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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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