Useful Linux Commands

Know Your Linux: Essential Commands for Ubuntu, CentOS, and Debian

System Information and Basic Commands:

1. Check Linux Version:

   • cat /etc/redhat-release - Check Linux version (for CentOS/RedHat).
   • hostnamectl - Display or change the system's hostname.
   • uname - Display system information (OS, kernel version).

2. Memory and Disk Usage:

   • cat /proc/meminfo - Check memory information.
   • df -Ph - Display disk space usage in human-readable format.
   • du -h /directory/ - Check disk usage of a directory.
   • du -hc --max-depth=1 /directory/ - Summary of disk usage.

File and Directory Operations:

1. Listing and Navigating Directories:

   • ls - List directory contents.
   • ls -R - List contents recursively (including subdirectories).
   • ls -a - Show hidden files.
   • ls -al - Detailed file information (permissions, size, etc.).
   • pwd - Display the current directory path.
   • cd /path/to/directory - Change directories.
   • cd .. - Move up one directory.
   • cd - - Go to the previous directory.
   • cd - Go to the home directory.

2. Creating and Removing Directories:

   • mkdir DirectoryName - Create a new directory.
   • rmdir DirectoryName - Remove an empty directory.
   • rm -r DirectoryName - Remove a directory and its contents.

3. File Operations:

   • touch filename.txt - Create a blank file.
   • cat filename.txt - View file contents.
   • cat > filename - Create a new file.
   • cp filename /path/to/destination/ - Copy files.
   • mv filename /new/location/ - Move or rename files.
   • ln -s target linkname - Create symbolic links between files.
   • chmod 755 filename - Change file permissions.
   • chown username filename - Change file ownership.

4. Comparing and Sorting Files:

   • diff file1 file2 - Compare files line by line.
   • cmp file1 file2 - Compare two files byte by byte.
   • comm file1 file2 - Compare two sorted files.
   • sort - Sort lines of a text file.

Advanced File and Archive Operations:

1. Archiving and Compression:

   • tar -cvf archive.tar /directory/ - Create an archive.
   • tar -xvf archive.tar - Extract an archive.
   • zip filename.zip files - Compress files into a .zip archive.
   • unzip filename.zip - Extract files from a .zip archive.

2. Concatenation and Case Conversion:

   • cat filename1 filename2 > filename3 - Merge two files.
   • cat filename | tr a-z A-Z > output.txt - Convert a file to uppercase.

Process and System Management:

1. Managing Users:

   • adduser newuser - Add a new user.
   • userdel newuser - Delete a user.
   • passwd newuser - Change a user's password.
   • usermod -aG sudo newuser - Add user to sudo group (Ubuntu/Debian).
   • usermod -aG wheel newuser - Add user to sudo group (CentOS).
   • groups newuser - Verify user's group.
   • su - newuser - Access the system as a new user.

2. Monitoring and Managing Processes:

   • ps - Display running processes.
   • kill PID - Terminate a process by its Process ID.
   • killall processname - Terminate all processes by name.
   • top - Monitor system resource usage in real time.

3. Services and Network:

   • service service_name start/stop/restart - Manage system services.
   • ifconfig - Configure or view network interfaces.
   • ping hostname - Test network connectivity to a host.
   • traceroute hostname - Trace the path packets take to a host.

Searching, Networking, and Miscellaneous:

1. Searching Files and Text:

   • locate filename - Find a file by name.
   • find /path -name filename.txt - Search for files within a directory.
   • grep "search_term" filename.txt - Search for patterns within files.
   • grep -r "search_term" /directory/ - Search recursively within a directory.

2. Networking and Downloads:

   • wget URL - Download files from the internet.
   • ssh user@host - Securely connect to a remote system.

3. System Updates and Package Management:

   • apt, yum, rpm, pacman - Install and manage software packages (varies by Linux distribution).
   • sudo command - Run commands with superuser privileges.
   • history - View command history of the current session.

Utilities and Miscellaneous:

1. File and Data Management:

   • df -Ph - Check disk space usage in human-readable format.
   • head -n 5 filename.txt - View the first five lines of a file.
   • tail -n 5 filename.txt - View the last five lines of a file.
   • cal - Display a calendar.

2. Firewall and Security:

   • ufw - Manage firewall rules (Ubuntu).
   • iptables - Manage packet filtering rules.

3. System and Command Information:

   • alias - Create shortcuts for commands.
   • wheris command - Locate the binary, source, and manual pages for a command.
   • whatis command - Display a one-line description of a command.

4. Low-level Operations:

   • dd if=input of=output - Convert and copy files at a low level.
   • export - Set environment variables for the current session.

Final Tips:

• Run Multiple Commands:

  • Use Command1; Command2 to run multiple commands sequentially.
  • Use Command1 && Command2 to run the second command only if the first is successful.

• Clear Terminal:

  • Use clear to clean the terminal screen.