How do you define and use functions in Bash? How do you pass arguments and return values?

Question

Accepted Answer

Functions in Bash help organize code into reusable blocks. There are two syntaxes for defining them:

function greet {
  echo "Hello, $1"
}

# Or the POSIX-compatible style:
greet() {
  echo "Hello, $1"
}

Arguments work just like script arguments. Inside the function, $1 is the first argument, $2 is the second, and so on. The $@ variable contains all arguments:

deploy() {
  local environment=$1
  local version=$2
  echo "Deploying version $version to $environment"
}

deploy production v1.2.3

Notice the local keyword. This makes variables local to the function, preventing them from polluting the global scope. Always use local for function variables.

Returning values from functions is tricky because the return command only accepts integers between 0 and 255, meant for exit codes:

check_status() {
  if [ -f /tmp/ready ]; then
    return 0  # Success
  else
    return 1  # Failure
  fi
}

if check_status; then
  echo "Ready"
fi

For returning actual data, use echo and command substitution:

get_timestamp() {
  date +%Y%m%d_%H%M%S
}

timestamp=$(get_timestamp)
echo "Backup file: backup_$timestamp.tar.gz"

Another pattern uses a global variable for output, though this is less clean:

get_info() {
  RESULT="some computed value"
}

get_info
echo "$RESULT"

The echo approach is generally preferred because it is more functional and avoids global state.