Functions

Bash · Reference cheat sheet

📋 Overview

Functions group reusable commands. They share the shell’s process (unless you spawn a subshell). Arguments arrive as $1…"$@". Prefer local for internals and explicit return status (0–255).

🔧 Core concepts

Functions cannot return structured data via return—print to stdout and capture with $(...), or use namerefs (declare -n) / globals carefully.

💡 Examples

Basic with locals:

greet() {
  local who="${1:-world}"
  echo "Hello, $who"
}
greet "Ada"

Return status vs output:

is_dir() {
  [[ -d "$1" ]]
}

path_join() {
  local a="$1" b="$2"
  echo "${a%/}/$b"
}

if is_dir "/tmp"; then
  out="$(path_join "/var" "log")"
fi

Nameref (bash 4.3+):

append() {
  local -n _arr=$1
  shift
  _arr+=("$@")
}
list=()
append list a b

Guard usage:

usage() { echo "usage: $0 <file>" >&2; }

main() {
  [[ $# -ge 1 ]] || { usage; return 2; }
  …
}
main "$@"

⚠️ Pitfalls

• local var=$(cmd) masks cmd’s exit status; split assign and check.
• Forgetting local leaks variables into the global scope.
• return outside a function exits the shell when sourced carefully—use only in functions.
• Recursion depth and performance: bash functions are not optimized like compiled code.
• Defining functions in a pipeline/subshell does not affect the parent shell.
• Name clashes with external commands: prefer project prefixes (myapp_).

🔗 Related

• variables.md
• exit_codes.md
• scripts_best_practices.md
• subshells.md
• getopts.md