Shell by Example: Functions

Shell by Example: Functions POSIX + Bash

Functions let you group commands together and reuse them throughout your script. They’re essential for organizing larger scripts. Define a function using the `name() { }` syntax. The `function` keyword is optional and not POSIX.	Edit `#!/bin/sh greet() { echo "Hello, World!" }`

Bash Bash supports an alternative syntax using the `function` keyword:	Edit `#!/bin/bash function greet { echo "Hello, World!" }`

Bash You can also combine both syntaxes:	Edit `#!/bin/bash function greet() { echo "Hello, World!" } another_greet_fn() { echo "Hello, World!" }`

Bash Call a function by using its name.	Edit `#!/bin/bash function greet() { echo "Hello, World!" } greet`
	Output: Hello, World!

Functions can accept arguments via `$1`, `$2`, etc.	Edit `#!/bin/sh greet_user() { echo "Hello, $1!" } greet_user "Alice" greet_user "Bob"`
	Output: Hello, Alice! Hello, Bob!

`$@` contains all arguments, `$#` is the count.	Edit `#!/bin/sh show_args() { echo "Number of arguments: $#" for arg in "$@"; do echo " - $arg" done } show_args one two three`
	Output: Number of arguments: 3 - one - two - three

Functions return exit status with `return`. Return 0 for success, non-zero for failure.	Edit `#!/bin/sh is_even() { if [ $(($1 % 2)) -eq 0 ]; then return 0 # success/true else return 1 # failure/false fi } if is_even 4; then echo "4 is even" fi if ! is_even 7; then echo "7 is odd" fi`
	Output: 4 is even 7 is odd

To return string values, use command substitution.	Edit `#!/bin/sh get_greeting() { echo "Hello, $1!" } message=$(get_greeting "World") echo "Got: $message"`
	Output: Got: Hello, World!

Variables in functions are global by default. Be careful with naming to avoid conflicts.	Edit `#!/bin/sh counter=0 increment() { counter=$((counter + 1)) } increment increment echo "Counter: $counter"`
	Output: Counter: 2

Bash Bash provides the `local` keyword for function-scoped variables that don’t affect the global namespace:	Edit `#!/bin/bash calculate() { local result=0 local i for i in "$@"; do result=$((result + i)) done echo "$result" } result="global" sum=$(calculate 1 2 3 4 5) echo "Sum: $sum" # Sum: 15 echo "Result: $result" # Result: global (unchanged)`
	Output: Sum: 15 Result: global

Functions must be defined before they’re called. Unlike some languages, shell scripts are interpreted top-to-bottom, so calling an undefined function fails.	Edit `#!/bin/sh # Calling an undefined function produces an error: undefined_func 2>&1 \|\| true # This works: define first, then call greet() { echo "Hello, $1!" } greet "World"`
	Output: /script: line 7: undefined_func: not found Hello, World!

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