tr -d 'e'

sed – substitution

sed s/Amazing/super/gi textfile.txt
sed s/originalword/replaceword/g filename
sed -n '3,5p' textfile.txt

rsync – remote sync

# -v = verbose
# -r = recursive
# -z = compress during transfer
# -h = human‑readable
# -P or --progress = shows real‑time progress
rsync -avzhe ssh --progress /root/pkgs root@127.0.0.1:/root/pkgs
rsync -zvh backup.tar.gz /tmp/backups/

curl – request URLs

# -o = save data to a file
# -L = follow redirect
# -C = resume an interrupted download
# -u = authentication
# -v = verbose
# --headers = specify request headers
# -d or --data = POST request body
# -X = specify the request type
# -k = disable certificate checking
curl -o logo.png https://reqbin.com/static/img/logo.png
curl --header 'Host: targetapplication.com' https://192.0.0.1:8080/
curl -X POST https://httpbin.org/post

Operations with compressed files – zless, zmore, zcat, zgrep

nc & telnet & netstat

nc -lv 1234         # start a listener on port 1234
nc -zv google.com 443 # check if port is open
nc -zv 10.0.2.4 1230‑1235 # scanning a port range
telnet host.com 1234 # connect to the port in the HOST
netstat -an | grep LISTEN