Command-Line Basics: Bash, CMD, PowerShell (ls, grep, awk, sed, cron jobs)

 To master the basics of the command line, you'll want to familiarize yourself with three popular shells:

1. Bash (Bourne Again Shell) – Common in Linux and macOS.
2. CMD (Command Prompt) – Windows command-line interpreter.
3. PowerShell – A more advanced command-line interface for Windows, with scripting capabilities.

Here’s a breakdown of essential commands for each of these:

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1. Bash Basics (Linux/macOS)

• Navigating Directories:

  • ls: List directory contents.

    bash
    
    ls           # Lists files in current directory
    ls -l        # Lists files with detailed info (permissions, owner, etc.)
    ls -a        # List all files, including hidden files
    

  • cd <directory>: Change directory.

    bash
    
    cd /path/to/folder    # Navigate to a specific directory
    cd ~                  # Go to home directory
    cd ..                 # Move one directory up
    

• Searching Files:

  • grep <pattern> <file>: Search for a pattern inside a file.

    bash
    
    grep "error" log.txt      # Finds occurrences of "error" in log.txt
    

  • find: Search for files in a directory hierarchy.

    bash
    
    find . -name "test.txt"   # Finds test.txt in current directory and subdirectories
    

• Text Processing:

  • awk: Process text files (often used for data extraction).

    bash
    
    awk '{print $1}' file.txt   # Prints the first column of a file
    

  • sed: Stream editor for modifying files.

    bash
    
    sed 's/old/new/g' file.txt   # Replaces "old" with "new" globally in the file
    

• Automation (Cron Jobs):

  • Cron Jobs: Automate tasks on Linux/macOS (for scheduled tasks).
    • To list cron jobs:

      bash
      
      crontab -l
      

    • Edit cron jobs:

      bash
      
      crontab -e
      

    • A cron job example to run a script every day at midnight:

      bash
      
      0 0 * * * /path/to/script.sh
      

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2. CMD (Windows)

• Navigating Directories:

  • dir: List files in the current directory.

    cmd
    
    dir           # Lists files in the current directory
    dir /a        # Shows all files, including hidden
    

  • cd <directory>: Change directory.

    cmd
    
    cd C:\path\to\folder    # Navigate to a specific directory
    cd ..                   # Move one directory up
    

• Searching Files:

  • findstr <pattern> <file>: Search for a pattern inside a file.

    cmd
    
    findstr "error" log.txt  # Finds occurrences of "error" in log.txt
    

• Text Processing:

  • For basic text manipulation, you might use batch scripts with commands like for, set, and echo, but CMD lacks tools like awk and sed by default.

• Automation (Task Scheduler):

  • For scheduled tasks in Windows, use Task Scheduler instead of cron:
    • Open Task Scheduler > Create Task > Set triggers and actions.

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3. PowerShell Basics (Windows)

• Navigating Directories:

  • Get-ChildItem (alias ls): List files in the current directory.

    powershell
    
    Get-ChildItem       # Lists files in the current directory
    ls                  # Alias for Get-ChildItem
    

  • Set-Location <path> (alias cd): Change directory.

    powershell
    
    Set-Location C:\path\to\folder   # Navigate to a specific directory
    cd C:\path\to\folder            # Alias for Set-Location
    

• Searching Files:

  • Select-String <pattern> <file>: Search for a pattern inside a file.

    powershell
    
    Select-String "error" log.txt   # Finds occurrences of "error" in log.txt
    

• Text Processing:

  • PowerShell has more advanced text manipulation features than CMD.
  • Select-Object, Where-Object, and ForEach-Object are commonly used for filtering and processing text.

    powershell
    
    Get-Content file.txt | Select-String "error"  # Filters out lines with "error"
    

• Automation (Task Scheduler):

  • Similar to CMD, PowerShell can create and manage tasks through Task Scheduler.
    • Use New-ScheduledTask cmdlets to automate tasks.

    powershell
    
    New-ScheduledTaskTrigger -Daily -At "6:00AM" -Action { PowerShell -Command "C:\path\to\script.ps1" }
    

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Tips for Mastery:

1. Practice Regularly: Start using the terminal every day. You can practice by doing basic file operations, text processing, and automation tasks.
2. Learn Scripting: Both PowerShell and Bash support scripting. Learn how to write simple scripts to automate repetitive tasks.
3. Read Documentation: Use man <command> in Bash to read the manual pages for commands (e.g., man ls), or Get-Help <command> in PowerShell (e.g., Get-Help Get-ChildItem).
4. Experiment with Real Projects: Try creating cron jobs or task scheduler jobs to run scripts that monitor system health or perform backups.