Functions: Declarations, Local Scope & Return Values

In Plain English

Functions modularize script logic into reusable, testable units. Inside functions, arguments are accessed via positional parameters (`$1`, `$2`), variables MUST be scoped with `local` to prevent global pollution, and values are returned via stdout or numeric exit codes (`return 0-255`).

The Mental Model

"A function is a mini-script inside your script: it takes inputs, works in its own private workspace (`local`), and produces an answer on stdout."

Deep Dive: How It Works

Syntax Styles: `function_name() { ... }` (POSIX standard) or `function name { ... }`. Prefer the POSIX syntax without the redundant `function` keyword.

Local Variable Scoping: Always write `local my_var="value"` inside functions.

Return vs Output: `return N` only returns an integer status code (0 = success, 1-255 = error); To return a string or data structure, print to stdout (`echo "$result"`) and capture with command substitution (`RES=$(my_func)`).

Syntax Blueprint

calculate_tax() {
  local amount="$1"
  local rate="$2"
  echo "$(( amount * rate / 100 ))"
}

TAX=$(calculate_tax 500 10)
echo "Calculated tax: $TAX"

Use local for variables and echo for string/data return values.

Core Rules to Remember

POSIX Syntax: `name() { ... }` is universal across bash, zsh, dash, and sh.

Local Scope Rule: Variables declared without `local` leak into global script state.

Positional Parameters: $1 is first arg, $2 is second, $# is arg count, and "$@" is all args as a list.

Common Beginner Traps & How to Fix Them

Mistake:Trying to return a string with `return "hello"`.

Why it happens: `return` in Bash only accepts integer status codes between 0 and 255.

How to fix: Use `echo "hello"` and capture with `RESULT=$(my_func)`.

Pro Tip: Pass array arguments to functions safely using `my_func "${ARRAY[@]}"` and consume using `local items=("$@")`.

Live Interactive Example

Hit Run Code to see it live

Modular Functions with Local Scope and Return Values

Git 2.45 / CLI

Output Console

Click "Run Code" to view the rendered output.

How it works: multiply computed 6*7 and returned 42 to stdout, captured into $RESULT.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Define and Call a Function

Define a function `greet()` that takes a name `$1` and echoes "Hello, $1!".
Call `greet "ZenCompiler"`.

Sandbox Output

Click "Run & Check" to test your solution.

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