Bash Functions

Defining a Function

A function is defined with the function keyword and a name, or more portably with just name() { ... }. The body is a block of commands between braces, and the function must be defined before it is called anywhere in the script.

#!/bin/bash
function greet() {
  echo "Hello from a function!"
}

greet
greet

Positional Parameters: $1, $2, and $@

Arguments passed to a function are available inside it as $1, $2, and so on, exactly like command-line arguments to a script. $# holds the count of arguments, and $@ expands to all of them as separate words. These parameters are local to the function call and do not affect the script's own $1, $2 outside it.

#!/bin/bash
introduce() {
  echo "Name: $1"
  echo "Role: $2"
  echo "Total arguments: $#"
}

introduce "Sam" "Engineer"

Returning Values

The return statement sets a function's exit status, a number from 0 to 255, and is meant for signaling success or failure -- 0 means success, anything else means an error. To return actual data like a string or number, print it with echo and capture it in the caller using command substitution $(function_name).

• return sets the exit status (0-255), checked with $? or in an if condition
• echo plus command substitution passes back computed data, e.g. result=$(add 2 3)
• local varname=value keeps a variable scoped to the function only
• Functions must be defined before they are called in the script

#!/bin/bash
add() {
  local sum=$(( $1 + $2 ))
  echo "$sum"
}

is_even() {
  if [ $(( $1 % 2 )) -eq 0 ]; then
    return 0
  else
    return 1
  fi
}

result=$(add 4 7)
echo "Sum: $result"

if is_even 6; then
  echo "6 is even"
fi