Linux Command Line Basics: Quick Reference and Operations Guide

Linux Command Line Basics

The Linux command line is a core tool for system administration. A command's basic syntax is:

command [options] [arguments] [path]

Square brackets [] indicate optional parts. If the path is omitted, the command operates on the current working directory.

Path Notation

Paths are either absolute or relative:

• Absolute path: Starts from the root directory /. Example: /home/shine refers to the shine subdirectory inside /home.
• Relative path: Starts from the current working directory. Example: If the current directory is /home, then shine/doc corresponds to the absolute path /home/shine/doc.

Common special symbols:

• . represents the current directory.
• .. represents the parent directory.
• ~ represents the current user's home directory (e.g., /home/shine).

Getting Command Help

If you are unsure how to use a command, consult its help:

• man command_name: View the full manual page.
• command_name --help or -h: Quickly view common options (e.g., ls --help).

Interrupting and Exiting Commands

During command execution, you can interrupt or exit using:

• Ctrl+C: Terminate the currently running foreground command.
• Ctrl+D: Send EOF (End-of-File), often used to exit the current shell or end input.
• exit, logout: Exit the current shell session.

Common Command Quick Reference

File and Directory Operations

• cd [path]: Change working directory.
  • cd / switches to the root directory.
  • cd or cd ~ returns to the current user's home directory.
• ls [options] [path]: List directory contents.
  • ls -a: Show all files (including hidden ones).
  • ls -l: Display in detailed list format.
• mkdir [-p] directory_name: Create a directory. The -p option creates parent directories if they don't exist.
• rm [options] file_or_directory: Remove files or directories.
  • rm -r: Recursively delete a directory and its contents.
  • rm -f: Force deletion without confirmation.
• mv source destination: Move or rename files/directories.
• cp [options] source destination: Copy files or directories.
  • cp -r: Recursively copy a directory and its contents.
• pwd: Print the absolute path of the current working directory.

Viewing and Editing File Content

• cat filename: Display the entire file content.
• less filename: View file content page by page (supports scrolling, searching).
• echo text: Output text or variable values (e.g., echo $PATH).
• nano or vi/vim: Text editors. Beginners may start with nano; vi/vim is more powerful but has a steeper learning curve.

File Permissions and Attributes

• chmod [options] permissions file: Change file permissions (e.g., chmod 755 script.sh).
• chown [options] user:group file: Change file owner and group.
• ln [-s] source_file link_file: Create a link. The -s option creates a soft (symbolic) link.

Compression and Extraction

• tar xvf file.tar: Extract a .tar file.
• tar xzvf file.tar.gz: Extract a .tar.gz or .tgz file.
• tar xjvf file.tar.bz2: Extract a .tar.bz2 file.
• unzip file.zip: Extract a ZIP file (requires unzip package).
• unrar x file.rar: Extract a RAR file (requires unrar package).

System Status Monitoring

• df -h: Display disk space usage in a human-readable format.
• du -sh [directory_or_file]: Check the size of a specified directory or file.
• top or htop: Dynamically view process status and system resource usage.
• free -h: Display memory usage.

Package Management

• RHEL/CentOS/Fedora: Use yum or dnf (newer versions).
• Debian/Ubuntu: Use apt or apt-get.

Other Useful Commands

• grep [pattern] file: Search for a pattern in a file.
• find path -name filename: Find files.
• sort file: Sort file contents.
• export VARIABLE=value: Set an environment variable.
• su [username]: Switch user (default is root).
• passwd [username]: Change a user's password.
• who or whoami: Show information about currently logged-in users.

Mastering these basic commands will enable you to perform daily operations and system administration efficiently in the Linux terminal. Practice regularly and use man and --help for detailed assistance.