Send a Wake-on-LAN (WOL) packet to a target machine via an always-on Linux host

wakeonlan.sh

#!/usr/bin/env zsh
# -----------------------------------------------------------------------------
# wakeonlan.sh
#
# Send a Wake-on-LAN (WOL) packet to a target machine via an always-on
# Linux host.
#
# Many computers that are powered off cannot receive WOL packets from
# outside their local network, e.g. when using Zerotier or Tailscale 
# VPN. This script solves that problem by SSHing into a machine that
# is always running (for example a home server, Raspberry Pi, or NAS
# host) and sending the WOL packet from there.
#
# Behaviour:
#   - A target hostname is looked up in an internal host → MAC address map.
#   - The script connects via SSH to the configured remote host.
#   - The remote host sends the WOL magic packet using the `wakeonlan` tool.
#
#   If the requested host is not found in the map, the script prints the list
#   of available hosts.
#
# Options:
#   --list
#       Show all configured hosts and their MAC addresses.
#
#   --remote HOST
#       Override the default remote machine used to send the WOL packet.
#
#   --dry-run
#       Print the SSH command that would be executed without running it.
#
# Usage:
#   wakeonlan.sh [--remote HOST] [--dry-run] [--list] <target-host>
#
# Examples:
#   wakeonlan.sh computer1.local
#   wakeonlan.sh --remote linuxhost.local computer2.local
#   wakeonlan.sh --dry-run computer3.local
#   wakeonlan.sh --list
#
# Requirements:
#   - zsh
#   - ssh client
#   - `wakeonlan` installed on the remote host
#
# Remote host setup example (Ubuntu/Debian):
#
#       sudo apt update
#       sudo apt install wakeonlan
#
# It is recommended to configure passwordless SSH authentication:
#
#       ssh-copy-id linuxhost.local
#
# so the script can run without prompting for a password.
#
# Notes:
#   - Hostnames are matched case-insensitively.
#   - The remote host must be on the same LAN as the target machines
#     for Wake-on-LAN to work.
#
# Installation (optional):
#   Copy the script to a directory on your PATH:
#
#       cp wakeonlan.sh /usr/local/bin/wakeonlan
#       chmod +x /usr/local/bin/wakeonlan
#
#   You can then run it from anywhere:
#
#       wakeonlan computer1.local
#
# Author: Jeremy Varnham https://github.com/jvarn
# License: MIT
# -----------------------------------------------------------------------------

set -euo pipefail

# -----------------------------------------------------------------------------
# Configuration (edit this section)
# -----------------------------------------------------------------------------
typeset -A HOSTS
HOSTS=(
  computer1.local   a0:b1:c2:d3:e4:f5
  computer2.local   a1:b2:c3:d4:e5:f6
  computer3.local   a2:b3:c4:d5:e6:f7
  computer4.local   a3:b4:c5:d6:e7:f8
)
REMOTE_HOST="linuxhost.local"
DRY_RUN=0

# -----------------------------------------------------------------------------
# Functions
# -----------------------------------------------------------------------------

usage() {
  cat <<'EOF'
Usage:
  wakeonlan.sh [--remote HOST] [--dry-run] [--list] <target-host>

Examples:
  wakeonlan.sh computer1.local
  wakeonlan.sh --remote linuxhost.local computer2.local
  wakeonlan.sh --dry-run computer3.local
  wakeonlan.sh --list
EOF
}

list_hosts() {
  echo "Known hosts:"
  for k in ${(ok)HOSTS}; do
    printf "  %-10s %s\n" "$k" "$HOSTS[$k]"
  done | sort
}

# -----------------------------------------------------------------------------
# Argument parsing
# -----------------------------------------------------------------------------

while [[ $# -gt 0 ]]; do
  case "$1" in
    -h|--help) usage; exit 0 ;;
    --list)    list_hosts; exit 0 ;;
    --dry-run) DRY_RUN=1; shift ;;
    --remote)
      [[ $# -ge 2 ]] || { echo "Error: --remote requires a host."; usage; exit 2; }
      REMOTE_HOST="$2"
      shift 2
      ;;
    --) shift; break ;;
    -*) echo "Error: Unknown option: $1"; usage; exit 2 ;;
    *)  break ;;
  esac
done

# -----------------------------------------------------------------------------
# Main logic
# -----------------------------------------------------------------------------

TARGET_HOST_RAW="${1:-}"
[[ -n "$TARGET_HOST_RAW" ]] || { echo "Error: Missing <target-host>."; usage; exit 2; }

TARGET_HOST="${TARGET_HOST_RAW:l}"   # lowercase in zsh
MAC="${HOSTS[$TARGET_HOST]:-}"

if [[ -z "$MAC" ]]; then
  echo "Error: Host '$TARGET_HOST_RAW' not found."
  echo
  list_hosts
  exit 1
fi

# Basic MAC sanity check
if ! [[ "$MAC" =~ '^([[:xdigit:]]{2}:){5}[[:xdigit:]]{2}$' ]]; then
  echo "Error: Stored MAC for '$TARGET_HOST' doesn't look valid: '$MAC'"
  exit 1
fi

REMOTE_CMD="command -v wakeonlan >/dev/null 2>&1 && wakeonlan '$MAC'"

if [[ "$DRY_RUN" -eq 1 ]]; then
  echo "Dry run:"
  echo "  ssh \"$REMOTE_HOST\" $REMOTE_CMD"
  exit 0
fi

ssh -o BatchMode=yes "$REMOTE_HOST" "$REMOTE_CMD" \
  || { echo "Error: Failed to send WOL via '$REMOTE_HOST' for '$TARGET_HOST' ($MAC)."; exit 1; }