Essential Linux Commands: Classification, Usage, and Tips

Command Classification

Internal commands – part of the shell interpreter.

External commands – independent executable files.

Distinguish internal vs external with type : <code>type cd cd is a shell builtin type cat cat is /bin/cat</code> Help for internal commands: <code>man cd help cd info cd</code> Help for external commands: <code>man cat info cat cat --help</code>

Command Syntax

Format: command [options] [arguments] <code>ls -al /</code>

Common Commands

ls – List files or directories

Options: -l – detailed list. -a – include hidden entries.

Examples:

mkdir aaa
cd aaa/
touch 1.txt

ls
1.txt

ls -a
. .. 1.txt

ls -l
total 0
-rw-rw-r--. 1 user user 0 Feb 7 15:03 1.txt

cd – Change directory

Examples:

Go to the user home directory:

cd
# or
cd ~

Enter directory aaa: cd aaa/ Return to previous directory: cd - Go up one level: cd .. Switch to another user's home:

cd ~test/

pwd – Print working directory

cd tmp
pwd

mkdir – Make directory

Option -p – create parent directories as needed.

Examples:

Create a single directory: mkdir bbb Create nested directories:

mkdir -p ccc/ddd

rmdir – Remove empty directory

rmdir bbb

Fails if the directory is not empty.

touch – Create an empty file

touch 1.txt

rm – Remove files or directories

Options: -f, --force – force deletion. -r, -R – recursive delete.

Examples:

Delete a file: rm 1.txt Delete a directory recursively:

rm -r ccc/

cat – Concatenate and display file contents

echo aaaa > 1.txt
cat 1.txt

head – Show the first N lines (default 10)

Option -n, --lines=[-]K – show first K lines; a leading - excludes the last K lines.

Examples:

Show first 10 lines of a log: head /var/log/messages Show first 5 lines: head -n 5 /var/log/messages Exclude the last 5 lines:

head -n -5 /var/log/messages

tail – Show the last N lines (default 10)

Option -n, --lines=[+]K – show last K lines; a leading + starts from line K to the end. -f – follow file growth. -F – like -f but continues after log rotation.

Examples:

Show last 10 lines: tail /var/log/messages Show last 5 lines: tail -n 5 /var/log/messages Follow a file:

tail -f /var/log/messages

more – Page through output

Option -num – number of lines per screen.

Commands: SPACE – next screen. q – quit. / – search. = – show line number. ! or :! – run a command in a subshell.

less – Page through output with backward scrolling

Commands: b – scroll back one screen. f – scroll forward one screen. / – search.

echo – Display a line of text

echo hello world

cp – Copy files

Copy a single file: cp 1.txt /cc Copy multiple files using brace expansion:

cp ./{1.txt,2.txt} /cc

mv – Move or rename files

Move a file: mv 1.txt /cc Move multiple files: mv ./{1.txt,2.txt} cc/ Rename a file:

mv 1.txt a.txt

find – Search for files

Find a specific file: find 1.txt Wildcard search:

find *.txt

wc – Count lines, words, characters

Options: -m – characters. -w – words. -l – lines.

wc 1.txt

grep – Search text using patterns

Option -i – ignore case.

grep -i a 1.txt

ln – Create links

Option -s – create symbolic link.

Default creates a hard link; hard links remain valid if the source file is deleted, while symbolic links become dangling.

ln 1.txt x.txt

1.txt

is the source; x.txt is the link.

System Management Commands

stat – Detailed file information (more than ls )

stat 1.txt

who – Show logged‑in users

who

whoami – Show current user

whoami

hostname – Show or set the host name

Show: hostname Temporarily change:

hostname tempname

uname – Show system information

uname -a

top – Interactive process viewer (press q to quit)

top

ps – Snapshot of processes

ps -aux

du – Estimate directory size

Option -h – human‑readable units.

du -h /home

df – Report file system disk space usage

Option -h – human‑readable.

df -h

ifconfig – Network interface configuration

Show current configuration: ifconfig Temporarily set IP address:

ifconfig eth0 192.168.1.125

ping – Test network connectivity

ping 192.168.1.1

netstat – Network statistics

Options: -a – all sockets. -l – listening services. -t – TCP connections. -p, --programs – show PID and program name.

netstat -altpn

clear – Clear the terminal screen

clear

alias – Define command shortcuts

alias showmeit="ps -aux"

unalias – Remove an alias

unalias showmeit

kill – Terminate processes

kill -9 5231

5231 is the process ID.

free – Show memory and swap usage

free

Packaging and Compression Commands

gzip

bzip2

tar

Tar options: -c, --create – create archive. -x, --extract, --get – extract archive. -z – gzip compression. -j – bzip2 compression. -v – verbose (show progress).

Create archive: tar -cvf 1.txt.tar 1.txt Extract archive: tar -xvf 1.txt.tar Extract to a specific directory: tar -xvf 1.txt.tar -C bb/ Gzip archive: tar -zcvf 1.txt.tar.gz 1.txt Extract gzip archive: tar -zxvf 1.txt.tar.gz 1.txt Bzip2 archive: tar -jcvf 1.txt.tar.gz 1.txt Extract bzip2 archive:

tar -jxvf 1.txt.tar.gz 1.txt

Shutdown / Reboot Commands

shutdown

– schedule shutdown or reboot. -r – reboot. -h – halt (no reboot). now – immediate action. halt – halt the system. reboot – reboot the system. poweroff – power off.

Linux Pipelines and Redirection

|

– pipe output of one command to the next. > – redirect output to a file (overwrite). >> – append output to a file.

Command substitution with backticks: echo `date` Single quotes suppress special characters: echo '$PATH' Double quotes allow variable expansion but preserve most characters:

echo "$PATH"

Good Linux Learning Habits

Read manual pages ( man) and other help documents.

Use the Tab key for auto‑completion.

Master useful shortcuts: Ctrl+C – interrupt current process. Ctrl+R – search command history. Ctrl+L – clear screen (same as clear).