What is systemd PID file?

Question

I want to run jar file as a daemon. So I have written a shell script to "start|stop|restart" the daemon. I didn't get a chance to its working status. Can I use this script without creating a PID file? Why do we need a PID file at all? In which case we should use PID file?

Below is my UNIT file.

[Unit]
Description=myApp
After=network.target

[Service]
Environment=JAVA_HOME=/opt/java/jdk8
Environment=CATALINA_HOME=/opt/myApp/
User=nzpap
Group=ngpap
ExecStart=/kohls/apps/myApp/myapp-scripts/myapp-deploy.sh
Restart=always

[Install]
WantedBy=multi-user.target

I did not gain info by browsing through the internet about PID concept.

Answer 1

Below is the documentation of the PIDFile=<path-to-pid> parameter supported by systemd.

The idea is that many existing services do a fork() and you have no control over whether the service should or not fork. So in that case you want to use this option.

Since you are creating your own service, the important part to read about is the second paragraph which tells you to not use a PID file at all. As you said: It's useless. Especially useless since you can get that PID using the following command:

systemctl [--user] show --property MainPID --value <service>

Note: the --user is used to query user specific services.

And within the .service file, you can access that PID with the special variable named $MAINPID as in:

ExecStop=/bin/kill "$MAINPID"

Further, any child of that process will be in a group using that same $MAINPID, so you can get the list of children using that same number like so:

pgrep --pgroup $MAINPID

This list may also appear when you check the process status:

systemctl status <service>

If that works, see the CGroup: ... field. This doesn't work for all services.

PIDFile=

Takes a path referring to the PID file of the service. Usage of this option is recommended for services where Type= is set to forking. The path specified typically points to a file below /run/. If a relative path is specified it is hence prefixed with /run/. The service manager will read the PID of the main process of the service from this file after start-up of the service. The service manager will not write to the file configured here, although it will remove the file after the service has shut down if it still exists. The PID file does not need to be owned by a privileged user, but if it is owned by an unprivileged user additional safety restrictions are enforced: the file may not be a symlink to a file owned by a different user (neither directly nor indirectly), and the PID file must refer to a process already belonging to the service.

Note that PID files should be avoided in modern projects. Use Type=notify or Type=simple where possible, which does not require use of PID files to determine the main process of a service and avoids needless forking.

Note 1: emphasis added by me.

Note 2: if you do use a PID file with systemd, it should be under /run/... or you must specify a full path (a path starting with a /).

Answer 2

A pidfile holds the main task. Because the main task is possible able to spawn more to handle requests or whatever. Two examples for that...

#pidof apache2
31583 31582 31579 31577 31576 31571 347
#pidof tinyproxy
6813 6812 6811 6810 6809 6808 6807 6806 6805 6804 6803 6802 6801 6800 6799 6798 6797 6796 6795 6794 6793
#ls /var/run/apache2/
apache2.pid  cgisock.347  
#cat /var/run/apache2/apache2.pid 
347
#cat /var/run/tinyproxy/tinyproxy.pid 
6793