Experimentalist

CLI: Handbook

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.

explainshellopen in new window demystify the complex CLI commands. cheat.shopen in new window and tldr.shopen in new window are also amazing resource for references.

Documentation

  • man - By Default the system documentation/manuals are documented in the man pages
# Usage:
man <COMMAND>
  • help - Lists all the internal commands of shell
# Usage:
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 )) # Arthematic 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 pipefile
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
# Usage:
ls -lrt > test.out
  • >> - STDOUT redirection to file, appends at destination
# Usage:
ls -lrt >> test.out
  • < - STDIN redirection from a file
# Usage:
wc < input_file.out
  • File Descriptors based redirection
# Usage:
# Redirect the STDERR to STDOUT
ls -lrt 2>&1
  • | - redirects STDOUT of one command into the STDIN of a second command
# Usage:
# Redirect the STDOUT to STDIN
ls -lrt | wc
  • 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
# Usage:
sleep 100 & # Run the command as a backgroud 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 fore-ground which has the JOB_ID
bg %<JOB_ID> # Resume the job in the back-ground which has the JOB_ID
ctrl+z # Pauses executing of the current running job
ctrl+c # terminates the current executing process

SecureShell - SSH

  • TCP/IP port 22
  • sshd - ssh server
    • package openssh-server
    • default sshd server config /etc/ssh/sshd_config
  • ssh - ssh client
    • package openssh-client
    • default ssh client config /etc/ssh/ssh_config or ~/ssh/config
# Usage:
# -v = verbose mode for debugging
# -4 = forces IPv4
# -6 = forces IPv6
# -i = specify the identity private keys
# -p = port

ssh user@HOST
ssh -i private_key_file user@HOST
ssh HOST # if the ssh keys are already copied in the server
  • Generating the SSH keys - ssh-keygen
# Usage:
# -t = type of key, dsa, ecdsa, ecdsa-sk, ed25519, ed25519-sk, rsa, rsa-sha2-512(DEFAULT)
# -b = Key length
# -f = Specify file to write to
# -C = comment
ssh-keygen -t rsa -b 4096
ssh-keygen -t ed25519
  • Securely copying public SSH keys - ssh-copy-id
# Usage:
# -i = Specify the ssh keys file
# -p = Port to login
ssh-copy-id -i ~/.ssh/id_rsa.pub user@HOST
  • scp & sftp

Other Linux Commands

ls - listing of files

# Usage:
# --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

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

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

head - starting content of the files

# Usage:
# -n = line numbers to be displayed in the head
# -c = bytes to be displayed from the file

head file.txt
head -n 10 file.txt
head -c 150 file.txt

tail - end of the content of files

# Usage:
# -f = real time updates to the files content appended to the end of file.
# -n = no of lines of tail to be displayed

tail file.txt
tail -f file.log

# Usage:
# -s = softlink/symlink

ln sourceFileForHardLink hardLinkFileNameWithPath
ln -s sourceFileForHardLink softLinkFileNameWithPath

chmod - change file permissions

  • User-Group-Others(--- --- ---)
  • Read-Write-Execute(rwx)
# Usage:
# -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

# Usage:
# -R = Recursive
# -V = Verbose

chown user:group directory

du - directory usage

# Usage:
# -h = human readable

du -h

df - disk filesystem

# Usage:
# -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 keeps 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)

grep - search for string

# Usage:
# --color = color for search results
# -r = recursive search
# -i = ignore case search
# -c = count the matching lines
# -E = search extended using regex
# -v = invert the search 
# -n = add the line numbers to the search results
# -w = exact search 
# -o = just show the matched strings, not the entire line
grep "test" test_file.txt
grep "key" *

find - find the files/directories etc

# Usage:
# -name = file the name
# -iname = file name search using ignorecase
# -type = type of search, d = directory, f = file
#         l = symbolic link, s = socket, b = block special
# -exec = command execute the command in searched files 
# -user = search the file with the specified user
# -size = size of files to be searched

find . -name somefile.txt
find /home -iname somefile.txt
find / -type f -perm 0777 -print -exec chmod 644 {} \;

ps - lists all the processes which are running.

# Usage:
# -A or -e = list all the process running for all users 
# -T = process 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

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

disown - delete/remove jobs so that it doesn't send SIGHUP signal

# Usage:
# -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

# Usage:
# -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

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

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

#Usage
# -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

#Usage
# -c = count freq 
# -d = print only the lines which are duplicated 
# -u = print all unique lines
# -i = check the frequency after ignoring the case

sort file2 | uniq -c
sort file2 | uniq -u
sort file2 | uniq -cd

cut - cut a field out

# Usage:
# -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

# Usage:
# -l = no of lines
# -w = no of words
# -m = no of characters

wc -l

tr - translate character

# Usage:
# -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

# Usage
# -v = verbose
# -r = recursive
# -z = compress during transfer
# -h = human-redable
# -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

# Usage
# -o = save data to a file
# -L = redirect follow
# -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

# Usage
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
Last Updated:
Contributors: Jubin Mathew