Useful Linux commands

This is a list of useful Linux commands. Most of them are basic and they can be used as a reference for developers.

files and directories

• ls => list all files
• ls -a => list all files along with hidden ones
• ls -R => list all files and folders recursively

• ls -l => list in long format (shows more info)

• rm {filename} => deletes a file

• mv {filename} {directory path} => moves a file

• mv {filename} {filename2} => renames a file

• cat {filename} => read contents of a file

• cat > {filename} => writes to file

• touch {filename} => creates a file

• mkdir {directory name} => makes a directory

• rmdir {directory name} => removes a directory

• rm -rf {directory name} => removes a directory recursively along with the files

• mv {dirname1} {path or dirname} => moves or renames directory

• history => list all commands used in the current session

• clear => clears the terminal screen

file permissions

File permissions will be provided to users, groups and others (in the same order). They can be provided as numbers or letters.

r - read, w - write, x - execute
u - user, g - group, o - other, a - all

0 - no permission
1 - execute
2 - write
3 - execute + write
4 - read
5 - read + execute
6 - read + write
7 - all

• chmod {number} {filename} => gives permissions to user,group and others.

Eg: chmod 777 ex.txt => gives all permissions to all users, groups and others.

'+' - gives permission
'-' - removes permission
'=' - assigns the given permission

Eg: chmod u=rw ex.txt => gives read/write to user
Eg: chmod a= ex.txt => removes all permission to all users.

• chgrp {group name} {filename} => changes group for a file
• chown {user} {filename} => changes owner of a file

input/ output directions

• '>' write (stdout)
  Eg: ls -al > {filename} => writes the output of "ls -al" to the file. If the file already exists, it is overwritten.

• '>>' append (stdout)
  Eg: echo "hi" >> {filename} => appends to the file

• '<' input

File descriptors : 0 - stdin, 1 - stdout, 2 - stderr

Error redirection:

This is used to redirect errors to a log file, rather than cluttering the terminal.

Eg: cat ex.txt 2> error.log => tries to read the file, if there is any error, it is written to error.log

Here, '2' is the file descriptor for stderr.

pg, more and less

Show large text in the terminal as scrollable chunks by piping it to the pg, more or less commands.

Eg: cat large_file.txt | less => gives a scrollable viewer.

grep

Search for a string in the output by piping to grep.

Eg: ls | grep key => shows files that have a string "key".

Flags:

• -i => case insensitive
• -c => displays only count
• -n => displays matching line and number
• -v => displays output that didn't match

Eg: ls | grep -cv key => displays count to output that didn't match

• grep can also accept regular expressions. Refer here

sort

Sort the contents

Eg: ls | sort => displays in alphabetic order

Flags:

• -r => reverse
• -n => numerical
• -f => case insensitive

Eg: ls -l | sort -nk2 => displays content numerically sorted on the second column. (number of hard links in this case)

variables

• VARIABLE=value => Set variables in the session.
• $VARIABLE => gets the value of a variable.
• unset VARIABLE => unsets the variable.

networking commands

• ping {ip} => checks the connection
• ssh -i {key} {username@ip} => connects to a remote computer securely. key is the private key file.
• scp -i {key.pem} {source} {dest} => copies a file. remote location should be host:file.
• lsof -i {protocol:port} => list all processes listening on a port.

Eg: lsof -i tcp:3000 lists all processes listening to port 3000

process management

• top => lists all running process
• kill {pid} => kills a process with the given process id

mount device

• lsblk => list all block devices
• mount {device id} {directory} => mount a device to a directory

Eg: mount /dev/sda4 /ex/mnt

Flags:

• -a => mounts all devices in /etc/fstab
• -l => lists all mounted devices

• -t => type of filesystem (eg: ext4)

• umount {device id} => unmounts a device

manual

• man {command name} => shows usage of a command

cover image by Sai Kiran Anagani