notification-queue-notifier

#!/usr/bin/env bash
#
# Any program can send notifications by just writing to a queue file, e.g.
#
#   $ do-stuff | other-stuff >> $HOME/.cache/notification-queue
#
# Notifications can be deferred in the sense that they will be held in the queue until the target
# user starts the process to read them. This means that the user can receive notifications from
# stuff that happened even before logging in.
#
# Any process from any user can send notifications as long as they have permission to write to the
# user queue file.
#
# The script runs forever reading the lines in the queue and showing the notifications.
#
# NOTE 1: This script should be started on user desktop login via platform-specific mechanisms,
#  e.g. FreeDesktop (aka XDG) Autostart spec on Linux.
#
# NOTE 2: `notify-desktop` is my own script to run cross-platform notifications. This allows me to
#  use this notification-queue system in any system supporting UNIX pipe files (should probably
#  work on Windows/WSL, but not tested). If you don't need this, simply use `notify-send` on Linux
#  from libnotify package (probably installed by default), or any tool for your OS.
#  See https://github.com/gerardbosch/dotfiles/blob/cbd9d92224b0fdf5516e24e68aa97760d470a049/home/bin/notify-desktop
#
set -euo pipefail

# A FIFO pipe file (not a regular file)
QUEUE=$HOME/.cache/notification-queue


checkSingletonOrDie() {
  # pgrep is limited to 15-char command name; -f looks for whole command line (may lead for false positives)
  # $$ used to exclude the PID of self instance
  if pgrep -f "$(basename "$0")" | grep -v "^$$\$" >/dev/null; then
    echo "$(basename "$0")"
    echo "Script is already running. Exiting."
    exit 1
  fi
}

# == Main ==

checkSingletonOrDie
basedir=$(dirname "$QUEUE")

test -d "$basedir" || mkdir -p "$basedir"
test -e "$QUEUE"   || mkfifo "$QUEUE"

while [[ -e "$QUEUE" ]]; do
  while read line; do
    #notify-send "[Notification queue]" "$line"    # <-- platform-specific (Linux)
    notify-desktop "[Notification queue]" "$line"  # <-- platform-independent
    sleep 10
  done < $QUEUE
done