Linux commands

Linux Basics

Linux is a free, open-source operating system used on servers, desktops, and embedded systems. It supports both command-line (CLI) and graphical (GUI) interfaces.

---

Key Concepts

• Case Sensitivity: Files and directories are case-sensitive.
• Everything is a File: Devices, processes, and sockets are represented as files.
• Multi-user Support: Multiple users can operate simultaneously.
• Multitasking: Multiple processes can run at the same time.
• Permissions: Access to files and directories is controlled by permissions.
• Processes: Each running program is a process with a unique PID.
• Shell: The command-line interface (e.g., Bash) interprets user commands.

---

File System Structure

Directory	Purpose
/	Root directory
/home	User home directories
/etc	System configuration files
/var	Variable files (logs, spool)
/usr	User programs and data
/bin	Essential user binaries
/sbin	System binaries
/tmp	Temporary files
/dev	Device files
/proc	Process and kernel information
/lib	Essential shared libraries
/opt	Optional application software packages
/mnt	Mount point for temporary filesystems
/media	Removable media (USB, CD-ROM)

---

Common Linux Commands

Command	Description
pwd	Print working directory
ls	List files and directories
ls -l	Long listing format
ls -a	Show hidden files
cd <dir>	Change directory
cd ~	Go to home directory
cd ..	Go up one directory
cp <src> <dst>	Copy files or directories
cp -r <src> <dst>	Copy directories recursively
mv <src> <dst>	Move or rename files/directories
rm <file>	Remove files
rm -r <dir>	Remove directories recursively
mkdir <dir>	Create a new directory
rmdir <dir>	Remove an empty directory
cat <file>	Display file contents
less <file>	View file contents page by page
more <file>	View file contents page by page
head <file>	Show first 10 lines of a file
tail <file>	Show last 10 lines of a file
tail -f <file>	Follow appended data in real time
touch <file>	Create an empty file or update timestamp
echo <text>	Print text to terminal
man <command>	Show manual for a command
info <command>	Show detailed info about a command
chmod <mode> <file>	Change file permissions
chown <user>:<group> <file>	Change file owner/group
ps	Show running processes
ps aux	Detailed process list
kill <pid>	Kill a process by PID
killall <name>	Kill processes by name
top	Display running processes
htop	Interactive process viewer
df -h	Show disk space usage
du -sh <dir>	Show directory size
free -h	Show memory usage
ifconfig / ip a	Show network interfaces
ping <host>	Test network connectivity
wget <url>	Download files from the web
curl <url>	Transfer data from/to a server
ssh <user>@<host>	Secure shell remote login
scp <src> <dst>	Secure copy files over SSH
tar -czvf <file.tar.gz> <dir>	Create compressed archive
tar -xzvf <file.tar.gz>	Extract compressed archive
zip <file.zip> <files>	Create zip archive
unzip <file.zip>	Extract zip archive
find <dir> -name <pattern>	Search for files
grep <pattern> <file>	Search text in files
history	Show command history
alias ll='ls -l'	Create command alias
sudo <command>	Run command as superuser
passwd	Change user password
date	Show current date and time
cal	Display a calendar
whoami	Show current user
uname -a	Show system information
reboot	Reboot the system
shutdown now	Shutdown the system immediately

---

File Permissions

• r: read
• w: write
• x: execute

Example: -rwxr-xr--

• Owner: rwx (read, write, execute)
• Group: r-x (read, execute)
• Others: r-- (read)

Change permissions:

chmod u+x file      # Add execute for user
chmod g-w file      # Remove write for group
chmod o+r file      # Add read for others
chmod 755 file      # rwxr-xr-x

Change ownership:

chown user file
chown user:group file

---

Process Management

• ps, top, htop: View processes
• kill <pid>: Terminate process
• jobs: List background jobs
• bg, fg: Move jobs to background/foreground
• &: Run command in background (command &)

---

Networking

• ifconfig / ip a: Show interfaces
• ping <host>: Test connectivity
• netstat -tuln: Show listening ports
• ss -tuln: Show socket statistics
• scp, ssh: Secure file transfer and remote login

---

Package Management

• Debian/Ubuntu: apt, dpkg
  • sudo apt update
  • sudo apt upgrade
  • sudo apt install <package>
  • sudo apt remove <package>
• RedHat/CentOS: yum, dnf, rpm
  • sudo yum install <package>
  • sudo dnf install <package>
  • sudo rpm -i <package.rpm>

---

Useful Tips

• Use tab for auto-completion.
• Use history to see previous commands.
• Use sudo to run commands as superuser.
• Use ctrl + c to stop a running command.
• Use ctrl + l to clear the terminal.
• Use !! to repeat the last command.
• Use !n to repeat command number n from history.
• Use clear to clear the terminal screen.
• Use exit to close the terminal session.

---

Editing Files

• nano <file>: Simple text editor
• vim <file>: Advanced text editor
• emacs <file>: Powerful text editor

---

Redirection and Pipes

• >: Redirect output to file (ls > out.txt)
• >>: Append output to file (echo hi >> out.txt)
• <: Use file as input (sort < file.txt)
• |: Pipe output to another command (ls | grep txt)

---

Environment Variables

• View: echo $HOME
• Set: export VAR=value
• List: env

---

Getting Help

• man <command>: Manual page for a command
• <command> --help: Most commands support this flag
• info <command>: More detailed help

---

System Monitoring

• top, htop: Monitor processes
• free -h: Memory usage
• df -h: Disk usage
• du -sh <dir>: Directory size
• uptime: System uptime

---

Users and Groups

• who: List logged-in users
• id: Show user and group IDs
• adduser <user>: Add new user
• deluser <user>: Delete user
• usermod -aG <group> <user>: Add user to group
• groups <user>: Show user groups

---

Scheduling Tasks

• crontab -e: Edit user cron jobs
• at <time>: Schedule one-time task

---

Disk Management

• lsblk: List block devices
• fdisk -l: List disk partitions
• mount <device> <dir>: Mount device
• umount <dir>: Unmount device

---

Archiving and Compression

• tar -czvf archive.tar.gz dir/: Create archive
• tar -xzvf archive.tar.gz: Extract archive
• gzip file: Compress file
• gunzip file.gz: Decompress file

---

Working with Files from the Terminal

• Write to a file:
  
  echo "text" > file.txt — Overwrites file with text
  
  echo "text" >> file.txt — Appends text to file

• Save output of a command to a file:
  
  ls > files.txt — Saves output of ls to files.txt

• Edit and save files interactively:

  • nano file.txt — Edit and save with Ctrl+O, then exit with Ctrl+X
  • vim file.txt — Enter insert mode with i, save with :w, quit with :q
  • emacs file.txt — Save with Ctrl+X Ctrl+S, exit with Ctrl+X Ctrl+C

• Close a file in an editor:

  • In nano: Ctrl+X
  • In vim: :q (quit), :wq (write and quit)
  • In emacs: Ctrl+X Ctrl+C

• Saving and closing files is editor-specific; always check the help (Ctrl+G in nano, :help in vim, C-h in emacs) for more shortcuts.