The case statement in shell scripting provides a clean and efficient way to handle multiple conditions without nested if-else statements. This guide will explore various uses of case statements with practical examples.

Basic Syntax

The basic syntax of a case statement is:

case EXPRESSION in
    PATTERN1)
        STATEMENTS
        ;;
    PATTERN2)
        STATEMENTS
        ;;
    *)
        DEFAULT STATEMENTS
        ;;
esac

Example 1: Country Capitals

This script demonstrates a simple case statement that returns the capital city of a given country:

#!/bin/bash

echo -n "Enter the country name: "
read country

echo -n "Capital of $country is "
case $country in
    "India")
        echo "New Delhi"
        ;;
    "Argentina")
        echo "Buenos Aires"
        ;;
    "Brazil")
        echo "Brasília"
        ;;
    *)
        echo "unknown"
        ;;
esac

Example 2: System Administration Menu

Here’s a practical example for system administrators:

#!/bin/bash

echo "System Administration Menu"
echo "1. Check disk space"
echo "2. Check memory usage"
echo "3. Check CPU info"
echo "4. Exit"

read -p "Enter your choice (1-4): " choice

case $choice in
    1)
        df -h
        ;;
    2)
        free -m
        ;;
    3)
        lscpu
        ;;
    4)
        echo "Exiting..."
        exit 0
        ;;
    *)
        echo "Invalid choice"
        ;;
esac

Example 3: File Type Checker

This script uses case statement with pattern matching to identify file types:

#!/bin/bash

read -p "Enter filename: " filename

case $filename in
    *.jpg|*.jpeg|*.png)
        echo "Image file"
        ;;
    *.txt|*.doc|*.pdf)
        echo "Document file"
        ;;
    *.mp3|*.wav)
        echo "Audio file"
        ;;
    *)
        echo "Unknown file type"
        ;;
esac

Example 4: Day of Week

Using case with command substitution:

#!/bin/bash

day=$(date +%u)

case $day in
    1)
        echo "It's Monday - Start of the work week"
        ;;
    2|3|4)
        echo "It's a regular work day"
        ;;
    5)
        echo "It's Friday - Almost weekend!"
        ;;
    6|7)
        echo "It's weekend!"
        ;;
    *)
        echo "Invalid day number"
        ;;
esac

Advanced Features

Multiple Patterns

You can match multiple patterns using the | operator:

case $answer in
    y|Y|yes|YES)
        echo "Proceeding..."
        ;;
    n|N|no|NO)
        echo "Aborting..."
        ;;
esac

Pattern Matching

Case supports pattern matching with wildcards:

case $filename in
    *.txt)
        echo "Text file"
        ;;
    data???)
        echo "Data file with 3 digits"
        ;;
    [Ll]og*)
        echo "Log file"
        ;;
esac

Best Practices

1. Always include a default case (*) to handle unexpected inputs
2. Use double quotes around variables to prevent word splitting
3. Keep related patterns together for better readability
4. Add comments for complex pattern matching
5. Use consistent indentation for better code structure

When to Use Case Statements

Case statements are particularly useful when:

• You have multiple conditions based on a single variable
• Pattern matching is needed
• Code readability is a priority
• You want to avoid complex nested if-else structures

Common Pitfalls

1. Forgetting the ;; terminator after each case
2. Missing the closing esac
3. Not handling the default case
4. Using complex expressions that should be handled by if statements

See Also

• Bash Scripting Guide
• Shell Scripting Tutorial
• Advanced Bash Scripting

Remember that while case statements are powerful, they’re best suited for simple pattern matching and equality comparisons. For more complex conditions, consider using if-else statements or other control structures.