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Bash Case Esac

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Mastering Bash's `case` Statement: A Comprehensive Guide



The Bash shell, a powerful command-line interpreter, offers numerous tools for scripting and automation. Among these, the `case` statement provides a concise and readable way to handle multiple conditional branches, offering a cleaner alternative to lengthy `if-elif-else` constructs. This article aims to provide a comprehensive understanding of the Bash `case` statement, exploring its syntax, functionality, and practical applications with illustrative examples.


Understanding the Syntax of `case`



The `case` statement in Bash follows a specific syntax designed for pattern matching. Its basic structure looks like this:

```bash
case variable in
pattern1)
commands ;;
pattern2)
commands ;;
) # Default case
commands ;;
esac
```

Let's break down the elements:

`case variable`: This initiates the `case` statement, specifying the variable whose value will be matched against the patterns.

`in`: This keyword separates the variable from the patterns.

`pattern1) commands ;;`: This represents a single case. `pattern1` is a pattern to be matched against the variable's value. If a match is found, the `commands` are executed. The `;;` signifies the end of the case. Multiple commands can be listed, each on a new line.

`) commands ;;`: This is the optional default case, indicated by the wildcard ``. If none of the preceding patterns match the variable's value, the commands within the default case are executed.

`esac`: This keyword marks the end of the `case` statement. It's essentially the counterpart of `case`.


Pattern Matching in `case` Statements



Bash's `case` statement supports powerful pattern matching capabilities using wildcard characters:

`` (asterisk): Matches any sequence of characters, including an empty string.

`?` (question mark): Matches any single character.

`[...]` (character set): Matches any single character within the specified set. Ranges are allowed (e.g., `[a-z]`).

`|` (pipe): Acts as an "or" operator, allowing you to match against multiple patterns within a single case.


Practical Examples:



Let's illustrate the usage of `case` with various scenarios:

Example 1: Simple Menu

```bash

!/bin/bash



read -p "Enter your choice (1-3): " choice

case $choice in
1)
echo "You chose option 1" ;;
2)
echo "You chose option 2" ;;
3)
echo "You chose option 3" ;;
)
echo "Invalid choice!" ;;
esac
```

Example 2: File Type Check

```bash

!/bin/bash



file="mydocument.pdf"
case $file in
.txt)
echo "Text file" ;;
.pdf)
echo "PDF file" ;;
.jpg|.jpeg)
echo "Image file" ;;
)
echo "Unknown file type" ;;
esac
```

Example 3: Using Character Sets

```bash

!/bin/bash



read -p "Enter a letter: " letter

case $letter in
[aeiou])
echo "Vowel" ;;
[A-Z])
echo "Uppercase letter" ;;
[a-z])
echo "Lowercase letter" ;;
)
echo "Not a letter" ;;
esac
```


Advantages of Using `case`



The `case` statement offers several advantages over nested `if-elif-else` structures:

Readability: `case` statements are often more concise and easier to read, especially when dealing with numerous conditions.

Maintainability: The structured format simplifies modification and debugging.

Efficiency: For simple conditional checks, `case` can be more efficient than nested `if` statements.


Conclusion



The Bash `case` statement provides a powerful and elegant method for handling multiple conditional branches within shell scripts. Its pattern-matching capabilities and structured syntax enhance code readability, maintainability, and potentially efficiency. By mastering `case`, you significantly improve your Bash scripting abilities, enabling you to write cleaner, more effective shell scripts.


FAQs



1. Can I use variables within patterns? Yes, you can use variables inside patterns by enclosing them in double quotes, for example: `case "$var" in "$pattern").`

2. What happens if multiple patterns match? Only the first matching pattern's commands will be executed.

3. Can I nest `case` statements? Yes, you can nest `case` statements within each other for complex logic.

4. Are there any performance differences between `case` and `if-elif-else`? For simple scenarios, the performance difference is negligible. For very complex conditional logic, `case` might offer slight performance advantages, but this depends on the specifics.

5. What is the best practice for handling errors in a `case` statement? Always include a default case (``) to handle unexpected input or unmatched patterns and provide informative error messages to the user.

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bash nested case syntax and - Unix & Linux Stack Exchange Stack Exchange Network. Stack Exchange network consists of 183 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

shell script - Using case and arrays together in bash - Unix 15 Dec 2017 · IFS=@ case "@${ARR[*]}@" in (*"@$1@"*) echo "Option is contained in the array";; (*) echo "Option is not contained in the array";; esac With bash -O extglob , zsh -o kshglob -o globsubst , you could define a helper that builds a pattern based on the elements of the array:

What does "esac" mean at the end of a bash case statement? Is it ... The esac keyword is indeed a required delimiter to end a case statement in bash and most shells used on Unix/Linux excluding the csh family. The original Bourne shell was created by Steve Bourne who previously worked on ALGOL68. This language invented this reversed word technique to delimit blocks. case/esac if/fi do/od

bash - How to specify AND / OR operators (conditions) for case ... 29 Oct 2019 · For completeness, while case has a | OR operator, it doesn't have an AND operator but if using shells with extended glob operators (ksh, zsh, bash), you can implement the AND in the pattern syntax:

case + how to implement equal or less or greater in case syntax @StéphaneChazelas - i like case. theres some cool stuff you can do with $((math)) and case - especially surrounding assignments in patterns that never happen until they need to - and you can even build parse trees that expand nested recursions if you populate the patterns with an alias chain. its the fastest way ive found to get a shell to do stuff like character translation and to …

bash - How can I use a variable as a case condition? - Unix 6 Oct 2015 · @JeffSchaller - it's not a bad idea a lot of times, and is maybe just the ticket in this case. i considered recommending it, too, but the read bit stopped me. in my opinion user input validation, which is what this appears to be, case patterns should not be at the top of the evaluation list, and should rather be pruned down to the * default pattern such that the only …

bash - Possible to match multiple conditions in one case … 3 Apr 2020 · You can use the ;;& conjunction. From man bash:. Using ;;& in place of ;; causes the shell to test the next pattern list in the statement, if any, and execute any associated list on a successful match.

bash - Using `case` to handle script arguments - Unix & Linux … 26 Nov 2014 · What does "esac" mean at the end of a bash case statement? Is it required? 4.

Bash, use case statement to check if the word is in the array 2 Sep 2022 · I am writing a script which must accept a word from a limited predefined list as an argument. I also would like it to have completion. I'm storing list in a variable to avoid duplication between complete and case. So I've written this, completion does work, but case statement doesn't. Why? One can't just make case statement parameters out of ...

bash - a simple case/esac construct exercise fails, why? - Unix 17 May 2021 · Bash treats the CR as part of the word esac␍, so this is not a reserved word, and the actual word esac is missing. CR characters are part of Windows line endings: a Unix line ending is the LF (line feed) character alone, whereas a Windows line ending is the two-character sequence CR+LF.