A case statement in shell scripting is a control structure used to test a variable against multiple patterns and execute corresponding code blocks. It offers a cleaner and more efficient alternative to lengthy if-then-elif chains. 1) Each pattern can be a literal, wildcard, or shorthand like [0-9]. 2) The ;; operator ends each case branch while ) acts as the default fallback. 3) Case statements are ideal for handling command-line arguments, user input, file extensions, or status codes. 4) Wildcards such as .log or combinations using | allow flexible pattern matching. 5) Omitting ;; causes unintended fall-through execution making it critical to include. 6) Including a default case ensures unexpected inputs are handled gracefully.

A case statement in shell scripting is a control structure that lets you test a variable against multiple patterns and execute different blocks of code depending on which pattern matches. It’s like a more readable and efficient alternative to long chains of if-then-elif statements, especially when dealing with many possible values.

Basic Syntax of a Case Statement

The general syntax looks like this:

case expression in
  pattern1)
    # commands to execute if expression matches pattern1
    ;;
  pattern2)
    # commands to execute if expression matches pattern2
    ;;
  *)
    # default case — runs if no other pattern matches
    ;;
esac

Each "pattern" can be a literal string, a wildcard (like *.txt), or even use shorthand like [0-9] for ranges. The ;; ends each case branch, and *) acts as the default or fallback option.

When You’d Use a Case Statement

Case statements are especially useful when:

• You’re parsing command-line arguments.
• You want to handle user input with several expected responses.
• You're matching file extensions, status codes, or simple conditions.

For example, imagine you have a script that accepts a single argument like "start", "stop", or "restart". A case statement makes handling those inputs clean and easy.

case "$1" in
  start)
    echo "Starting service..."
    ;;
  stop)
    echo "Stopping service..."
    ;;
  restart)
    echo "Restarting service..."
    ;;
  *)
    echo "Usage: $0 {start|stop|restart}"
    exit 1
    ;;
esac

This kind of structure keeps your logic organized and avoids messy nested if/else blocks.

Wildcards and Pattern Matching

One powerful feature of case statements is their ability to match patterns using wildcards:

• *) matches anything (used for default cases).
• *.log) could match any filename ending in .log.
• [Yy][Ee][Ss]) would match variations like "yes", "Yes", "YES".

Here's an example where we check file types based on extension:

filename="report.pdf"

case "$filename" in
  *.txt)
    echo "It's a text file."
    ;;
  *.jpg | *.png)
    echo "It's an image file."
    ;;
  *.pdf)
    echo "It's a PDF document."
    ;;
  *)
    echo "Unknown file type."
    ;;
esac

You can combine multiple patterns using | as an OR operator, making it flexible for various scenarios.

Tips and Common Pitfalls

• Make sure to end each block with ;;, or else execution will fall through to the next case.
• Keep patterns simple unless you really need complex matching.
• Always include a default case (*)) to catch unexpected inputs.

If you forget ;;, like this:

case "$choice" in
  yes)
    echo "You said yes"
  no)
    echo "You said no"
    ;;
esac

Then choosing "yes" will also run the "no" block — which is probably not what you want.

So yeah, the case statement is a handy tool in shell scripting when you need to compare one value against multiple possibilities. It’s clean, fast, and built right into bash and other common shells. Just remember to format it properly and cover all expected cases — and maybe a few unexpected ones too.

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