Case Statements

Case statements provide a cleaner, more readable alternative to long if-elif chains when checking a single variable against multiple values. They're perfect for menu systems, command parsing, file type detection, and any situation where you need multi-way branching based on a single value. Case statements use pattern matching, making them more powerful than simple equality checks!

Basic Case Statement Syntax

A case statement starts with case, tests a value against patterns, and ends with esac (case spelled backwards). Each pattern is followed by ) and ends with ;;.

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# Basic syntax
case variable in
    pattern1)
        # code for pattern1
        ;;
    pattern2)
        # code for pattern2
        ;;
    *)
        # default case (optional)
        ;;
esac

# Example
choice="yes"
case $choice in
    yes|y|YES|Y)
        echo "You chose yes"
        ;;
    no|n|NO|N)
        echo "You chose no"
        ;;
    *)
        echo "Invalid choice"
        ;;
esac

Pattern Matching

Case statements support glob patterns, making them powerful for matching multiple values and ranges.

# File extension check
filename="image.jpg"
case $filename in
    *.jpg|*.jpeg)
        echo "JPEG image"
        ;;
    *.png)
        echo "PNG image"
        ;;
    *.gif)
        echo "GIF image"
        ;;
    *)
        echo "Unknown file type"
        ;;
esac

# Character class example
char="a"
case $char in
    [a-z])
        echo "Lowercase letter"
        ;;
    [A-Z])
        echo "Uppercase letter"
        ;;
    [0-9])
        echo "Digit"
        ;;
    *)
        echo "Other character"
        ;;
esac

Multiple Patterns

You can combine multiple patterns using | (pipe) to match any of them. This is cleaner than separate if-elif conditions.

# Multiple patterns with |
action="START"
case $action in
    start|START|begin|BEGIN)
        echo "Starting service..."
        ;;
    stop|STOP|halt|HALT)
        echo "Stopping service..."
        ;;
    restart|RESTART|reboot|REBOOT)
        echo "Restarting service..."
        ;;
    *)
        echo "Unknown action"
        ;;
esac

Practical Examples

Case statements are commonly used in real-world scripts:

# Menu system
PS3="Choose an option: "
read choice
case $choice in
    1)
        echo "Starting..."
        ;;
    2)
        echo "Stopping..."
        ;;
    3)
        echo "Restarting..."
        ;;
    q|Q|quit|QUIT)
        echo "Exiting..."
        exit 0
        ;;
    *)
        echo "Invalid option"
        ;;
esac

# Command parsing
command="$1"
case $command in
    install|i)
        echo "Installing..."
        ;;
    uninstall|remove|r)
        echo "Uninstalling..."
        ;;
    status|s)
        echo "Checking status..."
        ;;
    *)
        echo "Usage: script.sh [install|uninstall|status]"
        exit 1
        ;;
esac

Common Mistakes

# Wrong - missing ;;
case $var in
    pattern1)
        echo "Matched"
    # Missing ;; causes fall-through!
    pattern2)
        echo "Also matches"
        ;;
esac

# Correct - each case ends with ;;
case $var in
    pattern1)
        echo "Matched"
        ;;  # Required!
    pattern2)
        echo "Also matches"
        ;;
esac

# Wrong - == doesn't work in case
case $var in
    == "value")  # Syntax error
        echo "Match"
        ;;
esac

# Correct - just use the pattern
case $var in
    value)  # Matches exact string
        echo "Match"
        ;;
    "value with spaces")  # Quote if needed
        echo "Match"
        ;;
esac

# Case uses GLOB patterns, not regex
filename="test123.txt"

# Correct - glob patterns
case $filename in
    test*.txt)  # Matches test, then any chars, then .txt
        echo "Match"
        ;;
esac

# Wrong - regex syntax doesn't work
case $filename in
    test[0-9]+.txt)  # + doesn't work in glob
        echo "Won't match as expected"
        ;;
esac

Exercise: Create a File Type Detector

#!/bin/bash
# File type detector

filename="$1"

case "$filename" in
    *.jpg|*.jpeg|*.JPG|*.JPEG)
        echo "$filename: JPEG image"
        ;;
    *.png|*.PNG)
        echo "$filename: PNG image"
        ;;
    *.sh|*.bash)
        echo "$filename: Shell script"
        ;;
    *.py)
        echo "$filename: Python script"
        ;;
    *.txt|*.md|*.log)
        echo "$filename: Text file"
        ;;
    *)
        echo "$filename: Unknown file type"
        ;;
esac

Summary

What's Next?

Excellent! Case statements make multi-way decisions much cleaner. Next, we'll dive into For Loops - one of the most commonly used control structures for iterating over lists, arrays, files, and ranges. You'll learn different for loop styles and when to use each!