Shell by Example: Functions

Shell by Example: Functions POSIX

	`#!/bin/sh`
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.	`greet() { echo "Hello, World!" }`
Bash Bash supports an alternative syntax using the `function` keyword:	`function greet { echo "Hello, World!" }`
You can also combine both syntaxes:	`function greet() { echo "Hello, World!" } another_greet_fn() { echo "Hello, World!" }`
Call a function by using its name.	`greet`
Functions can accept arguments via $1, $2, etc.	`greet_user() { echo "Hello, $1!" } greet_user "Alice" greet_user "Bob"`
$@ contains all arguments, $# is the count.	`show_args() { echo "Number of arguments: $#" for arg in "$@"; do echo " - $arg" done } show_args one two three`
Functions return exit status with `return`. Return 0 for success, non-zero for failure.	`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`
To return string values, use command substitution.	`get_greeting() { echo "Hello, $1!" } message=$(get_greeting "World") echo "Got: $message"`
Variables in functions are global by default. Be careful with naming to avoid conflicts.	`counter=0 increment() { counter=$((counter + 1)) } increment increment echo "Counter: $counter"`
Bash Bash provides the `local` keyword for function-scoped variables that don’t affect the global namespace:	`calculate() { local result=0 local i for i in "$@"; 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)`
Functions must be defined before they’re called. Define all functions at the top of your script.

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