Writing a Unit File

A unit is a small text file

To make systemd manage your own program, write a unit file in /etc/systemd/system/. It has sections in square brackets: [Unit] describes it, [Service] says how to run it (ExecStart is the command), and [Install] says when it should start at boot:

sudo tee /etc/systemd/system/heartbeat.service > /dev/null <<'EOF'
[Unit]
Description=Write a heartbeat every few seconds

[Service]
ExecStart=/bin/sh -c 'while true; do date > /run/heartbeat; sleep 3; done'

[Install]
WantedBy=multi-user.target
EOF
cat /etc/systemd/system/heartbeat.service

In the terminal

student@vulntech:~$ sudo tee /etc/systemd/system/heartbeat.service > /dev/null <<'EOF'
> [Unit]
> Description=Write a heartbeat every few seconds
> 
> [Service]
> ExecStart=/bin/sh -c 'while true; do date > /run/heartbeat; sleep 3; done'
> 
> [Install]
> WantedBy=multi-user.target
> EOF
[sudo] password for student:
student@vulntech:~$ cat /etc/systemd/system/heartbeat.service
[Unit]
Description=Write a heartbeat every few seconds

[Service]
ExecStart=/bin/sh -c 'while true; do date > /run/heartbeat; sleep 3; done'

[Install]
WantedBy=multi-user.target

Tell systemd, then start it

After writing or changing a unit, run systemctl daemon-reload so systemd re-reads its files. Then start it like any other service and check it's working:

sudo systemctl daemon-reload
sudo systemctl start heartbeat
systemctl is-active heartbeat
cat /run/heartbeat

In the terminal

student@vulntech:~$ sudo systemctl daemon-reload
[sudo] password for student:
student@vulntech:~$ sudo systemctl start heartbeat
student@vulntech:~$ systemctl is-active heartbeat
active
student@vulntech:~$ cat /run/heartbeat
Sun Oct 18 06:52:35 UTC 2026

Tip: Type=oneshot with RemainAfterExit=yes suits a job that runs once and finishes (a backup, a setup step) rather than a program that stays running. The default, Type=simple, is for a long-running program like the one above.

Remember: A unit file in /etc/systemd/system/NAME.service has [Unit], [Service] (with ExecStart=) and [Install] sections. Run systemctl daemon-reload after editing it, then start/enable it.