Exercise: Write your own service

Write your own service

• Write a service unit and enable it.
• See Restart=on-failure bring a killed service back.
• Override a setting with a drop-in and diagnose status 203.

Start the exercise

On workstation, start the exercise. It installs the program /usr/local/bin/reporter.sh on servera: a loop that appends a heartbeat line to /var/log/reporter.log every $INTERVAL seconds (default 10).

[student@workstation ~]$ lab start sa-unit-files

Later tasks assume that you are logged in to the server they name: ssh student@servera from workstation, then sudo -i for a root shell.

Write the unit and start it

Create /etc/systemd/system/reporter.service that runs the script, restarts it on failure after 2 seconds, and is wanted by multi-user.target. Check it with systemd-analyze verify, reload systemd, then enable and start it.

[root@servera ~]# vim /etc/systemd/system/reporter.service
[root@servera ~]# cat /etc/systemd/system/reporter.service
[Unit]
Description=Write a heartbeat line to a report file
After=network.target

[Service]
ExecStart=/usr/local/bin/reporter.sh
Restart=on-failure
RestartSec=2

[Install]
WantedBy=multi-user.target
[root@servera ~]# systemd-analyze verify /etc/systemd/system/reporter.service
[root@servera ~]# systemctl daemon-reload
[root@servera ~]# systemctl enable --now reporter
Created symlink /etc/systemd/system/multi-user.target.wants/reporter.service → /etc/systemd/system/reporter.service.
[root@servera ~]# systemctl is-active reporter
active

Read what it writes

Wait a little, then check the log file and the service's status.

[root@servera ~]# tail -3 /var/log/reporter.log
16:37:00 reporter alive
16:37:10 reporter alive
16:37:20 reporter alive

Crash it

Kill the service's main process with signal 9. After a few seconds, is it running again? What does the journal say?

[root@servera ~]# kill -9 $(systemctl show -p MainPID --value reporter)
[root@servera ~]# sleep 3; systemctl is-active reporter
active
[root@servera ~]# journalctl -u reporter --no-pager | tail -3
Oct 03 16:37:18 servera.lab.example.com systemd[1]: reporter.service: Failed with result 'signal'.
Oct 03 16:37:20 servera.lab.example.com systemd[1]: reporter.service: Scheduled restart job, restart counter is at 1.
Oct 03 16:37:20 servera.lab.example.com systemd[1]: Started Write a heartbeat line to a report file.

systemd noticed the unclean exit, waited RestartSec, and started a new copy. Compare what you did in the last chapter: a plain background job stays dead.

Speed it up with a drop-in

Without touching the original unit, set INTERVAL=2 through a drop-in file called 10-interval.conf. Restart and confirm that new lines appear every 2 seconds.

[root@servera ~]# mkdir -p /etc/systemd/system/reporter.service.d
[root@servera ~]# printf '[Service]\nEnvironment=INTERVAL=2\n' > /etc/systemd/system/reporter.service.d/10-interval.conf
[root@servera ~]# systemctl daemon-reload
[root@servera ~]# systemctl restart reporter
[root@servera ~]# sleep 5; tail -3 /var/log/reporter.log
16:37:13 reporter alive
16:37:15 reporter alive
16:37:17 reporter alive
[root@servera ~]# systemctl cat reporter | tail -3
# /etc/systemd/system/reporter.service.d/10-interval.conf
[Service]
Environment=INTERVAL=2

Break it and read the error

Change ExecStart= in the unit to /usr/local/bin/nothere.sh, reload, and restart. Find the failure in the status and in the journal. Then put the correct path back.

[root@servera ~]# sed -i 's#^ExecStart=.*#ExecStart=/usr/local/bin/nothere.sh#' /etc/systemd/system/reporter.service
[root@servera ~]# systemctl daemon-reload
[root@servera ~]# systemctl restart reporter
[root@servera ~]# systemctl status reporter | grep Process
    Process: 988 ExecStart=/usr/local/bin/nothere.sh (code=exited, status=203/EXEC)
[root@servera ~]# journalctl -u reporter -n 1 --no-pager
Oct 03 16:37:21 servera.lab.example.com systemd[988]: reporter.service: Failed at step EXEC spawning /usr/local/bin/nothere.sh: No such file or directory
[root@servera ~]# sed -i 's#^ExecStart=.*#ExecStart=/usr/local/bin/reporter.sh#' /etc/systemd/system/reporter.service
[root@servera ~]# systemctl daemon-reload
[root@servera ~]# systemctl restart reporter
[root@servera ~]# systemctl is-active reporter
active

Grade and finish

Check your work with lab grade sa-unit-files, then put the exercise away with lab finish sa-unit-files.