Unit 9: Scheduled tasks and persistence · Lesson 3 of 6 · 14 min
systemd timers
The modern replacement for cron: read when a timer fires and which service it starts.
mission
- You are
- You are completing the schedule inventory.
- Situation
- Not every scheduled job lives in cron. systemd timers start services on a calendar.
- Goal
- Find the custom timer and what it runs.
- Hand in
- The time of the nightly backup and its command.
A timer and its service
- backup.timerOnCalendar=02:30
- backup.serviceUser and ExecStart
- The scriptWhat really runs
List enabled timers
ls /etc/systemd/system/timers.target.wants/
apt-daily-upgrade.timer
apt-daily.timer
backup.timer
dpkg-db-backup.timer
e2scrub_all.timer
fstrim.timer
motd-news.timer
you should see One custom timer among the package ones.
ls- Lists the files in a folder. Built into Linux and macOS.
Read the timer and its service
A timer starts the service with the same name unless it says otherwise.
grep -E '^(OnCalendar|Persistent)' /etc/systemd/system/backup.timer OnCalendar=*-*-* 02:30:00 Persistent=true grep -E '^(User|ExecStart)' /etc/systemd/system/backup.service User=root ExecStart=/opt/scripts/report.sh
you should see Every night at 02:30 as root. Persistent means a missed run happens at the next boot.
defender tip On a running host, systemctl list-timers shows the last and next run of each.
grep- Prints the lines of text that match a pattern. Built into Linux and macOS.
List every timer and its state
This works even without systemd running, because it reads the unit files.
systemctl list-unit-files --type=timer --no-pager | head -n 9
UNIT FILE STATE PRESET
apt-daily-upgrade.timer enabled enabled
apt-daily.timer enabled enabled
backup.timer enabled enabled
dpkg-db-backup.timer enabled enabled
e2scrub_all.timer enabled enabled
fstrim.timer enabled enabled
motd-news.timer enabled enabled
systemd-sysupdate-reboot.timer disabled enabled
you should see backup.timer enabled alongside the standard ones.
systemctl- Starts, stops and inspects system services. Built into Linux systems that use systemd.
head- Prints the first lines of a file or of piped text. Built into Linux and macOS.
What a defender takes away
- 1Timers live next to services in /etc/systemd/system.
- 2OnCalendar sets the schedule.
- 3Review timers with the same questions as cron: who, what, when.
Questions
0 of 2 solved
Answer each one from what you find on your lab machine.