21 Essential Linux Commands Every Developer Should Master

This article summarizes the 21 most frequently used Linux commands, often asked in interviews, and serves as a quick reference guide.

1. Files and Directories

cd

Change the current directory.

cd /home – go to /home directory

cd .. – go up one level

cd ../.. – go up two levels

cd – go to the user’s home directory

cd ~user1 – go to user1’s home directory

cd - – return to the previous directory

pwd

Display the current working directory.

ls

List files and directories.

ls – list files

ls -l – detailed list

ls -a – include hidden files

ls -R – recursive listing

ls [0-9] – list names containing digits

cp

Copy files.

-a – preserve attributes

-p – preserve properties (useful for backup)

-i – prompt before overwriting

-r – copy directories recursively

-u – copy only when source is newer

mv

Move or rename files/directories.

-f – force overwrite

-i – prompt before overwriting

-u – overwrite only if source is newer

rm

Remove files or directories.

-f – ignore nonexistent files, no warnings

-i – interactive mode, ask before deleting

-r – recursive delete (dangerous for directories)

2. Viewing File Content

cat

Display file contents; often combined with pipelines.

cat file1 – show entire file

tac file1 – reverse order

cat -n file1 – show line numbers

more file1 – paginate long files

head -n 2 file1 – first two lines

tail -n 2 file1 – last two lines

tail -n +1000 file1 – from line 1000 onward

cat filename | head -n 3000 | tail -n +1000 – lines 1000‑3000

cat filename | tail -n +3000 | head -n 1000 – lines 3000‑3999

3. File Search

find

find / -name file1 – search from root

find / -user user1 – files owned by user1

find /usr/bin -type f -atime +100 – files not accessed in 100 days

find /usr/bin -type f -mtime -10 – files modified within 10 days

whereis halt – locate binary, source, or man page

which halt – show full path of executable

find /var/mail/ -size +50M -exec rm {} \;

4. File Permissions

chmod

ls -lh – display permissions

chmod ugo+rwx directory1 – grant read/write/execute to all

chmod go-rwx directory1 – revoke permissions from group and others

chown

chown user1 file1 – change file owner

chown -R user1 directory1 – change owner recursively

chown user1:group1 file1 – change owner and group

chgrp

chgrp group1 file1 – change group ownership

5. Text Processing

grep

Search for patterns in files.

grep Aug /var/log/messages – find "Aug"

grep ^Aug /var/log/messages – lines starting with "Aug"

grep [0-9] /var/log/messages – lines containing digits

grep Aug -R /var/log/* – recursive search

sed 's/string1/string2/g' example.txt – replace text

sed '/^$/d' example.txt – delete empty lines

paste

paste file1 file2 – merge files column‑wise

paste -d '+' file1 file2 – merge with '+' delimiter

sort

sort file1 file2 – sort contents

sort file1 file2 | uniq – unique lines

sort file1 file2 | uniq -u – lines only in one file

sort file1 file2 | uniq -d – common lines

comm

comm -1 file1 file2 – exclude file1 lines

comm -2 file1 file2 – exclude file2 lines

comm -3 file1 file2 – exclude common lines

6. Archiving and Compression

tar

Create, list, or extract archives; can use compression options.

-c – create archive

-t – list archive contents

-x – extract archive

-j – use bzip2

-z – use gzip

-v – verbose output

-f filename – specify archive file

-C dir – change to directory before operation

Examples:

tar -jcv -f filename.tar.bz2 … – create bzip2 archive

tar -jtv -f filename.tar.bz2 – list contents

tar -jxv -f filename.tar.bz2 -C /dest – extract

Other tools: bunzip2, bzip2, gunzip, gzip, rar, zip, unzip.

7. System Shutdown and Reboot

shutdown -h now – halt now

init 0 – halt

telinit 0 – halt

shutdown -h hh:mm & – schedule shutdown

shutdown -c – cancel scheduled shutdown

shutdown -r now – reboot now

reboot – reboot

logout – log out

time – measure command execution time

8. Process Management

jps

Show Java processes and their IDs.

ps

Display process information.

-A – all processes

-a – all without controlling terminal

-u – processes of a specific user

-x – extended listing

-l – long format

ps aux    # show all processes
ds ax    # show processes without terminal
ds -lA   # detailed list
ds axjf  # show process tree

kill

Send signals to processes by PID or job number.

killall

Send signals to all processes matching a name.

top

Real‑time system monitor, similar to Windows Task Manager.

Ways to kill a process:

Graphical interface

kill -9 pid – force kill

killall -9 program_name

pkill program_name

Check process ports:

netstat -tunlp | grep PORT_NUMBER