laxman

Posted on Nov 15, 2020 • Edited on Nov 17, 2020

Useful Linux commands

#linux #devops

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 {srcFileName} {destFileName} => renames a file
cp {srcFileName} {destFileName} => copies file
cp -R {srcDir} {destDir} => copies directory
cat {filename} => read contents of a file
cat > {filename} => writes to file
touch {filename} => creates a file, touch is basically used to change file timestamps. Refer here
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
pwd => displays the path of the current working directory.

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.

working with pipes

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
dig {hostname} => performs a dns lookup
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

Top comments (7)

Jon Randy 🎖️ • Nov 16 '20 • Edited

touch {filename} => creates a file

Not really - touch is a command used to update the access date and/or modification date of a computer file or folder. The fact that it creates the file if it doesn't exist is purely a side effect. There are many ways to create a new file