Advanced Linux Shell Scripting for DevOps Engineers

Shell scripting is an essential skill for system administrators, developers, and anyone working with Linux/Unix-based systems. It allows you to automate repetitive tasks, manipulate files, and enhance productivity by writing small programs. Let’s dive into Shell scripting, from the basics to advanced topics.

1. What is Shell?

The shell is a command-line interpreter that provides a way for users to interact with the operating system. Some popular shells are:

• Bash (Bourne Again Shell) – Default on most Linux systems.

• Zsh – A more feature-rich shell.

• Ksh (KornShell) – Useful in many Unix systems.

In shell scripting, we write commands in a file (script) which the shell can interpret.

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2. Getting Started with a Simple Shell Script

1. Creating a Script: Create a text file and include commands you want to execute.

     #!/bin/bash
    echo "Hello, World!"
   

2. Execution:

   • Make the script executable: chmod +x script_name.sh

   • Run the script: ./script_name.sh

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3. Basic Shell Commands in Scripts

• echo: Prints text to the terminal.

    echo "This is a message"
  

• Variables: Store data for reuse.

    NAME="John"
    echo "My name is $NAME"
  

• Command Substitution: Capture the output of commands.

    CURRENT_DATE=$(date)
    echo "Today is $CURRENT_DATE"
  

• Comments: Use # for comments.

    # This is a comment
  

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4. Conditional Statements (if-else)

If-Else Structure

• Syntax:

    if [ condition ]; then
      # Commands
    elif [ another_condition ]; then
      # Commands
    else
      # Commands
    fi
  

• Example:

    AGE=25
    if [ $AGE -ge 18 ]; then
      echo "Adult"
    else
      echo "Minor"
    fi
  

Comparison Operators:

• -eq (equal), -ne (not equal)

• -lt (less than), -gt (greater than)

• -le (less than or equal to), -ge (greater than or equal to)

String Comparison:

• = (equal), != (not equal), -z (empty string)

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5. Loops

For Loop:

• Syntax:

    for item in list; do
      # Commands
    done
  

• Example:

    for i in {1..5}; do
      echo "Iteration $i"
    done
  

• While Loop:

• Syntax:

    while [ condition ]; do
      # Commands
    done
  

• Example:

    COUNTER=1
    while [ $COUNTER -le 5 ]; do
      echo "Count: $COUNTER"
      ((COUNTER++))
    done
  

Until Loop:

• Syntax:

    until [ condition ]; do
      # Commands
    done
  

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6. Functions in Shell Scripts

Functions allow us to write reusable blocks of code.

• Syntax:

    codefunction_name() {
      # Commands
    }
  

• Example:

    codegreet() {
      echo "Hello, $1!"
    }
  
    greet "Alice"
  

  In this example, $1 represents the first argument passed to the function.

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7. Handling Input and Output

Reading Input from the User:

read -p "Enter your name: " NAME
echo "Welcome, $NAME"

Redirecting Output:

• >: Redirects output to a file (overwrites).

• >>: Appends output to a file.

    echo "This is a log entry" >> logfile.txt
  

Redirecting Errors:

• 2>: Redirects standard error.

    ls nonexistentfile 2> error.log
  

Piping: Passes output of one command as input to another.

ls | grep "filename"

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8. Advanced Shell Scripting

Arrays in Shell:

• Defining an Array:

    MY_ARRAY=("item1" "item2" "item3")
  

• Accessing Elements:

    echo ${MY_ARRAY[1]}  # Output: item2
  

• Looping Through Arrays:

    for ITEM in "${MY_ARRAY[@]}"; do
      echo $ITEM
    done
  

Case Statements:

• Syntax:

    case $variable in
      pattern1)
        # Commands ;;
      pattern2)
        # Commands ;;
      *)
        # Default commands ;;
    esac
  

• Example:

    read -p "Enter a number between 1 and 3: " NUMBER
    case $NUMBER in
      1)
        echo "You entered one" ;;
      2)
        echo "You entered two" ;;
      3)
        echo "You entered three" ;;
      *)
        echo "Invalid number" ;;
    esac
  

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9. File Handling

• Creating and Writing Files:

    echo "This is a test" > testfile.txt
  

• Reading Files:

    while read line; do
      echo $line
    done < filename.txt
  

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10. Trap Command

Trap allows you to catch signals and perform cleanups or special commands when the script is interrupted.

• Syntax:

    trap "commands" signal
  

• Example:

    trap "echo 'Caught CTRL+C'; exit" SIGINT
  

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11. Working with Cron Jobs

Shell scripts can be automated using cron.

• Cron Job Format:

    * * * * * /path/to/script.sh
  

  • The format represents minute hour day-of-month month day-of-week.

• Example (Every day at 5 PM):

    0 17 * * * /path/to/backup.sh
  

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12. Debugging Shell Scripts

• Using set command:

  • set -x: Enables debugging (prints commands before execution).

  • set +x: Disables debugging.

• Example:

    set -x
    echo "This will be debugged"
    set +x
    echo "This will not"
  

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13. Shell Scripting Best Practices

• Use Comments: Explain your code.

• Error Handling: Use ||, &&, and if blocks to manage errors.

• Always Check Exit Status: $? gives the status of the last command executed (0 means success).

    cp file.txt /backup || echo "Copy failed!"
  

• Use Functions: Break down tasks into functions for better code structure.

• Avoid Hardcoding: Use variables and arguments.

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14. Conclusion

Mastering shell scripting requires practice, but with this foundation, you can write scripts that automate your daily tasks, handle complex workflows, and improve productivity. Start with simple scripts and progressively explore more advanced topics like file handling, arrays, and automation with cron.

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