Functions

A function is a named group of commands you can run like a command. Functions keep scripts organized and avoid repeating code.

Real-world analogy: a function is a recipe card. Write it once, then say "make pancakes" whenever you need them.

Syntax

name() {
  commands
}

Example

greet() {
  echo "Hello, $1!"
}

greet "Ada"
greet "Linus"

Expected output

Hello, Ada!
Hello, Linus!

• A function is defined before it's used, then called by name, like any command.
• Inside a function, $1, $2, $#, and "$@" are the function's arguments, not the script's.

A useful pattern: logging helpers

log()   { echo "[INFO] $*"; }
warn()  { echo "[WARN] $*" >&2; }
die()   { echo "[ERROR] $*" >&2; exit 1; }

log "Starting backup"
warn "Disk is 85% full"
[[ -d /definitely/missing ]] || die "Backup folder not found"
log "This line never runs"

[INFO] Starting backup
[WARN] Disk is 85% full
[ERROR] Backup folder not found

$* joins all arguments into one string — right for messages. die prints an error and stops the script, which makes checks short and readable: command || die "message".

Organizing a script with functions

A common layout: define functions first, then call a main function at the bottom with all the script's arguments:

usage() {
  echo "Usage: backup.sh <source> <destination>"
}

main() {
  if (( $# != 2 )); then
    usage
    return 1
  fi
  echo "Backing up $1 to $2"
}

main "photos" "/mnt/backup"

Backing up photos to /mnt/backup

Reading from main down, the script's flow is clear at a glance.

Common mistakes

• Calling a function before its definition — Bash reads scripts top to bottom, so it isn't known yet: command not found.
• Expecting exit inside a function to only leave the function. exit ends the whole script; use return to leave just the function.

Practice

Write a function square that prints the square of its first argument, and call it with 7.

square() {
  echo $(( $1 * $1 ))
}
square 7

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