function

Define a function with the specified name

$ function [func_name] { [echo "Function contents here"]; }

$ func_name

$ [func_name]() { [echo "Function contents here"]; }

$ help function

function name [()] {
  commands
}

name() {
  commands
}

In Bash and compatible shells like ksh and zsh, the function keyword defines shell functions—named blocks of code executed like commands for modularity and reuse in scripts or interactive sessions.

Functions encapsulate logic, accept arguments via $1, $2, etc., and return status codes (0-255) via return or exit. Use local for variables scoped to the function. They support recursion and can call other functions.

Defined functions persist in the current shell until unset -f name or shell exit. Export with export -f name for subshell inheritance. List with declare -f or typeset -f.

This enhances scripting beyond aliases, enabling complex logic like loops and conditionals. Prefer the portable name() syntax over function for broader compatibility.

Non-POSIX; function keyword unsupported in strict /bin/sh. Use name() form for portability.
Conflicts with existing commands/builtins shadow them.

function greet {
  echo "Hello, $1!"
}

greet Alice # Hello, Alice!

function sum {
  local total=$(( $1 + $2 ))
  echo $total
}

sum 5 3 # 8

Originated in KornShell (ksh88, 1988); Bash adopted in v1.0 (1989). POSIX.1-2008 mandates name() without function. Evolved for recursion, exporting in later Bash versions.

bash(1), ksh(1), declare(1), typeset(1), unset(1)