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Prasad
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Useful Linux commands

#linux

This post has been originally posted on my blog site here

In this post, we are going to take a look on some essential and useful linux / unix commands that are being used for day to day operations

Commands for directory operations

ls

This command is use to list content in folder

ls [options] [directory or path]
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example , if I am inside /etc/ folder, and if I execute command then I can see all folders inside /etc/ folder as below

root@Prabhu:/etc# ls PackageKit cron.daily fstab kernel mime.types polkit-1 selinux sysctl.d X11 cron.hourly fuse.conf landscape mke2fs.conf profile sensors.d systemd adduser.conf cron.monthly gai.conf ld.so.cache modprobe.d profile.d sensors3.conf terminfo alternatives cron.weekly glvnd ld.so.conf modules protocols services timezone apparmor cron.yearly gnutls ld.so.conf.d modules-load.d python3 sgml tmpfiles.d apparmor.d crontab gprofng.rc ldap mtab python3.12 shadow ubuntu-advantage
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option : -la

ls command have many options, some useful options are as below

ls -la
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where

-l : use a long listing format
-a, --all: do not ignore entries starting with .

Example:

root@Prabhu:/etc# ls -la total 808 drwxr-xr-x 88 root root 4096 Jun 11 22:51 . drwxr-xr-x 22 root root 4096 Jun 11 22:46 .. -rw------- 1 root root 0 Sep 27 2024 .pwd.lock -rw-r--r-- 1 root root 837 Sep 27 2024 .resolv.conf.systemd-resolved.bak -rw-r--r-- 1 root root 208 Sep 27 2024 .updated drwxr-xr-x 2 root root 4096 Sep 27 2024 PackageKit drwxr-xr-x 7 root root 4096 Sep 27 2024 X11 -rw-r--r-- 1 root root 3444 Jul 5 2023 adduser.conf drwxr-xr-x 2 root root 4096 Sep 27 2024 alternatives
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option : -lt

if you want to sort the list content by time then use -lt option

ls -lt
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where
-t: sort by time, newest first; see --time

Example:

root@Prabhu:/etc# ls -lt total 792 -rw-r--r-- 1 root root 17607 Jun 11 22:51 ld.so.cache drwxr-xr-x 4 root root 4096 Jun 11 22:51 ssl drwxr-xr-x 2 root root 4096 Jun 11 22:51 gnutls drwxr-xr-x 2 root root 4096 Jun 11 22:50 init.d
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option: -ltr

if you want to sort the list content by time in reverse order then use -ltr option

ls -ltr
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where

Example:

root@Prabhu:/etc# ls -ltr total 792 -rw-r--r-- 1 root root 45 Jan 24 2020 bash_completion -rw-r--r-- 1 root root 12813 Mar 27 2021 services -rw-r--r-- 1 root root 34 Aug 2 2022 ld.so.conf -rw-r--r-- 1 root root 367 Aug 2 2022 bindresvport.blacklist -rw-r--r-- 1 root root 552 Oct 13 2022 pam.conf -rw-r--r-- 1 root root 3144 Oct 17 2022 protocols
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where:
-r, --reverse: reverse order while sorting

There are many other options, but I use above options in my day to day life

pwd

This command is used to print full path of current directory

pwd [options]
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Example

root@Prabhu:/etc# pwd /etc
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It has several options as below
-L, --logical: use PWD from environment, even if it contains symlinks
-P, --physical: avoid all symlinks

cd

This command is used for navigation

cd [directory path where you want to go]
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example, if I am inside /etc/ folder and I want to go to pm folder inside same directory then I will use following command

root@Prabhu:/etc# pwd /etc root@Prabhu:/etc# cd pm root@Prabhu:/etc/pm#
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if you want to go to some other folder of specific path then we can pass full directory path as below

root@Prabhu:/etc/pm# pwd /etc/pm root@Prabhu:/etc/pm# cd /var/run/log root@Prabhu:/var/run/log#
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cd command also provides some shortcuts as below

cd : returns to current user's home directory

cd .. : go to one level up

cd - : go to previous directory

mkdir

This command is used to create new directory

mkdir dir_name
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Example

root@Prabhu:~/prasad# mkdir testing root@Prabhu:~/prasad# ls testing root@Prabhu:~/prasad#
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rmdir

This command is used to remove empty directory

rmdir dir_name
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Example

root@Prabhu:~/prasad# ls testing root@Prabhu:~/prasad# rmdir testing root@Prabhu:~/prasad# ls root@Prabhu:~/prasad#
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Note: this command can only delete empty folders, if you have any file / folder inside this folder then this command cannot be used.

Commands for file operations

touch

This command is used to create new empty file in folder or to update timestamp of existing file

touch [option] file_name.extension
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Example:

root@Prabhu:~/prasad# ls root@Prabhu:~/prasad# root@Prabhu:~/prasad# touch test.txt root@Prabhu:~/prasad# ls test.txt root@Prabhu:~/prasad#
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cp

This command is used to copy files and folders from one location to another location

cp [options] source destination
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Example

root@Prabhu:~# cp prasad/test.txt shell/ root@Prabhu:~# cd shell/ root@Prabhu:~/shell# ls -ltr total 32 -rwxr-xr-x 1 root root 16 Jun 11 23:33 test.txt root@Prabhu:~/shell#
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If you want to copy whole directory including its files and sub folders then need to use option -r along with this.

Example: if I want to copy folder /etc/ssh along with its sub folders and files then execute commands as below

root@Prabhu:~# root@Prabhu:~# cp -r /etc/ssh/ prasad root@Prabhu:~# cd prasad root@Prabhu:~/prasad# ls -ltr total 8 -rwxrwxrwx 1 root root 16 Jun 11 23:31 test.txt drwxr-xr-x 3 root root 4096 Jun 11 23:38 ssh root@Prabhu:~/prasad# cd ssh root@Prabhu:~/prasad/ssh# ls -ltr total 8 drwxr-xr-x 2 root root 4096 Jun 11 23:38 ssh_config.d -rw-r--r-- 1 root root 1649 Jun 11 23:38 ssh_config root@Prabhu:~/prasad/ssh#
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mv

This command is used to move the file or folder from one location to another or to rename the file

mv source destination
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Example
if you want to move file from one folder to another

root@Prabhu:~# mv prasad/test.txt shell/test.txt root@Prabhu:~# cd shell root@Prabhu:~/shell# ls -ltr total 12 -rwxrwxrwx 1 root root 16 Jun 11 23:31 test.txt root@Prabhu:~/shell#
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if you want to rename the file, then keep destnation same but change filename while execute mv command

root@Prabhu:~/shell# mv test.txt test2.txt root@Prabhu:~/shell# ls -ltr total 12 -rwxrwxrwx 1 root root 16 Jun 11 23:31 test2.txt
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cat

This command is used to print the content of file

cat [options] filename
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Example, if you want to see content in file

root@Prabhu:~/shell# cat file.txt this is prasad root@Prabhu:~/shell#
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Example, if you want to read the content of one file and put it into another file as below

root@Prabhu:~/shell# cat test2.txt > file.txt root@Prabhu:~/shell# less file.txt this is prasad file.txt (END)
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grep (Global regualar expression print)

This command is very helpful if you want to search specific word from files

grep [options] pattern [filename]
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Example: to search keyword in any specific file

root@Prabhu:~/shell# grep "prasad" file.txt this is prasad
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if you want to display line number in search result

root@Prabhu:~/shell# grep -n "prasad" file.txt 1: this is prasad root@Prabhu:~/shell#
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if you want to display count of number matches your pattern then use command sa below

root@Prabhu:~/shell# grep -c "prasad" file.txt 1 root@Prabhu:~/shell#
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if you want to check if any specific keyword exists in which all files in folder

root@Prabhu:~/shell# grep -l "prasad" * exit.sh file.txt returnCode.sh test2.txt
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find

This command is used to search for file and directories based on name, type, size, data etc

find [path] -options [expression]
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where
Path : where to start for sarch
Expression: Criteria like filename, size etc
Example, if you want to search if file with specific name in folder "shell"

root@Prabhu:~# find shell -name "file.txt" shell/file.txt root@Prabhu:~#
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if you want to search by type/ extension

root@Prabhu:~# find shell -name *.txt shell/file.txt shell/test2.txt root@Prabhu:~#
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if you want to search all files with specific permissions

root@Prabhu:~# find shell -perm 777 shell/for.sh shell/functions.sh shell/if.sh shell/returnCode.sh shell/test2.txt shell/exit.sh root@Prabhu:~#
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use grep and find together to search for specific keyword in file

root@Prabhu:~# find ./ -type f -name "*.txt" -exec grep "prasad" {} \; this is prasad root@Prabhu:~#
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awk

This command is used for pattern scanning and processing

awk options 'pattern {action}' input_file > output_file
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Example
Print all lines in file

root@Prabhu:~/shell# awk '{print}' file.txt this is prasad root@Prabhu:~/shell#
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print specific column

root@Prabhu:~/prasad# awk '{print $1,$4}' sample.txt What Ipsum? Lorem simply Why use It long Where come Contrary belief, The of Where get There variations root@Prabhu:~/prasad#
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sed

This command used to replace content in the file

sed 's/old_content/new_content/' file_name
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Example, let's replace 'prasad' by 'prabhu' in file.txt

root@Prabhu:~/shell# cat file.txt this is prasad
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as you see file contain this is prasad

now execute sed command to replace

root@Prabhu:~/shell# sed 's/prasad/prabhu/' file.txt this is prabhu root@Prabhu:~/shell#
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less

This command is used to read / print few lines in files, then we can enter space to remaining content and so on

Below is syntax

less file_name
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1: enter q to exit fron reading mode
2: if you want to search anything, then enter / followed by keyword example /prasad then press enter
3: if you are in search mode, and you want to find next occurrence then press n
4: if you are in search mode, and you want to find previous occurrence then press shift + n

head

This command is used to print first few lines in file
Below is syntax

head file_name
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tail

This command is used to print last few lines in file
Below is syntax

tail file_name
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diff

This command is used to check difference in two files in same folder or different folder

diff file1 file2
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example, if I want to find difference in file.txt and test2.txt in same folder

root@Prabhu:~/shell# diff file.txt test2.txt 1c1 < this is prasad --- > this is prabhu root@Prabhu:~/shell#
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Access & User operations

sudo

This command enables non-root user who are part of sudo group to execute administrative operations
Syntax is as below

sudo [options] command
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if you want to act as sudo then execute following command

sudo su
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if you want to modify any file which need admin previlages then use command as below

sudo nano file.txt
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We can perform many such operations using sudo but you should be part of sudo group in order to use this command

whoami

Use this command to check current logged in user

root@Prabhu:~/prasad# whoami root root@Prabhu:~/prasad#
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chmod

this command used to change file/ directory permissions
Syntax

chmod [options][permission][file / directory]
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Example if I want to modify permission for sample.txt, below is current permission befor change

root@Prabhu:~/prasad# ls -la total 16 drwxr-xr-x 3 root root 4096 Jun 12 22:45 . drwx------ 8 root root 4096 Jun 12 23:11 .. -rw-r--r-- 1 root root 3094 Jun 12 22:37 sample.txt root@Prabhu:~/prasad#
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now read-write permissions to all users for this file

root@Prabhu:~/prasad# ls -la total 16 drwxr-xr-x 3 root root 4096 Jun 12 22:45 . drwx------ 8 root root 4096 Jun 12 23:11 .. -rwxrwxrwx 1 root root 3094 Jun 12 22:37 sample.txt
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chown

this command is used to change ownership of file / directory, below is its syntax

chown [options] newowner:newgroup [filename]
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disk related operations

df

this command is used to check disk usage and used spaced in your machine
example

root@Prabhu:~# df Filesystem 1K-blocks Used Available Use% Mounted on none 5038196 0 5038196 0% /usr/lib/modules/6.6.87.1-microsoft-standard-WSL2 none 5038196 4 5038192 1% /mnt/wsl drivers 123857916 121770020 2087896 99% /usr/lib/wsl/drivers /dev/sdd 1055762868 1675256 1000384140 1% / none 5038196 80 5038116 1% /mnt/wslg none 5038196 0 5038196 0% /usr/lib/wsl/lib rootfs 5033176 2664 5030512 1% /init none 5038196 520 5037676 1% /run none 5038196 0 5038196 0% /run/lock none 5038196 0 5038196 0% /run/shm none 5038196 76 5038120 1% /mnt/wslg/versions.txt none 5038196 76 5038120 1% /mnt/wslg/doc C:\ 123857916 121770020 2087896 99% /mnt/c D:\ 976760828 350942804 625818024 36% /mnt/d tmpfs 5038196 16 5038180 1% /run/user/0 root@Prabhu:~#
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du

This command is used to check size of directory in KB
Example

root@Prabhu:~# cd shell/ root@Prabhu:~/shell# du 40 . root@Prabhu:~/shell#
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Process related operations

top

Command shows all running processes in system and their hardware consumption

root@Prabhu:~/shell# top top - 23:26:44 up 1:29, 2 users, load average: 0.01, 0.02, 0.00 Tasks: 30 total, 1 running, 29 sleeping, 0 stopped, 0 zombie %Cpu(s): 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st MiB Mem : 9840.2 total, 9310.8 free, 493.3 used, 196.3 buff/cache MiB Swap: 3072.0 total, 3072.0 free, 0.0 used. 9346.9 avail Mem PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1 root 20 0 21740 12240 9296 S 0.0 0.1 0:00.94 systemd 2 root 20 0 3060 1792 1792 S 0.0 0.0 0:00.02 init-systemd(Ub 7 root 20 0 3076 1792 1792 S 0.0 0.0 0:00.00 init 65 root 19 -1 50420 15752 14984 S 0.0 0.2 0:00.48 systemd-journal 118 root 20 0 25004 6272 4992 S 0.0 0.1 0:00.15 systemd-udevd 192 systemd+ 20 0 21452 12800 10624 S 0.0 0.1 0:00.11 systemd-resolve 193 systemd+ 20 0 91020 7552 6784 S 0.0 0.1 0:00.18 systemd-timesyn 205 root 20 0 4236 2560 2432 S 0.0 0.0 0:00.01 cron
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ps

this command shows summarized status of running processes in system

root@Prabhu:~/shell# ps PID TTY TIME CMD 1566 pts/2 00:00:00 sudo 1567 pts/2 00:00:00 su 1568 pts/2 00:00:00 bash 1597 pts/2 00:00:00 ps root@Prabhu:~/shell#
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kill

This command is used to kill any process, we need to pass process id to kill that perticular process

kill PID
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Other utility commands

uname

this command shows information about system

root@Prabhu:~/shell# uname Linux
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To display all setting, use -a

root@Prabhu:~/shell# uname -a Linux Prabhu 6.6.87.1-microsoft-standard-WSL2 #1 SMP PREEMPT_DYNAMIC Mon Apr 21 17:08:54 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux root@Prabhu:~/shell#
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hostname

Use this command to get hostname of your system

root@Prabhu:~/shell# hostname Prabhu
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time

Use this command to check system date time settings

root@Prabhu:~/shell# time real 0m0.000s user 0m0.000s sys 0m0.000s root@Prabhu:~/shell#
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ip

this command is used to get list and manage network related parameters

ip [options] command
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Example, To display ip address of current system

root@Prabhu:~/shell# ip address show 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet 127.0.0.1/8 scope host lo valid_lft forever preferred_lft forever inet 10.255.255.254/32 brd 10.255.255.254 scope global lo valid_lft forever preferred_lft forever inet6 ::1/128 scope host valid_lft forever preferred_lft forever 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000 link/ether 00:15:5d:48:7f:77 brd ff:ff:ff:ff:ff:ff inet 172.28.43.90/20 brd 172.28.47.255 scope global eth0 valid_lft forever preferred_lft forever inet6 fe80::215:5dff:fe48:7f77/64 scope link valid_lft forever preferred_lft forever root@Prabhu:~/shell#
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