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Linux Command Line Basics

1. ls (List)

• Purpose: Lists directory contents
• Usage: ls [options] [directory]
• Common options:
  • -l: Long format
  • -a: Show hidden files
  • -h: Human-readable file sizes

$ ls
Documents  Downloads  Pictures

$ ls -l
total 12
drwxr-xr-x 2 user user 4096 May 15 10:00 Documents
drwxr-xr-x 2 user user 4096 May 15 10:00 Downloads
drwxr-xr-x 2 user user 4096 May 15 10:00 Pictures

2. pwd (Print Working Directory)

• Purpose: Displays the current working directory
• Usage: pwd

$ pwd
/home/user

3. cd (Change Directory)

• Purpose: Changes the current working directory
• Usage: cd [directory]
• Special directories:
  • ..: Parent directory
  • .: Current directory
  • ~: Home directory

$ pwd
/home/user
$ cd Documents
$ pwd
/home/user/Documents
$ cd ..
$ pwd
/home/user

4. mkdir (Make Directory)

• Purpose: Creates new directories
• Usage: mkdir [options] directory_name
• Common option:
  • -p: Create parent directories as needed

$ mkdir new_folder
$ ls
Documents  Downloads  new_folder  Pictures

$ mkdir -p parent/child/grandchild
$ ls -R parent
parent:
child

parent/child:
grandchild

parent/child/grandchild:

5. mv (Move)

• Purpose: Moves or renames files and directories
• Usage: mv [options] source destination

$ touch file.txt
$ ls
file.txt
$ mv file.txt newfile.txt
$ ls
newfile.txt
$ mv newfile.txt Documents/
$ ls Documents
newfile.txt

6. cp (Copy)

• Purpose: Copies files and directories
• Usage: cp [options] source destination
• Common option:
  • -r: Copy directories recursively

$ cp newfile.txt backup.txt
$ ls
backup.txt  newfile.txt

$ cp -r Documents Docs_backup
$ ls
backup.txt  Docs_backup  Documents  newfile.txt

7. rm (Remove)

• Purpose: Removes files and directories
• Usage: rm [options] file/directory
• Common options:
  • -r: Remove directories and their contents recursively
  • -f: Force removal without prompting

$ rm backup.txt
$ ls
Docs_backup  Documents  newfile.txt

$ rm -r Docs_backup
$ ls
Documents  newfile.txt

8. touch

• Purpose: Creates empty files or updates file timestamps
• Usage: touch file_name

$ touch new_file.txt
$ ls -l
total 0
-rw-r--r-- 1 user user 0 May 15 11:00 new_file.txt

9. ln (Link)

• Purpose: Creates links between files
• Usage: ln [options] target link_name
• Common option:
  • -s: Create a symbolic link

$ ln -s /path/to/file.txt link_to_file
$ ls -l
total 0
lrwxrwxrwx 1 user user 17 May 15 11:05 link_to_file -> /path/to/file.txt

10. clear

• Purpose: Clears the terminal screen
• Usage: clear

$ clear
# Terminal screen is now cleared

Remember to practice these commands regularly to become proficient in using the Linux command line interface!

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More Linux Command Line Tools

Let’s expand our Linux command-line knowledge with these crucial tools:

1. cat (Concatenate)

• Purpose: Displays the contents of files on the terminal
• Usage: cat [options] file1 file2 ...
• Common options:
  • -n: Number lines
  • -b: Number non-empty lines

$ cat file.txt
This is the content of file.txt

$ cat -n file.txt
     1  This is the content of file.txt

2. echo (Echo)

• Purpose: Prints any text that follows the command
• Usage: echo [options] string
• Common options:
  • -e: Enable interpretation of backslash escapes
  • -n: Do not print a trailing newline

$ echo "Hello, world!"
Hello, world!

$ echo -e "Hello, \nworld!"
Hello,
world!

3. less (Less)

• Purpose: Displays paged outputs in the terminal, allowing you to scroll through large files or outputs
• Usage: less [options] file
• Navigation:
  • Space: Scroll down one page
  • b: Scroll up one page
  • Enter: Scroll down one line
  • q: Quit less

$ less large_file.txt
# You can now scroll through the file using the navigation keys.

4. man (Manual)

• Purpose: Access manual pages for all Linux commands
• Usage: man [command]

$ man ls
# Displays the manual page for the 'ls' command.

5. uname (Unix Name)

• Purpose: Gets basic information about the operating system
• Usage: uname [options]
• Common options:
  • -a: Display all information
  • -s: Display the kernel name

$ uname -a
Linux hostname 5.10.0-10-amd64 #21~20.04.1-Ubuntu SMP Fri Jul 1 17:06:08 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux

$ uname -s
Linux

6. whoami (Who Am I)

• Purpose: Displays the current user’s username
• Usage: whoami

$ whoami
user

7. tar (Tape Archive)

• Purpose: Creates, extracts, and manipulates archive files
• Usage: tar [options] [archive-file] [files]
• Common options:
  • -c: Create archive
  • -x: Extract archive
  • -t: List archive contents
  • -z: Use gzip compression
  • -f: Specify archive file

$ tar -czvf my_archive.tar.gz file1.txt file2.txt
# Creates a compressed archive named 'my_archive.tar.gz' containing 'file1.txt' and 'file2.txt'

$ tar -xzvf my_archive.tar.gz
# Extracts the contents of the archive.

8. grep (Global Regular Expression Print)

• Purpose: Searches for lines matching a regular expression
• Usage: grep [options] pattern [file]
• Common options:
  • -i: Ignore case
  • -v: Invert the match (show lines that don’t match)

$ grep "hello" file.txt
# Displays lines in 'file.txt' that contain "hello".

$ grep -i "world" file.txt
# Displays lines containing "world" regardless of case.

9. head (Head)

• Purpose: Displays the first specified number of lines from a file
• Usage: head [options] [number] file
• Common option:
  • -n: Specify the number of lines to display

$ head file.txt
# Displays the first 10 lines of 'file.txt'.

$ head -n 5 file.txt
# Displays the first 5 lines of 'file.txt'.

10. tail (Tail)

• Purpose: Displays the last specified number of lines from a file
• Usage: tail [options] [number] file
• Common option:
  • -n: Specify the number of lines to display

$ tail file.txt
# Displays the last 10 lines of 'file.txt'.

$ tail -n 5 file.txt
# Displays the last 5 lines of 'file.txt'.

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Advanced Linux Command Line Tools

Let’s dive deeper into the world of Linux command-line tools with these advanced utilities:

1. diff (Difference)

• Purpose: Finds the difference between two files
• Usage: diff [options] file1 file2
• Common options:
  • -u: Unified output format
  • -c: Context output format
  • -q: Quiet output, only reports if files differ

$ diff file1.txt file2.txt
# Displays the differences between 'file1.txt' and 'file2.txt'.

$ diff -u file1.txt file2.txt
# Displays the differences in unified format.

2. cmp (Compare)

• Purpose: Compares two files and reports if they are identical
• Usage: cmp [options] file1 file2
• Common option:
  • -s: Silent output, only reports if files differ

$ cmp file1.txt file2.txt
# Reports if 'file1.txt' and 'file2.txt' are identical.

$ cmp -s file1.txt file2.txt
# Silently reports if 'file1.txt' and 'file2.txt' differ.

3. comm (Common)

• Purpose: Combines the functionality of diff and cmp to compare two files
• Usage: comm [options] file1 file2
• Common options:
  • -1: Suppress lines unique to file1
  • -2: Suppress lines unique to file2
  • -3: Suppress lines common to both files

$ comm file1.txt file2.txt
# Displays lines unique to 'file1.txt', 'file2.txt', and common to both.

$ comm -1 file1.txt file2.txt
# Suppresses lines unique to 'file1.txt'.

4. sort (Sort)

• Purpose: Sorts the content of a file while outputting
• Usage: sort [options] file
• Common options:
  • -n: Sort numerically
  • -r: Reverse sort
  • -u: Unique sort

$ sort file.txt
# Sorts the content of 'file.txt' alphabetically.

$ sort -n file.txt
# Sorts the content of 'file.txt' numerically.

5. export (Export)

• Purpose: Exports environment variables in Linux
• Usage: export [variable_name]=[value]
• Common usage:
  • export PATH=$PATH:/new/path

$ export MY_VAR="Hello, world!"
# Exports the environment variable 'MY_VAR' with the value "Hello, world!".

$ echo $MY_VAR
Hello, world!

6. zip (Zip)

• Purpose: Zips files in Linux
• Usage: zip [options] archive_name file1 file2 ...
• Common options:
  • -r: Recursively zip directories
  • -q: Quiet output

$ zip my_archive.zip file1.txt file2.txt
# Zips 'file1.txt' and 'file2.txt' into 'my_archive.zip'.

$ zip -r my_archive.zip directory/
# Recursively zips the contents of 'directory/' into 'my_archive.zip'.

7. unzip (Unzip)

• Purpose: Unzips files in Linux
• Usage: unzip [options] archive_name
• Common options:
  • -q: Quiet output
  • -d: Extract to a specific directory

$ unzip my_archive.zip
# Unzips the contents of 'my_archive.zip' to the current directory.

$ unzip -d /path/to/directory my_archive.zip
# Unzips the contents of 'my_archive.zip' to '/path/to/directory'.

8. ssh (Secure Shell)

• Purpose: Establishes a secure connection to a remote server
• Usage: ssh [options] user@host
• Common options:
  • -p: Specify the port number
  • -i: Specify the identity file

$ ssh user@remote_host
# Establishes a secure connection to 'remote_host' as 'user'.

$ ssh -p 2222 user@remote_host
# Establishes a secure connection to 'remote_host' on port 2222.

9. service (Service)

• Purpose: Manages system services (start, stop, restart, etc.)
• Usage: service [service_name] [action]
• Common actions:
  • start: Starts the service
  • stop: Stops the service
  • restart: Restarts the service

$ service httpd start
# Starts the 'httpd' service.

$ service httpd stop
# Stops the 'httpd' service.

10. ps (Process Status)

• Purpose: Displays active processes
• Usage: ps [options]
• Common options:
  • -a: Displays all processes
  • -u: Displays processes owned by the specified user
  • -x: Displays processes without a controlling terminal

$ ps -a
# Displays all active processes.

$ ps -u user
# Displays processes owned by 'user'.

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System Commands

1. kill (Kill a Process)

• Purpose: Sends a signal to a process to terminate it.
• Usage: kill [signal] [PID]
• Common signals:
  • SIGTERM (15): Gracefully stop a process.
  • SIGKILL (9): Immediately terminate a process without cleanup.
  • SIGHUP (1): Reload a process.

$ kill -9 1234
# Immediately terminates the process with PID 1234.

$ kill -TERM 1234
# Gracefully stops the process with PID 1234.

$ kill -l
# Lists all available signals.

To find the PID, you can use commands like ps, pgrep, or pidof.

2. killall (Kill Processes by Name)

• Purpose: Terminates processes by their name.
• Usage: killall [signal] process_name
• Common usage:
  • killall -9 firefox
    • Immediately terminates all instances of the Firefox process.

$ killall firefox
# Terminates all Firefox processes with the default SIGTERM signal.

$ killall -9 firefox
# Immediately terminates all Firefox processes.

This command is useful for killing multiple processes with the same name without needing to specify each PID.

3. df (Display Disk Filesystem Information)

• Purpose: Displays information about disk space usage.
• Usage: df [options]
• Common options:
  • -h: Human-readable format.
  • -T: Display file system type.

$ df -h
# Displays disk space usage in a human-readable format.

$ df -T
# Displays the type of each file system along with usage information.

This command helps you understand how much disk space is used and available on each mounted file system.

4. mount (Mount File Systems)

• Purpose: Attaches a file system to a directory.
• Usage: mount [options] device_name mount_point
• Common options:
  • -t: Specify the file system type.
  • -o: Specify mount options.

$ mount /dev/sdb1 /mnt
# Mounts the device /dev/sdb1 to the /mnt directory.

$ mount -t ext4 /dev/sdb1 /mnt
# Mounts an ext4 file system from /dev/sdb1 to /mnt.

This command is essential for making file systems available for use by the operating system.

5. chmod (Change File Permissions)

• Purpose: Changes the permissions of files or directories.
• Usage: chmod [permissions] file/directory
• Common permissions:
  • u: User
  • g: Group
  • o: Other
  • r: Read
  • w: Write
  • x: Execute

$ chmod 755 file.txt
# Sets the permissions to rwxr-x (owner has full permissions, group and others have read and execute).

$ chmod u+x file.txt
# Adds execute permission for the owner.

This command is used to manage access control for files and directories.

6. chown (Change Ownership)

• Purpose: Changes the ownership of files or directories.
• Usage: chown [user]:[group] file/directory
• Common usage:
  • chown user file.txt
    • Changes the owner of the file to the specified user.
  • chown user:group file.txt
    • Changes both the owner and group of the file.

$ chown user file.txt
# Changes the owner of 'file.txt' to 'user'.

$ chown user:group file.txt
# Changes the owner and group of 'file.txt' to 'user' and 'group', respectively.

This command is used to manage file and directory ownership.

7. ifconfig (Display Network Interfaces)

• Note: ifconfig is deprecated in favor of ip command in modern Linux systems.
• Purpose: Displays network interface information.
• Usage: ifconfig [interface]
• Common usage:
  • ifconfig eth0
    • Displays information about the eth0 interface.

$ ifconfig eth0
# Displays information about the eth0 network interface.

For modern systems, use the ip command instead:

$ ip addr show
# Displays detailed information about all network interfaces.

8. traceroute (Trace Network Hops)

• Purpose: Traces the route of packets to a network host.
• Usage: traceroute [options] host
• Common options:
  • -i: Specify the network interface.
  • -n: Suppress DNS lookups.

$ traceroute example.com
# Traces the network path to example.com.

$ traceroute -n example.com
# Traces the network path to example.com without performing DNS lookups.

This command helps in diagnosing network issues by showing the path packets take to reach a destination.

9. wget (Download Files)

• Purpose: Downloads files from the internet.
• Usage: wget [options] URL
• Common options:
  • -b: Run in the background.
  • -o: Specify the output file name.

$ wget https://example.com/file.txt
# Downloads the file from the specified URL.

$ wget -b https://example.com/file.txt
# Downloads the file in the background.

This command is useful for downloading files from the web directly from the command line.

10. ufw (Uncomplicated Firewall)

• Purpose: Manages the firewall rules in a simple way.
• Usage: ufw [options]
• Common options:
  • enable: Enables the firewall.
  • disable: Disables the firewall.
  • allow: Allows traffic on a specific port or service.
  • deny: Denies traffic on a specific port or service.

$ ufw enable
# Enables the firewall.

$ ufw allow 80
# Allows HTTP traffic on port 80.

$ ufw deny 22
# Denies SSH traffic on port 22.

This command is used to manage firewall rules in Ubuntu and other compatible systems.

11. iptables (Base Firewall)

• Purpose: The base firewall utility that other firewalls interface with.
• Usage: iptables [options]
• Common options:
  • -A: Append a rule to a chain.
  • -D: Delete a rule from a chain.
  • -L: List the rules in a chain.

$ iptables -A INPUT -p tcp --dport 80 -j ACCEPT
# Allows HTTP traffic on port 80.

$ iptables -D INPUT -p tcp --dport 80 -j ACCEPT
# Deletes the rule allowing HTTP traffic on port 80.

$ iptables -L
# Lists all the current firewall rules.

This command is used to manage the underlying firewall rules and is often used by other firewall utilities like ufw.

Understanding and using these commands will help you manage various aspects of your Linux system, from process management and file system operations to network diagnostics and firewall configuration.

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Package Managers

1. apt (Advanced Package Tool)

• Purpose: Package manager for Debian-based systems (e.g., Ubuntu).
• Usage: apt [options] command
• Common commands:
  • install: Install packages.
  • remove: Remove packages.
  • update: Update the package list.
  • upgrade: Upgrade installed packages.

$ sudo apt update
# Updates the package list.

$ sudo apt install package_name
# Installs the specified package.

$ sudo apt remove package_name
# Removes the specified package.

2. pacman (Package Manager)

• Purpose: Package manager for Arch Linux.
• Usage: pacman [options] command
• Common commands:
  • -S: Synchronize packages.
  • -R: Remove packages.
  • -Syu: Sync and update the system.

$ sudo pacman -Syu
# Synchronizes the package database and updates the system.

$ sudo pacman -S package_name
# Installs the specified package.

$ sudo pacman -R package_name
# Removes the specified package.

3. yum (Yum)

• Purpose: Package manager for Red Hat-based systems (e.g., CentOS, Fedora).
• Usage: yum [options] command
• Common commands:
  • install: Install packages.
  • remove: Remove packages.
  • update: Update installed packages.

$ sudo yum update
# Updates all installed packages.

$ sudo yum install package_name
# Installs the specified package.

$ sudo yum remove package_name
# Removes the specified package.

4. rpm (RPM Package Manager)

• Purpose: Package manager for Red Hat-based systems.
• Usage: rpm [options]
• Common options:
  • -i: Install a package.
  • -e: Remove a package.
  • -q: Query the package database.

$ sudo rpm -i package_name.rpm
# Installs the specified RPM package.

$ sudo rpm -e package_name
# Removes the specified package.

$ rpm -q package_name
# Queries the package database for information about the specified package.

System and User Management

5. sudo (Superuser Do)

• Purpose: Allows a permitted user to execute a command as the superuser.
• Usage: sudo [command]

$ sudo apt update
# Executes the 'apt update' command with superuser privileges.

$ sudo useradd new_user
# Adds a new user with superuser privileges.

6. cal (Calendar)

• Purpose: Displays a calendar.
• Usage: cal [options]

$ cal
# Displays the current month's calendar.

$ cal 2023
# Displays the calendar for the entire year 2023.

7. alias (Alias)

• Purpose: Creates custom shortcuts for regularly used commands.
• Usage: alias name='command'

$ alias ll='ls -l'
# Creates an alias 'll' for the 'ls -l' command.

$ alias gs='git status'
# Creates an alias 'gs' for the 'git status' command.

8. dd (Disk Dump)

• Purpose: Copies data from one file to another, often used for creating bootable USB sticks.
• Usage: dd [options] if=input_file of=output_file
• Common options:
  • if: Input file.
  • of: Output file.
  • bs: Block size.

$ sudo dd if=/path/to/image.iso of=/dev/sdX bs=4M
# Creates a bootable USB stick from an ISO image.

9. whereis (Where Is)

• Purpose: Locates the binary, source, and manual pages for a command.
• Usage: whereis [command]

$ whereis ls
# Displays the path to the 'ls' binary and its manual page.

10. whatis (What Is)

• Purpose: Displays a brief description of a command.
• Usage: whatis [command]

$ whatis ls
# Displays a brief description of the 'ls' command.

11. top (Top)

• Purpose: Displays active processes live with their system usage.
• Usage: top

$ top
# Opens the top utility, displaying system processes and their resource usage.

12. useradd (User Add)

• Purpose: Adds a new user.
• Usage: useradd [options] username
• Common options:
  • -m: Create the user’s home directory.
  • -s: Specify the user’s shell.

$ sudo useradd -m new_user
# Adds a new user with a home directory.

13. usermod (User Modify)

• Purpose: Modifies existing user data.
• Usage: usermod [options] username
• Common options:
  • -aG: Add the user to supplementary groups.
  • -s: Change the user’s shell.

$ sudo usermod -aG sudo new_user
# Adds the user to the 'sudo' group.

$ sudo usermod -s /bin/bash new_user
# Changes the user's shell to bash.

14. passwd (Password)

• Purpose: Creates or updates passwords for existing users.
• Usage: passwd [username]

$ sudo passwd new_user
# Sets a new password for the specified user.

$ passwd
# Changes the password for the current user.

15. systemctl (System Control)

$ sudo systemctl start httpd
# Starts the 'httpd' service.

$ sudo systemctl stop httpd
# Stops the 'httpd' service.

$ sudo systemctl enable httpd
# Enables the 'httpd' service to start automatically at boot.

16. journalctl (System Journal)

• Purpose: Accesses the system journal, which logs system events.
• Usage: journalctl [options]
• Common options:
  • -f: Follow the log output.
  • -u: Filter by unit name.
  • -p: Filter by priority level.

$ journalctl -f
# Displays the system journal and follows the log output.

$ journalctl -u httpd
# Displays the journal entries related to the 'httpd' service.

17. free (Free Memory)

• Purpose: Displays information about available memory.
• Usage: free [options]
• Common options:
  • -h: Human-readable format.

$ free -h
# Displays memory usage in a human-readable format.

18. du (Disk Usage)

• Purpose: Displays disk space usage for files and directories.
• Usage: du [options] file/directory
• Common options:
  • -h: Human-readable format.
  • -a: Display disk usage for all files.

$ du -h /home
# Displays disk usage for the '/home' directory in a human-readable format.

$ du -a /home
# Displays disk usage for all files and directories within '/home'.

These commands are essential for managing packages, system resources, and user accounts on a Linux system. Practice using them to become proficient in administering your environment.

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