Triggering regular tasks with Systemd
Alarm Clock
Systemd can start timers that automatically perform tasks at specified times. The configuration files are known as timer units.
You might want to use your Linux system to automatically create a backup every evening and rotate the log files at regular intervals. In most distributions, time-controlled tasks are handled by the Cron daemon. But Systemd is an interesting alternative to Cron. Systemd controls the startup process of most distributions, and it can also trigger time-controlled and recurring tasks.
Service Providers
The first task is to tell Systemd which task to perform. To do this, you create a configuration file, the Service Unit. Listing 1 shows an example.
Listing 1
Service Unit
[Unit] Description=Create a backup of the system [Service] ExecStart=/usr/bin/backup.sh /mnt
A service unit is a text file divided into several sections. The [Service]
section is required. ExecStart=
is followed by the command to be executed by the system. In Listing 1, Systemd would simply run a script that backs up the system to the /mnt
directory. The [Unit]
section adds some metadata. In the simplest case, Description=
is followed by a description of the task.
Service Units usually tell Systemd which services to boot when the system starts. (See the article on Systemd units elsewhere in this issue.) Systemd also supports additional sections and settings. However, since the system just needs to schedule the task, these settings are not (absolutely) necessary. In particular, you can leave out the complete [Install]
section.
Save the newly created service unit to /etc/systemd/system
. The filename corresponds to the (internal) name of the service unit. It must be unique among all service units and end with .service
, as in backup.service
. Systemd can also start existing service units or service units supplied by the distribution on a time-controlled basis. In this case, simply make a note of the filename of the service file.
Tick-Tock
To avoid burning the cake to a crisp, most hobby bakers set a kitchen timer. In a similar way, you need to set a separate timer for a task you wish to assign to Systemd.
First, create a new text file in the /etc/system/system
subdirectory. The text file should have the same filename as the service unit you created earlier, but it ends with .timer
. In the example, the file would be named backup.timer
. In Systemd speak, the file with the .timer
extension is known as the timer unit. In the timer unit, you describe when the timer should "go off," at which point, Systemd will start the backup.
The structure of a timer unit is very similar to that of a service unit. As the example from Listing 2 shows, it typically consists of three sections: [Unit]
is followed by general information about the timer. In Listing 2, this information would include a Description=
that serves mainly as a reminder for the user. Make a note on why the timer exists and what actions it triggers.
Listing 2
Timer Unit
[Unit] Description=Create a daily backup of the system [Timer] OnCalendar=*-*-* 18:15:00 Persistent=true RandomizedDelaySec=2h [Install] ZWantedBy=timers.target
Current Events
In the next section, [Timer]
, you tell Systemd when to start the task. Make a note of this time after OnCalendar=
in the notation weekday year-month-day hour:minutes: seconds. The setting OnCalendar=Fr 2018-11-30 12:00:00
tells Systemd to create the backup on Friday, November 30, 2018 at noon precisely. You can omit unnecessary information, such as the day of the week or the seconds.
Normally, you will not want Systemd to run the task once only, but repeat it. To set up a repeating event, you can simply list the corresponding days, dates, and times separated by commas. In the example from the first line of Listing 3, Systemd starts the backup November 30, 2018 at 1AM and 12 Noon.
Listing 3
Date and Time
OnCalendar=2018-11-30 01,12:00:00 OnCalendar=2018-01..12-01 01,12:00:00 OnCalendar= 2018-*-01 01,12:00:00
You can also abbreviate the number ranges with two dots ..
, which means that you do not have to list all the months, for example. The entry from the second line of Listing 3, tells Systemd to take action on the first day of each month. If the statement applies to all months, you can also use the wildcard *
(line three).
The *-*-*
entry from Listing 2 tells Systemd to run the backup every day at 18:15 in every month and every year.
Buy this article as PDF
(incl. VAT)
Buy Linux Magazine
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters
Support Our Work
Linux Magazine content is made possible with support from readers like you. Please consider contributing when you’ve found an article to be beneficial.
News
-
Canonical Bumps LTS Support to 12 years
If you're worried that your Ubuntu LTS release won't be supported long enough to last, Canonical has a surprise for you in the form of 12 years of security coverage.
-
Fedora 40 Beta Released Soon
With the official release of Fedora 40 coming in April, it's almost time to download the beta and see what's new.
-
New Pentesting Distribution to Compete with Kali Linux
SnoopGod is now available for your testing needs
-
Juno Computers Launches Another Linux Laptop
If you're looking for a powerhouse laptop that runs Ubuntu, the Juno Computers Neptune 17 v6 should be on your radar.
-
ZorinOS 17.1 Released, Includes Improved Windows App Support
If you need or desire to run Windows applications on Linux, there's one distribution intent on making that easier for you and its new release further improves that feature.
-
Linux Market Share Surpasses 4% for the First Time
Look out Windows and macOS, Linux is on the rise and has even topped ChromeOS to become the fourth most widely used OS around the globe.
-
KDE’s Plasma 6 Officially Available
KDE’s Plasma 6.0 "Megarelease" has happened, and it's brimming with new features, polish, and performance.
-
Latest Version of Tails Unleashed
Tails 6.0 is based on Debian 12 and includes GNOME 43.
-
KDE Announces New Slimbook V with Plenty of Power and KDE’s Plasma 6
If you're a fan of KDE Plasma, you'll be thrilled to hear they've announced a new Slimbook with an AMD CPU and the latest version of KDE Plasma desktop.
-
Monthly Sponsorship Includes Early Access to elementary OS 8
If you want to get a glimpse of what's in the pipeline for elementary OS 8, just set up a monthly sponsorship to help fund its continued existence.