Master Essential Linux Commands: Navigate Files, Manage Permissions, and Automate Tasks

1. Files and Directories

1. cd command

Used to change the current directory; the argument is the path to switch to, which can be absolute or relative.

cd /home – enter /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

2. pwd command

pwd displays the current working directory.

3. ls command

Lists files and directories.

ls – list files in the directory

ls -l – detailed list

ls -a – include hidden files

ls -R – recursive listing

ls [0-9] – list names containing numbers

4. cp command

Copies files; can copy multiple files to a directory.

-a – preserve attributes

-p – preserve attributes, similar to -a, often used for backup

-i – prompt before overwriting

-r – recursive copy for directories

-u – copy only when source is newer

5. mv command

Moves or renames files and directories.

-f – force overwrite without prompting

-i – prompt before overwriting

-u – overwrite only if source is newer

6. rm command

Removes files or directories.

-f – force, ignore nonexistent files

-i – interactive, prompt before removal

-r – recursive removal, dangerous for directories

2. Viewing File Content

7. cat command

Displays the content of text files; can be combined with more or less. cat file1 – view from the first byte tac file1 – view in 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

8. find command

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

Delete files larger than 50 M:

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

4. Permissions

9. chmod command

ls -lh – display permissions

chmod ugo+rwx directory1 – give read, write, execute to user, group, others

chmod go-rwx directory1 – remove all permissions for group and others

10. chown command

chown user1 file1 – change owner

chown -R user1 directory1 – change owner recursively

chown user1:group1 file1 – change owner and group

11. chgrp command

chgrp group1 file1 – change group

5. Text Processing

12. grep command

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/stringa1/stringa2/g' example.txt – replace text sed '/^$/d' example.txt – delete empty lines

13. paste command

paste file1 file2

– merge files column‑wise paste -d '+' file1 file2 – merge with "+" delimiter

14. sort command

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 – duplicate lines

15. comm command

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

16. tar command

-c – create archive

-t – list archive contents

-x – extract

-j – use bzip2

-z – use gzip

-v – verbose

-f filename – specify file

-C dir – change to directory

tar -jcv -f filename.tar.bz2 … – compress

tar -jtv -f filename.tar.bz2 – list

tar -jxv -f filename.tar.bz2 -C … – extract

bunzip2 file1.bz2 – decompress

bzip2 file1 – compress

gunzip file1.gz – decompress

gzip file1 – compress

gzip -9 file1 – maximum compression

rar a file1.rar test_file – create rar

rar a file1.rar file1 file2 dir1 – compress multiple

rar x file1.rar – extract

zip file1.zip file1 – create zip

unzip file1.zip – extract zip

zip -r file1.zip file1 file2 dir1 – recursive zip

7. System and Shutdown

shutdown -h now – halt

init 0 – halt

telinit 0 – halt

shutdown -h hh:mm – schedule halt

shutdown -c – cancel scheduled halt

shutdown -r now – reboot

reboot – reboot

logout – log out

time – measure command execution time

8. Process Management

17. jps command

Shows Java processes and their IDs.

18. ps command

-A – all processes

-a – all processes not attached to a terminal

-u – processes of a specific user

-x – extended information

-l – long format

ps aux    # view all processes
ps ax     # view non‑terminal processes
ps -lA    # detailed list of all processes
ps axjf   # view process tree

19. kill command

Sends a signal to a job or PID.

20. killall command

Sends a signal to all processes with a given name.

21. top command

Real‑time system monitor similar to Windows Task Manager.

Graphical method

kill -9 pid – force kill

killall -9 program_name

pkill program_name

Check process ports:

netstat -tunlp|grep PORT_NUMBER