Linux Learning Notes

Welcome to my Linux learning notes. These notes cover Linux fundamentals through advanced system administration topics.

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Directory Structure (/ Directory)

🔧 System and Configuration

Folder	Meaning
/bin	Essential binaries (commands) needed for the system to boot and run (e.g., ls, cp, mv)
/sbin	System binaries for root/admin tasks (e.g., reboot, fsck)
/etc	Configuration files (e.g., network, services, users, passwords)
/lib, /lib64	Libraries needed by binaries in /bin and /sbin
/boot	Files needed to boot the system (e.g., the kernel, GRUB)
/dev	Special files for devices (e.g., /dev/sda for disks)
/proc	Virtual filesystem with kernel and process info (e.g., /proc/cpuinfo)
/sys	Virtual filesystem with hardware and kernel info
/run	Runtime data for processes after boot (temporary, stored in RAM)

🧑‍💻 Users and Applications

Folder	Meaning
/home	Home directories of users (e.g., /home/alex)
/root	Home directory for the root user (not in /home)
/usr	User-installed software and data (like /usr/bin, /usr/lib, etc.)
/opt	Optional software packages (often 3rd party)
/var	Variable data like logs, mail, databases (e.g., /var/log)
/media	Mount point for external drives (e.g., USB, CDs)
/mnt	Temporary mount point (used by admins to mount anything manually)
/srv	Data for services like web or FTP servers (e.g., /srv/http)

🗑️ Temporary Stuff

Folder	Meaning
/tmp	Temporary files, usually cleared on reboot

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Understanding Linux Basics

File and Directory Management

1. ls - List Files and Directories

• Purpose: Displays the files and directories in your current directory.
• Usage:
  • ls – List files in the current directory.
  • ls -l – List files in long format (shows permissions, owner, file size, etc.).
  • ls -a – List all files, including hidden files (those starting with a dot ., like .bashrc).

2. cd - Change Directory

• Purpose: Change the current working directory.
• Usage:
  • cd /path/to/directory – Go to a specific directory (absolute path).
  • cd directory_name – Go to a subdirectory inside your current location.
  • cd .. – Move up one level (parent directory).
  • cd ~ – Go to your home directory (shortcut).

3. pwd - Print Working Directory

• Purpose: Shows the full path of your current directory.
• Usage:
  • pwd – Shows your current directory’s path.

4. mkdir - Make Directory

• Purpose: Creates a new directory.
• Usage:
  • mkdir directory_name – Creates a new directory.
  • mkdir -p path/to/directory – Create parent directories if they don’t exist.

5. rmdir - Remove Empty Directory

• Purpose: Removes an empty directory.
• Usage:
  • rmdir directory_name – Removes the empty directory.

6. rm - Remove Files or Directories

• Purpose: Delete files or directories.
• Usage:
  • rm file_name – Deletes a file.
  • rm -r directory_name – Deletes a directory and all its contents (be careful!).
  • rm -f file_name – Force delete without asking for confirmation.

7. cp - Copy Files or Directories

• Purpose: Copies files or directories from one place to another.
• Usage:
  • cp source_file destination – Copy a file to a new location.
  • cp -r source_directory destination – Copy a directory and its contents.

8. mv - Move or Rename Files/Directories

• Purpose: Moves files or directories, or renames them.
• Usage:
  • mv source_file destination – Move or rename a file.
  • mv old_name new_name – Rename a file or directory.

9. touch - Create a New Empty File

• Purpose: Creates an empty file or updates the timestamp of an existing file.
• Usage:
  • touch filename – Creates a new empty file or updates the file’s last modified time.

File Content Operations

10. cat - Concatenate and Display File Contents

• Purpose: Shows the contents of a file on the screen.
• Usage:
  • cat filename – Display the content of the file.
  • cat file1 file2 – Concatenate and display content from multiple files.

11. more - View File Contents Page-by-Page

• Purpose: View content of a file one page at a time (useful for large files).
• Usage:
  • more filename – View file contents one page at a time.
  • Use space to move to the next page and q to quit.

12. less - View File Contents (Improved Version of more)

• Purpose: Similar to more but gives more control, like scrolling up and down.
• Usage:
  • less filename – View file contents and scroll through.
  • Use arrow keys to scroll up/down, q to quit.

13. head - View the First Few Lines of a File

• Purpose: Displays the first 10 lines (by default) of a file.
• Usage:
  • head filename – Show the first 10 lines of a file.
  • head -n 20 filename – Show the first 20 lines of a file.

14. tail - View the Last Few Lines of a File

• Purpose: Displays the last 10 lines (by default) of a file.
• Usage:
  • tail filename – Show the last 10 lines of a file.
  • tail -n 20 filename – Show the last 20 lines of a file.
  • tail -f filename – View a file and keep updating as it grows (useful for log files).

15. file - Identify File Type

• Purpose: Identifies the file type (e.g., text, executable, image).
• Usage:
  • file filename – Shows the type of file.

Search and Find

16. find - Search for Files and Directories

• Purpose: Finds files or directories based on conditions like name, size, date, etc.
• Usage:
  • find /path/to/search -name "filename" – Search for files with a specific name.
  • find /path/to/search -type d – Search for directories only.
  • find /path/to/search -size +1G – Find files larger than 1GB.

17. grep - Search for Text in Files

• Purpose: Search for specific text within files.
• Usage:
  • grep "search_term" filename – Find lines in the file that contain the search term.
  • grep -r "search_term" /path/to/dir – Search recursively in a directory.

File Permissions and Ownership

18. chmod - Change File Permissions

• Purpose: Changes the read, write, and execute permissions of a file or directory.
• Usage:
  • chmod 755 filename – Set read, write, execute permissions for owner and read, execute for others.
  • chmod u+x filename – Add execute permission to the file for the owner.

19. chown - Change File Ownership

• Purpose: Change the owner and/or group of a file or directory.
• Usage:
  • chown user:group filename – Change the owner and group of a file.
  • chown user filename – Change the owner of a file.

20. chgrp - Change Group Ownership

• Purpose: Change the group ownership of a file or directory.
• Usage:
  • chgrp group filename – Change the group of a file.

System Utilities

21. man - Manual Pages (Help)

• Purpose: View the manual or documentation for a command.
• Usage:
  • man command – View the manual for a specific command.
  • Example: man ls – Shows the manual for the ls command.

22. echo - Print Text or Variables

• Purpose: Print text to the terminal, or show the value of a variable.
• Usage:
  • echo "Hello, World!" – Prints “Hello, World!” to the screen.
  • echo $HOME – Prints the value of the HOME variable (your home directory path).

23. sudo - Execute a Command as a Superuser

• Purpose: Execute commands that require administrative (root) privileges.
• Usage:
  • sudo command – Run a command with superuser privileges.
  • Example: sudo apt update – Update package lists (on Debian-based systems like Ubuntu).

24. exit - Exit the Terminal

• Purpose: Close the current terminal session.
• Usage:
  • exit – Exit the terminal (or close the terminal window).

Package Managers

Package managers are tools used to install, update, and remove software packages in Linux.

APT (Advanced Package Tool) – Debian-based systems (e.g., Ubuntu)

• sudo apt update – Update package lists.
• sudo apt install package_name – Install a package.
• sudo apt remove package_name – Remove a package.
• sudo apt upgrade – Upgrade installed packages.

YUM/DNF – Red Hat-based systems (e.g., Fedora, CentOS)

• sudo yum install package_name – Install a package (older systems).
• sudo dnf install package_name – Install a package (Fedora and newer CentOS).
• sudo yum remove package_name – Remove a package.
• sudo dnf upgrade – Upgrade installed packages.

Zypper – SUSE-based systems

• sudo zypper install package_name – Install a package.
• sudo zypper remove package_name – Remove a package.
• sudo zypper update – Upgrade installed packages.

Environment Variables

Environment variables store information about the system environment and configuration.

Common Environment Variables:

• HOME – Your home directory (e.g., /home/username).
• USER – The current user.
• PATH – A list of directories where executable programs are stored.
• SHELL – The path of the current shell (e.g., /bin/bash).
• LANG – The system’s language settings (e.g., en_US.UTF-8).

Viewing Environment Variables:

• echo $VARIABLE_NAME – Show the value of an environment variable.
• Example: echo $PATH – Displays the directories listed in the PATH variable.

Setting Environment Variables:

• export VARIABLE_NAME=value – Set an environment variable temporarily.
• Example: export EDITOR=nano – Set nano as the default text editor for the session.

Text Editors

Nano

• Purpose: A simple terminal-based text editor.
• Usage:
  • nano filename – Open a file in the nano editor.
• Commands:
  • Ctrl + O – Save the file.
  • Ctrl + X – Exit the editor.
  • Ctrl + K – Cut a line.
  • Ctrl + U – Paste a line.

Vim

• Purpose: A powerful, mode-based text editor.
• Usage:
  • vim filename – Open a file in vim.
• Commands:
  • i – Enter insert mode (for editing).
  • Esc – Exit insert mode.
  • :w – Save the file.
  • :q – Quit vim.
  • :wq – Save and quit.

Tips and Tricks

• Tab Completion: When typing commands or file names, press Tab to auto-complete.
• Up/Down Arrow: Use the up and down arrow keys to scroll through your previous commands.
• Ctrl + C: Stops a running command or process.
• Ctrl + Z: Puts a process in the background (suspend it).

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User & System Administration

User Management

1. useradd – Add a New User

• Purpose: Create a new user on the system.
• Usage:

  sudo useradd username
  sudo useradd -m -s /bin/bash username  # Create user with home directory and specific shell

• Note: m creates the home directory, and s specifies the shell.

2. usermod – Modify an Existing User

• Purpose: Modify user account settings.
• Usage:

  sudo usermod -aG groupname username  # Add user to a group
  sudo usermod -L username  # Lock user account
  sudo usermod -U username  # Unlock user account

3. userdel – Delete a User

• Purpose: Remove a user from the system.
• Usage:

  sudo userdel username  # Remove user but not their home directory
  sudo userdel -r username  # Remove user and their home directory

Group Management

4. groupadd – Add a New Group

• Purpose: Create a new group.
• Usage:

  sudo groupadd groupname

5. groupdel – Delete a Group

• Purpose: Remove a group from the system.
• Usage:

  sudo groupdel groupname

Password Management

6. passwd – Change User Password

• Purpose: Change a user’s password.
• Usage:

  sudo passwd username  # Change password of a specific user
  passwd  # Change the password of the current user

7. chage – Change User Password Expiry Information

• Purpose: Modify the password expiry details for a user.
• Usage:

  sudo chage -l username  # List password expiry information
  sudo chage -M 90 username  # Set password expiry to 90 day

User Information

8. id – Display User and Group Information

• Purpose: Show user and group information for a user.
• Usage:

  id username  # Show user and group info for username

9. groups – Show Groups a User Belongs To

• Purpose: List all the groups a user is a part of.
• Usage:

  groups username  # Show groups for a user

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Disk Management

Disk Space Usage

1. df – Display Disk Space Usage

• Purpose: Shows disk space usage on mounted filesystems.
• Usage:
  • df – Show disk space usage.
  • df -h – Show disk space in a human-readable format (MB, GB).

2. du – Disk Usage of Files and Directories

• Purpose: Shows disk usage for files or directories.
• Usage:
  • du filename – Show the disk usage of a file.
  • du -sh /path/to/directory – Show total disk usage of a directory in a human-readable format.

Partition Management

3. fdisk – Partition Table Manipulation

• Purpose: Create, delete, or modify disk partitions.
• Usage:
  • sudo fdisk -l – List all partitions on the system.
  • sudo fdisk /dev/sda – Open partitioning tool for a specific disk (replace /dev/sda with your disk).

Mounting Filesystems

4. mount – Mount a Filesystem

• Purpose: Mount a storage device or partition.
• Usage:
  • sudo mount /dev/sda1 /mnt – Mount a partition (/dev/sda1) to a directory (/mnt).
  • mount – List all mounted filesystems.

5. umount – Unmount a Filesystem

• Purpose: Unmount a filesystem or device.
• Usage:
  • sudo umount /mnt – Unmount the device mounted at /mnt.

Block Devices

6. lsblk – List Block Devices

• Purpose: Lists all block devices (like hard drives, partitions).
• Usage:
  • lsblk – List block devices with details like size and mount point.

Filesystem Operations

7. mkfs – Create a Filesystem

• Purpose: Create a new filesystem on a partition.
• Usage:
  • sudo mkfs.ext4 /dev/sda1 – Format /dev/sda1 with the ext4 filesystem.

8. fsck – File System Check

• Purpose: Check and repair filesystems.
• Usage:
  • sudo fsck /dev/sda1 – Check and repair the filesystem on /dev/sda1.

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Security Administration

File Permissions and Ownership

1. chmod – Change File Permissions

• Purpose: Change file permissions for users (read, write, execute).
• Usage:

  chmod 755 file  # rwxr-xr-x permissions for the file
  chmod u+x file  # Add execute permission for the user
  chmod -R 777 dir  # Recursively set permissions to 777 (rwx for all)

2. chown – Change File Owner and Group

• Purpose: Change the owner and/or group of a file or directory.
• Usage:

  sudo chown user:group file  # Change owner and group
  sudo chown -R user:group dir  # Change ownership recursively

3. chgrp – Change Group Ownership

• Purpose: Change the group ownership of a file or directory.
• Usage:

  sudo chgrp group file  # Change the group of the file

4. setfacl – Set File Access Control Lists

• Purpose: Set more granular file permissions (for specific users/groups).
• Usage:

  setfacl -m u:username:rwx file  # Grant read, write, execute to user
  getfacl file  # View ACLs for a file

File Integrity & Auditing

5. auditd – Linux Audit Daemon

• Purpose: Record security-relevant events.
• Usage:
  • Install the auditing tools: sudo apt install auditd
  • Configure logging: sudo auditctl -w /etc/passwd -p wa (Monitor changes to /etc/passwd).
  • View logs: sudo ausearch -f /etc/passwd

6. AIDE – Advanced Intrusion Detection Environment

• Purpose: Check system files for integrity and changes.
• Usage:
  • Install: sudo apt install aide
  • Initialize database: sudo aideinit
  • Check file integrity: sudo aide --check

Firewalls and Security

7. ufw – Uncomplicated Firewall

• Purpose: Manage firewall settings in a simple way.
• Usage:

  sudo ufw enable  # Enable firewall
  sudo ufw disable  # Disable firewall
  sudo ufw allow 22  # Allow SSH (port 22)
  sudo ufw deny 80  # Block HTTP (port 80)
  sudo ufw status  # Check firewall status

8. iptables – Configure Linux Kernel Firewall

• Purpose: Advanced firewall configuration.
• Usage:

  sudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT  # Allow SSH
  sudo iptables -A INPUT -p tcp --dport 80 -j DROP  # Block HTTP
  sudo iptables -L  # List firewall rules

9. fail2ban – Intrusion Prevention Tool

• Purpose: Protect from brute-force attacks by banning IPs.
• Usage:
  • Install: sudo apt install fail2ban
  • Check status: sudo fail2ban-client status
  • Configure: Edit /etc/fail2ban/jail.local

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Process Management

Process Viewing and Control

1. ps – Display Processes

• Purpose: Shows a snapshot of current processes.
• Usage:
  • ps – Display processes for the current shell.
  • ps aux – Show all processes running on the system.
  • ps -ef – Display process details in a full format.

2. top – Task Manager

• Purpose: Provides a dynamic view of system processes.
• Usage:
  • top – Start the task manager. Shows CPU usage, memory usage, and more.
  • q – Quit the top command.

3. kill – Terminate a Process

• Purpose: Kill a process by PID (Process ID).
• Usage:
  • kill PID – Kill a process using its PID.
  • kill -9 PID – Force kill a process (use with caution).

4. htop – Improved Task Manager

• Purpose: An interactive process viewer (more user-friendly than top).
• Usage:
  • htop – Start the htop process viewer.
  • Use arrow keys to navigate and F9 to kill a process.

Job Control

5. bg – Resume a Job in the Background

• Purpose: Resume a paused job in the background.
• Usage:
  • bg – Run the most recent job in the background.
  • bg %1 – Run job number 1 in the background.

6. fg – Bring a Job to the Foreground

• Purpose: Bring a background job to the foreground.
• Usage:
  • fg – Bring the most recent job to the foreground.
  • fg %1 – Bring job number 1 to the foreground.

7. jobs – List Background Jobs

• Purpose: Show a list of jobs running in the background.
• Usage:
  • jobs – List jobs with their status.

Process Priority

8. nice & renice – Change Process Priority

• Purpose: Set or change the priority of a process.
• Usage:
  • nice -n 10 command – Start a command with lower priority.
  • renice -n 10 PID – Change the priority of a running process.

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Networking in Linux

Basic Networking Commands

1. ip – View or Configure Network Interfaces

• Purpose: Shows or configures network interfaces and routing.
• Usage:
  • ip addr – Display network interface details (IP addresses).
  • ip link – Show or manipulate network interfaces.
  • ip route – Show or modify the routing table.

2. ping – Test Network Connectivity

• Purpose: Sends ICMP echo requests to test connectivity.
• Usage:
  • ping hostname_or_IP – Ping a host to check if it’s reachable.
  • ping -c 4 google.com – Ping Google 4 times and then stop.

3. traceroute – Trace the Route Packets Take

• Purpose: Shows the route packets take to a network host.
• Usage:
  • traceroute google.com – Trace the route to Google’s servers.

4. netstat – Network Statistics

• Purpose: Displays network connections, routing tables, and interface statistics.
• Usage:
  • netstat – Show current network connections.
  • netstat -tuln – Show listening ports.

5. ifconfig – Configure Network Interfaces

• Purpose: Display or configure network interfaces.
• Usage:
  • ifconfig – Show network interface details.
  • ifconfig eth0 up – Bring the interface eth0 up.

Remote Access

6. ssh – Secure Shell for Remote Login

• Purpose: Connect to a remote machine securely.
• Usage:
  • ssh user@hostname_or_IP – Connect to a remote server.
  • Example: ssh alice@192.168.1.100

7. scp – Secure Copy (Remote File Transfer)

• Purpose: Copy files between hosts over SSH.
• Usage:
  • scp file user@hostname:/path/to/destination – Copy a file to a remote server.

File Transfer and Web Access

8. wget – Download Files from the Web

• Purpose: Download files via HTTP, HTTPS, or FTP.
• Usage:
  • wget http://example.com/file – Download a file from the web.

9. curl – Transfer Data with URLs

• Purpose: Interact with web resources, download or send data.
• Usage:
  • curl http://example.com – Fetch the content from a URL.

DNS Management

10. nslookup – Query DNS Servers

• Purpose: Look up DNS records for a domain.
• Usage:
  • nslookup example.com – Get DNS information for the domain.

Network Configuration & Monitoring

11. nmcli – NetworkManager Command-Line Interface

• Purpose: Manage network connections.
• Usage:

  nmcli device status  # Show status of all network devices
  nmcli connection show  # List all network connections
  nmcli device wifi list  # List available Wi-Fi networks

12. ss – Socket Stat

• Purpose: Display socket connections (similar to netstat but faster).
• Usage:

  ss -tuln  # Show listening TCP and UDP sockets
  ss -s  # Show summary of socket statistics

13. nmap – Network Exploration and Security Auditing Tool

• Purpose: Scan and discover hosts/services on a network.
• Usage:

  nmap 192.168.1.1  # Scan a single IP
  nmap -p 1-65535 192.168.1.1  # Scan all ports on an IP

14. tcpdump – Network Packet Analyzer

• Purpose: Capture network traffic and analyze it.
• Usage:

  sudo tcpdump -i eth0  # Capture traffic on eth0
  sudo tcpdump -i eth0 port 80  # Capture HTTP traffic
  sudo tcpdump -w capture.pcap  # Save captured traffic to file

15. ethtool – Network Interface Controller (NIC) Configuration

• Purpose: Display or change network interface settings.
• Usage:

  sudo ethtool eth0  # Display information about eth0
  sudo ethtool -s eth0 speed 1000 duplex full  # Set NIC speed to 1000 Mbps (full-duplex)

Network Routing & NAT

16. ip route – View or Modify Network Routing

• Purpose: Manage and inspect routing tables.
• Usage:

  ip route show  # Show routing table
  ip route add default via 192.168.1.1  # Set default gateway
  ip route del 192.168.1.0/24  # Delete a route

17. sysctl – View and Change Kernel Parameters at Runtime

• Purpose: Modify kernel parameters (like network settings).
• Usage:

  sysctl net.ipv4.ip_forward  # Check if IP forwarding is enabled
  sudo sysctl -w net.ipv4.ip_forward=1  # Enable IP forwarding

18. iptables for NAT (Network Address Translation)

• Purpose: Set up port forwarding or NAT.
• Usage:

  sudo iptables -t nat -A PREROUTING -p tcp --dport 80 -j DNAT --to-destination 192.168.1.100:80  # Port forwarding

19. bridge – Network Bridge Configuration

• Purpose: Create and manage network bridges.
• Usage:

  sudo ip link add name br0 type bridge  # Create a bridge
  sudo ip link set dev eth0 master br0  # Add interface to bridge
  sudo ip link set dev br0 up  # Bring the bridge up