Linux Commands for DevOps Engineers

Here’s a list of basic Linux commands for beginners, tailored for aspiring DevOps engineers. Each command is followed by a simple explanation:

1. pwd (Print Working Directory)

• Usage: pwd

• Explanation: Displays the current directory you're in. It helps you confirm where you are in the file system.

2. ls (List Directory Contents)

• Usage: ls

• Common options:

  • ls -l: Lists files in long format (shows permissions, size, etc.).

  • ls -a: Lists all files, including hidden ones (those starting with .).

• Explanation: Lists all files and directories in your current directory. Useful for navigating and viewing file structure.

3. cd (Change Directory)

• Usage: cd [directory]

  • Example: cd /var/log

• Explanation: Moves you into the specified directory. Essential for navigating through the file system.

4. touch (Create a New File)

• Usage: touch [filename]

  • Example: touch myfile.txt

• Explanation: Creates a new, empty file. Useful for quickly creating files.

5. mkdir (Make Directory)

• Usage: mkdir [directory name]

  • Example: mkdir mynewfolder

• Explanation: Creates a new directory/folder. Helps organize files.

6. rm (Remove File or Directory)

• Usage:

  • rm [file]: Remove a file.

  • rm -r [directory]: Remove a directory and its contents recursively.

  • Example: rm -r myfolder

• Explanation: Deletes files or directories. Be careful when using this command since it’s irreversible.

7. cp (Copy Files or Directories)

• Usage:

  • cp [source] [destination]: Copy a file.

  • cp -r [source] [destination]: Copy a directory recursively.

  • Example: cp file.txt /backup/file.txt

• Explanation: Copies files or directories from one location to another.

8. mv (Move or Rename Files)

• Usage:

  • mv [source] [destination]: Move or rename a file or directory.

  • Example: mv file.txt /backup/ or mv oldname.txt newname.txt

• Explanation: Moves files from one location to another, or renames files/directories.

9. cat (Concatenate and Display File Content)

• Usage: cat [filename]

  • Example: cat myfile.txt

• Explanation: Displays the content of a file in the terminal. Good for quickly viewing files.

10. nano or vim (Text Editors)

• Usage:

  • nano [filename]: Opens a file in the Nano text editor.

  • vim [filename]: Opens a file in the Vim text editor.

• Explanation: Text editors in the terminal for editing configuration files, scripts, etc.

11. grep (Search for Patterns in Files)

• Usage: grep [pattern] [file]

  • Example: grep 'error' /var/log/syslog

• Explanation: Searches for a specific pattern or string in files. Very useful for searching logs or large text files.

12. find (Find Files and Directories)

• Usage: find [path] -name [filename]

  • Example: find / -name "myfile.txt"

• Explanation: Finds files and directories in the file system based on their name or other properties.

13. chmod (Change File Permissions)

• Usage: chmod [permissions] [file]

  • Example: chmod 755 script.sh

• Explanation: Changes the permissions of a file or directory. The 755 means read, write, and execute for the owner, and read and execute for others.

14. ps (Process Status)

• Usage: ps or ps aux

  • Example: ps aux | grep apache

• Explanation: Displays the running processes on the system. ps aux shows all processes with detailed information. Use with grep to filter for specific processes.

15. kill (Terminate a Process)

• Usage: kill [PID]

  • Example: kill 12345

• Explanation: Terminates a process by its Process ID (PID). Used to stop unresponsive programs.

16. df (Disk Usage)

• Usage: df -h

• Explanation: Shows the available disk space on mounted file systems. The -h option makes the output human-readable (in GB, MB, etc.).

17. du (Disk Usage of Files/Directories)

• Usage: du -h [path]

  • Example: du -h /var/log

• Explanation: Shows the disk space used by files and directories. The -h option provides human-readable sizes.

18. tar (Archive Files)

• Usage:

  • To create an archive: tar -cvf [archive.tar] [files]

  • To extract an archive: tar -xvf [archive.tar]

  • Example: tar -cvf myarchive.tar myfolder/

• Explanation: Compresses or extracts files into/from an archive. Very useful for backups and transferring multiple files.

• Creating a Tar Archive

  1. Open Your Terminal.

  2. Navigate to the Directory: Change to the directory containing the files you want to archive. For example:

      bashCopy codecd /path/to/directory
     

  3. Create a Tar Archive: Use the following command to create a tar archive named archive.tar from files file1.txt and file2.txt:

      bashCopy codetar -cvf archive.tar file1.txt file2.txt
     

     • -c: Create a new archive.

     • -v: Verbose output (lists files being processed).

     • -f: Specifies the filename of the archive.

Extracting a Tar Archive

To extract the contents of archive.tar, use the following command:

    bashCopy codetar -xvf archive.tar

• -x: Extract files from the archive.

• -v: Verbose output.

• -f: Specifies the filename of the archive.

Example Summary

• Create Archive:

    bashCopy codetar -cvf archive.tar file1.txt file2.txt
  

• Extract Archive:

    bashCopy codetar -xvf archive.tar
  

19. ssh (Secure Shell)

• Usage: ssh [user]@[server_ip]

  • Example: ssh user@192.168.1.10

• Explanation: Securely connects to a remote server via SSH. This is one of the key tools for DevOps engineers to manage remote infrastructure.

• Connecting to an EC2 Instance via SSH

  1. Open Terminal: Open your terminal on your local machine.

  2. Navigate to Key Pair: Change to the directory where your .pem file is stored:

      bashCopy codecd /path/to/your/keypair/
     

  3. Set Key Permissions: Ensure your key pair file has the correct permissions:

      bashCopy codechmod 400 your-key-pair.pem
     

  4. Connect to EC2 Instance: Use the SSH command to connect. Replace the placeholders with your values:

     

  5. Accept the Host Key: Type yes if prompted to confirm the authenticity of the host.

Summary

• Use SSH to securely access your EC2 instance using your key pair.

20. sudo (Run as Superuser)

• Usage: sudo [command]

  • Example: sudo apt update

• Explanation: Runs commands with superuser (root) privileges. Always use with caution, as it allows you to make system-wide changes.

21. apt or yum (Package Managers)

• Usage:

  • apt: For Ubuntu/Debian-based systems.

  • yum: For CentOS/RedHat-based systems.

  • Example: sudo apt install nginx or sudo yum install nginx

• Explanation: Manages the installation, update, and removal of software packages.

22. systemctl (Manage System Services)

• Usage:

  • systemctl start [service]: Starts a service.

  • systemctl stop [service]: Stops a service.

  • systemctl restart [service]: Restarts a service.

  • Example: sudo systemctl restart nginx

• Explanation: Used to start, stop, or manage system services like web servers, databases, etc.

23. top (Real-Time Process Monitoring)

• Usage: top

• Explanation: Displays real-time system information such as CPU, memory usage, and running processes. Great for monitoring system health.

24. crontab (Schedule Tasks)

• Usage: crontab -e

• Explanation: Opens the cron scheduler to define automated tasks (jobs) that run at specified intervals. Essential for automation in DevOps.

• Using crontab

  1. Open Your Terminal:

  2. Edit Your Crontab: To edit your crontab file, use the following command:

      bashCopy codecrontab -e
     

  3. Crontab Syntax: Each line in the crontab file represents a scheduled task and follows this syntax:

      markdownCopy code* * * * * command_to_run
     

     The five asterisks represent the following time fields:

     • Minute (0-59)

     • Hour (0-23)

     • Day of the Month (1-31)

     • Month (1-12)

     • Day of the Week (0-7) (Sunday can be represented as 0 or 7)

For example, to run a script every day at 5:30 AM, you would write:

        javascriptCopy code30 5 * * * /path/to/your/script.sh

4. Common Examples:

   • Run a script every minute:

       bashCopy code* * * * * /path/to/your/script.sh
     

   • Run a backup script every day at 2 AM:

       bashCopy code0 2 * * * /path/to/backup.sh
     

   • Run a command every Sunday at midnight:

       bashCopy code0 0 * * 0 /path/to/your/script.sh
     

5. View Your Crontab: To view the scheduled tasks in your crontab, run:

    bashCopy codecrontab -l
   

6. Remove Your Crontab: If you want to delete all your scheduled tasks, use:

    bashCopy codecrontab -r
   

Example Summary

• Command to Edit Crontab:

    bashCopy codecrontab -e
  

• Example Crontab Entry:

    bashCopy code30 5 * * * /path/to/your/script.sh  # Runs the script every day at 5:30 AM