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Understanding case in Linux (Bash Scripting)

The case statement in Linux (specifically in bash scripting) is a control flow statement that allows pattern-based branching. It is similar to a switch statement in other programming languages and is used to simplify conditional structures when comparing a variable against multiple values or patterns.

Basic Syntax

case variable in
    pattern1)
        commands
        ;;
    pattern2)
        commands
        ;;
    *)
        commands
        ;;
esac

Explanation:

  1. variable: The variable or value being tested.
  2. pattern: A value or a wildcard pattern to match against the variable.
  3. commands: The commands to execute if the variable matches the pattern.
  4. ;;: Marks the end of the commands for a pattern.
  5. *: A wildcard pattern that matches anything (default case).
  6. esac: Ends the case statement.

Example 1: Basic Usage

#!/bin/bash

echo "Enter a choice: start, stop, or restart"
read action

case $action in
    start)
        echo "Starting the service..."
        ;;
    stop)
        echo "Stopping the service..."
        ;;
    restart)
        echo "Restarting the service..."
        ;;
    *)
        echo "Invalid choice!"
        ;;
esac

Output:

  • Input start → Outputs Starting the service...
  • Input invalid → Outputs Invalid choice!

Pattern Matching

The case statement supports patterns, including wildcards.

Example 2: Using Wildcards

#!/bin/bash

read -p "Enter a file extension (e.g., .sh, .txt, .jpg): " ext

case $ext in
    *.sh)
        echo "This is a shell script file."
        ;;
    *.txt)
        echo "This is a text file."
        ;;
    *.jpg|*.png)
        echo "This is an image file."
        ;;
    *)
        echo "Unknown file type."
        ;;
esac

Multiple Patterns

You can use the | operator to combine multiple patterns.

Example 3: Combining Patterns

#!/bin/bash

read -p "Enter a day of the week: " day

case $day in
    Monday|Tuesday|Wednesday|Thursday|Friday)
        echo "It's a weekday."
        ;;
    Saturday|Sunday)
        echo "It's the weekend!"
        ;;
    *)
        echo "That's not a valid day."
        ;;
esac

Using Case for Numbers

Case statements can also handle numerical patterns.

Example 4: Ranges with Numbers

#!/bin/bash

read -p "Enter a number (1-10): " num

case $num in
    [1-3])
        echo "You entered a number between 1 and 3."
        ;;
    [4-6])
        echo "You entered a number between 4 and 6."
        ;;
    [7-9]|10)
        echo "You entered a number between 7 and 10."
        ;;
    *)
        echo "Invalid number."
        ;;
esac

Default Case

The * wildcard is used as the default case when none of the specified patterns match.

Example 5: Default Case

#!/bin/bash

read -p "Enter a command (start/stop/restart): " cmd

case $cmd in
    start)
        echo "Service starting..."
        ;;
    stop)
        echo "Service stopping..."
        ;;
    restart)
        echo "Service restarting..."
        ;;
    *)
        echo "Command not recognized."
        ;;
esac

Advantages of case Statement

  1. Readable: Simplifies complex if-else chains.
  2. Efficient: Matches patterns directly without multiple condition evaluations.
  3. Flexible: Allows wildcards, ranges, and multiple patterns.

Key Notes

  • Patterns must end with a ) character.
  • Each pattern can include multiple matching criteria separated by |.
  • Always end a case block with ;; for each pattern.
  • Use * as the default case to handle unexpected input.

By using case, you can create cleaner and more maintainable scripts for handling user input or managing different conditions in Linux.