One of the core features of any Linux distribution's init system is the ability to automatically start services, which can be daemons for system functions (cron for automatic script running, D-Bus for inter-process communication, desktop greeters like GDM and SDDM to launch your GUI/login screen, etc.), but also servers for things like databases, websites, and SSH connections (SSHD). Usually, a software package will come with a system service pre-made and ready to activate, but you can also write your own services for custom scripts!
We now use this functionality for our own meetings to automatically get the Orange Pi connected to ssh.stuylinux.org!
systemd/system
To get a sense of the extent of what systemd services are running on your system, just run systemctl.
You'll see userspace services like mysql.service and sshd.service for the servers we know and love, but also tons of important system functions, like hwclock.service, dev-*.mount for tons of the special virtual devices in /dev/, systemd-udevd.service to make /dev work as it does, systemd-journald.service for logs, systemd-networkd.service for networking, and many many others (some would argue too many...).
There are also .target files like getty.target for login prompts on TTY, cryptsetup.target for encrypted disks, local-fs.target to make sure your local filesystems are mounted, and most importantly multi-user.target, which specifies the userspace services that should be run when the system is booted up.
To see how these services are defined, look in subfolders of /usr/lib/systemd/ (pre-installed services) and /etc/systemd/ (customized services and more). By convention, the /etc/systemd/system/ directory is where we will be writing our service.
Making our own service
Before we can do anything, we need to have some code that can serve as our service. Copy this busy loop into a file (something like busy.sh) and run chmod +x busy.sh to mark it executable.
#!/bin/sh
while [ true ]
do
echo hi
sleep 1
done
To test that this works, try running ./busy.sh.
To turn this into a systemd service file, write a .service file into /etc/systemd/system/ using your favorite text editor.
For this case, we'll just write a /etc/systemd/system/busy.service file.
[Unit]
Description=Sample busy loop.
[Service]
ExecStart=/bin/sh /root/busy.sh # replace this with your file location
Restart=on-failure
RestartSec=10s
[Install]
WantedBy=multi-user.target
If you'd like your service to automatically restart when it crashes, you can use the Restart=on-failure directive to tell systemd to do so. It is recommended to set a RestartSec interval so systemd doesn't repeatedly spam your system with a buggy process.
In order for systemd to automatically start your service when the rest of the system is ready, you need to set the WantedBy block as shown below.
[Install]
WantedBy=multi-user.target
One interesting directive you make like to have, in case your service depends on something else (like networking), is the After directive.
For the Official Stuy Linux SSH.StuyLinux.Org Starting Service:tm:, we use After=sshd.service to make sure the SSH server is ready for forwarding.
Activating the service
To have systemd rescan all of the service files, run
systemctl daemon-reload
and now you can run
systemctl enable busy.service
to start the service!
Run systemctl status busy to check on how your service is running.
To temporarily stop the service, run
systemctl stop busy
or to disable it... run
systemctl disable busy
To restart the service, you can run
systemctl restart busy
or if you want to start but not reset the service, you can instead just run
systemctl start busy
And now you're a real systemd system admin!
Oh, just one more thing...
You might have noticed that you've seen this type of syntax before, and you're not wrong! Systemd syntax is inspired by .ini files on... Windows.