Basics of Linux

1. What is Linux?

Linux is an operating system kernel.

A complete Linux operating system usually consists of:

• Linux kernel
• Shell
• System utilities
• Package manager
• Libraries
• Applications

Examples of Linux distributions:

• Debian
• Ubuntu
• Fedora
• Arch Linux
• Alpine Linux

2. Terminal

The terminal lets you interact with Linux using commands.

pwd

Shows your current directory.

ls

Lists files.

cd /path/to/directory

Changes directory.

clear

Clears the terminal.

3. Files and Directories

Create a directory:

mkdir projects

Create a file:

touch hello.txt

Copy:

cp hello.txt backup.txt

Move or rename:

mv hello.txt hello.md

Delete:

rm hello.md

Delete a directory:

rm -r projects

4. Important Directories

Linux has a single filesystem tree starting at /.

/
├── bin      → essential commands
├── boot     → boot files
├── dev      → devices
├── etc      → configuration
├── home     → user directories
├── lib      → libraries
├── media    → removable media
├── mnt      → temporary mounts
├── opt      → optional software
├── proc     → process/kernel information
├── root     → root user's home
├── run      → runtime data
├── sbin     → system commands
├── tmp      → temporary files
├── usr      → user applications/files
└── var      → changing data

Your home directory is usually:

/home/username

Shortcut:

~

5. Absolute vs Relative Paths

Absolute path:

/home/marvin/projects

Relative path:

projects

Current directory:

.

Parent directory:

..

Home directory:

~

6. Reading Files

Print a file:

cat file.txt

Read interactively:

less file.txt

First lines:

head file.txt

Last lines:

tail file.txt

Follow a changing file:

tail -f app.log

7. Searching

Find files:

find . -name "*.txt"

Search text:

grep "hello" file.txt

Recursive search:

grep -r "hello" .

8. Permissions

Check permissions:

ls -l

Example:

-rwxr-xr--

Permissions are divided into:

owner   group   others
rwx     r-x     r--

Meaning:

r = read
w = write
x = execute

Change permissions:

chmod +x script.sh

9. Users

Show current user:

whoami

Show user information:

id

Run a command as root:

sudo command

Root has unrestricted system access.

10. Processes

List processes:

ps

More detailed:

ps aux

Interactive process viewer:

top

Terminate a process:

kill PID

Force termination:

kill -9 PID

11. Environment Variables

View a variable:

echo $PATH

Set a variable:

export NAME="value"

List environment variables:

env

PATH tells the shell where to look for executable programs.

12. Pipes

Commands can pass output to other commands.

ls | grep ".txt"

Example:

ps aux | grep nginx

The | operator sends the output of one command to another.

13. Redirection

Write output to a file:

echo hello > file.txt

Append:

echo world >> file.txt

Redirect errors:

command 2> error.log

Redirect everything:

command > output.log 2>&1

14. Shell

The shell interprets commands.

Common shells:

• Bash
• Zsh
• Fish

Check your shell:

echo $SHELL

Example:

ls -la

The shell:

1. Reads the command
2. Parses it
3. Finds the program
4. Starts the program
5. Displays its output

15. Package Managers

Package managers install software.

Debian/Ubuntu:

sudo apt update
sudo apt install curl

Fedora:

sudo dnf install curl

Arch:

sudo pacman -S curl

16. Networking

Show network interfaces:

ip addr

Show routes:

ip route

Test connectivity:

ping example.com

Make HTTP requests:

curl https://example.com

Show listening ports:

ss -tulpn

17. Disk Usage

Show filesystem usage:

df -h

Show directory size:

du -sh directory

Find large files:

du -ah . | sort -h

18. Archives

Create a tar archive:

tar -cf archive.tar directory/

Extract:

tar -xf archive.tar

Create compressed archive:

tar -czf archive.tar.gz directory/

Extract:

tar -xzf archive.tar.gz

19. Services

Modern Linux systems commonly use systemd.

Check a service:

systemctl status nginx

Start:

sudo systemctl start nginx

Stop:

sudo systemctl stop nginx

Enable at boot:

sudo systemctl enable nginx

Restart:

sudo systemctl restart nginx

20. Logs

View system logs:

journalctl

Follow logs:

journalctl -f

Logs for a service:

journalctl -u nginx

21. Useful Commands

22. The Linux Mental Model

Think of Linux as:

Hardware
   ↓
Linux kernel
   ↓
System calls
   ↓
Programs / services
   ↓
Shell
   ↓
You

The kernel manages:

• CPU
• Memory
• Processes
• Filesystems
• Devices
• Networking
• Security

The shell is simply a program that lets you interact with the system.

23. Good Commands to Learn First

pwd
ls
cd
mkdir
touch
cp
mv
rm
cat
less
grep
find
chmod
sudo
ps
kill
df
du
curl
systemctl
journalctl

Master these first. Everything else builds on them.