Difference between revisions of "Systemd/Timers"
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Revision as of 06:30, 12 April 2018
Timers are systemd unit files whose name ends in
.timer that control
.service files or events. Timers can be used as an alternative to cron (read #As a cron replacement). Timers have built-in support for calendar time events, monotonic time events, and can be run asynchronously.
Timers are systemd unit files with a suffix of
.timer. Timers are like other unit configuration files and are loaded from the same paths but include a
[Timer] section which defines when and how the timer activates. Timers are defined as one of two types:
- Realtime timers (a.k.a. wallclock timers) activate on a calendar event, the same way that cronjobs do. The option
OnCalendar=is used to define them.
- Monotonic timers activate after a time span relative to a varying starting point. They stop if the computer is temporarily suspended or shut down. There are number of different monotonic timers but all have the form:
OnTypeSec=. Common monotonic timers include
For a full explanation of timer options, see the. The argument syntax for calendar events and time spans is defined in .
.timer file, a matching
.service file exists (e.g.
.timer file activates and controls the
.service file. The
.service does not require an
[Install] section as it is the timer units that are enabled. If necessary, it is possible to control a differently-named unit using the
Unit= option in the timer's
$ systemctl list-timers
NEXT LEFT LAST PASSED UNIT ACTIVATES Thu 2014-07-10 19:37:03 CEST 11h left Wed 2014-07-09 19:37:03 CEST 12h ago systemd-tmpfiles-clean.timer systemd-tmpfiles-clean.service Fri 2014-07-11 00:00:00 CEST 15h left Thu 2014-07-10 00:00:13 CEST 8h ago logrotate.timer logrotate.service
- To list all timers (including inactive), use
systemctl list-timers --all.
- The status of a service started by a timer will likely be inactive unless it is currently being triggered.
- If a timer gets out of sync, it may help to delete its
/var/lib/systemd/timers. These are zero length files which mark the last time each timer was run. If deleted, they will be reconstructed on the next start of their timer.
A service unit file can be scheduled with a timer out-of-the-box. The following examples schedule
foo.service to be run with a corresponding timer called
A timer which will start 15 minutes after boot and again every week while the system is running.
[Unit] Description=Run foo weekly and on boot [Timer] OnBootSec=15min OnUnitActiveSec=1w [Install] WantedBy=timers.target
A timer which starts once a week (at 12:00am on Monday). When activated, it triggers the service immediately if it missed the last start time (option
Persistent=true), for example due to the system being powered off:
[Unit] Description=Run foo weekly [Timer] OnCalendar=weekly Persistent=true [Install] WantedBy=timers.target
When more specific dates and times are required,
OnCalendar events uses the following format:
DayOfWeek Year-Month-Day Hour:Minute:Second
An asterisk may be used to specify any value and commas may be used to list possible values. Two values separated by
.. indicate a contiguous range.
In the below example the service is run the first four days of each month at 12:00 PM, but only if that day is a Monday or a Tuesday.
OnCalendar=Mon,Tue *-*-01..04 12:00:00
To run a service on the first Saturday of every month, use:
OnCalendar=Sat *-*-1..7 18:00:00
More information is available in.
OnCalendartime specifications can be tested in order to verify their validity and to calculate the next time the condition would elapse when used on a timer unit file with the
calendaroption of the systemd-analyze utility. For example, one can use
systemd-analyze calendar weeklyor
systemd-analyze calendar "Mon,Tue *-*-01..04 12:00:00".
- Special event expressions like
weeklyrefer to specific start times and thus any timers sharing such calendar events will start simultaneously. Timers sharing start events can cause poor system performance if the timers' services compete for system resources. The
RandomizedDelaySecoption in the
[Timer]section avoids this problem by randomly staggering the start time of each timer. See .
Transient .timer units
One can use
systemd-run to create transient
.timer units. That is, one can set a command to run at a specified time without having a service file. For example the following command touches a file after 30 seconds:
# systemd-run --on-active=30 /bin/touch /tmp/foo
One can also specify a pre-existing service file that does not have a timer file. For example, the following starts the systemd unit named
someunit.service after 12.5 hours have elapsed:
# systemd-run --on-active="12h 30m" --unit someunit.service
Seefor more information and examples.
As a cron replacement
Although cron is arguably the most well-known job scheduler, systemd timers can be an alternative.
The main benefits of using timers come from each job having its own systemd service. Some of these benefits are:
- Jobs can be easily started independently of their timers. This simplifies debugging.
- Each job can be configured to run in a specific environment (see ).
- Jobs can be attached to cgroups.
- Jobs can be set up to depend on other systemd units.
- Jobs are logged in the systemd journal for easy debugging.
Some things that are easy to do with cron are difficult to do with timer units alone:
- Creation: to set up a timed job with systemd you need to create two files and run
systemctlcommands, compared to adding a single line to a crontab.
- Emails: there is no built-in equivalent to cron's
MAILTOfor sending emails on job failure. See the next section for an example of setting up a similar functionality using
You can set up systemd to send an e-mail when a unit fails. Cron sends mail to
MAILTO the job outputs to stdout or stderr, but many jobs are setup to only output on error. First you need two files: an executable for sending the mail and a .service for starting the executable. For this example, the executable is just a shell script using
#!/bin/bash /usr/bin/sendmail -t <<ERRMAIL To: $1 From: systemd <root@$HOSTNAME> Subject: $2 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 $(systemctl status --full "$2") ERRMAIL
Whatever executable you use, it should probably take at least two arguments as this shell script does: the address to send to and the unit file to get the status of. The .service we create will pass these arguments:
[Unit] Description=status email for %i to user [Service] Type=oneshot ExecStart=/usr/local/bin/systemd-email address %i User=nobody Group=systemd-journal
user is the user being emailed and
address is that user's email address. Although the recipient is hard-coded, the unit file to report on is passed as an instance parameter, so this one service can send email for many other units. At this point you can start
firstname.lastname@example.org to verify that you can receive the emails.
Then simply edit the service you want emails for and add
OnFailure=status-email-user@%n.service to the
%n passes the unit's name to the template.
- If you set up SSMTP security according to SSMTP#Security the user
nobodywill not have access to
/etc/ssmtp/ssmtp.conf, and the
systemctl start email@example.com will fail. One solution is to use
rootas the User in the
- If you try to use
mail -s somelogs addressin your email script,
mail -Ssendwait -s somelogs address.
Using a crontab
Several of the caveats can be worked around by installing a package that parses a traditional crontab to configure the timers.
Also, like with crontabs, a unified view of all scheduled jobs can be obtained with
systemctl. See #Management.
- Fedora Project wiki page on systemd calendar timers
- Gentoo wiki section on systemd timer services
- systemd-cron-next — tool to generate timers/services from crontab and anacrontab files
- systemd-cron — provides systemd units to run cron scripts; using systemd-crontab-generator to convert crontabs