systemdnotify

systemdnotify is a command-line utility that allows users to send status notifications to systemd. It's primarily used by shell scripts and other applications to inform systemd about their current state, such as starting, running, stopping, or completion. This mechanism is a fundamental part of systemd's service management, enabling it to track the lifecycle of processes and units.

When a service or script uses systemdnotify, it typically sends messages to the systemd manager via a special file descriptor, often referred to as `NOTIFY_SOCKET`. These messages can convey various states. For example, a service might send `READY=1` to indicate that it has successfully initialized and is ready to accept connections. Conversely, it might send `STOPPING=1` to signal that it is in the process of shutting down.

The use of systemdnotify is crucial for systemd's ability to provide features like watchdog timers and automatic restarts. If a service configured to use systemdnotify stops reporting its status, systemd might interpret this as a failure and take predefined actions, such as restarting the service or logging an error. This makes systemdnotify an important tool for building robust and observable system services. Most modern daemons and systemd units leverage this mechanism to integrate seamlessly with the systemd ecosystem.