systemd/Timers

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Systemd is capable of taking on a significant subset of the functionality of Cron through built in support for calendar time events (from systemd version 197) as well as monotonic time events. As of systemd version 212, it also has the foundations for being an anacron replacement through the use of the Persistent and OnCalendar options in timers.

Introduction

While Cron has been a stalwart on the Linux landscape for years, it still provides no way to detect job failures, establish job dependencies, or allocate processes to cgroups. If you require any of this functionality, systemd provides a good structure to set up job scheduling. While doing so is slightly more cumbersome than relying on dcron or cronie, the benefits are not insignificant:

  • Last status and logging outputs can be got through journalctl. This enables proper debugging
  • Systemd has a lot of options which are useful for setting the environment for the job to be done properly (eg IOSchedulingPriority, Nice or JobTimeoutSec)
  • These jobs can be made to depend on other systemd units if required

Hourly, daily, weekly, monthly and yearly events

One strategy which can be used for recreating this functionality is through timers which call in targets. All services which need to be run hourly can be called in as dependencies of these targets. The strategy mentioned here has been detailed first in this blogpost.

Note: There is an AUR package systemd-cronAUR which uses a slightly different strategy to do what is being done here. If you wish to use this package, then the following scripts might need to be modified and their locations changed for them to work properly.

First, the creation of a few directories is required:

# mkdir /etc/systemd/system/timer-{hourly,daily,weekly,monthly,yearly}.target.wants

The following files will need to be created in the paths specified in order for this to work.

Hourly events

/etc/systemd/system/timer-hourly.timer
[Unit]
Description=Hourly Timer

[Timer]
OnBootSec=5min
OnCalendar=hourly
Unit=timer-hourly.target

[Install]
WantedBy=basic.target
/etc/systemd/system/timer-hourly.target
[Unit]
Description=Hourly Timer Target
StopWhenUnneeded=yes

Daily events

/etc/systemd/system/timer-daily.timer
[Unit]
Description=Daily Timer

[Timer]
OnBootSec=10min
OnCalendar=daily
Persistent=true
Unit=timer-daily.target

[Install]
WantedBy=basic.target
/etc/systemd/system/timer-daily.target
[Unit]
Description=Daily Timer Target
StopWhenUnneeded=yes

As of systemd 212, the functionality of anacron is implemented in any timer which utilizes the OnCalendar option through the use of the Persistent option. If you would like to understand this functionality better, please see the systemd.timer(5) man page.

Weekly events

/etc/systemd/system/timer-weekly.timer
[Unit]
Description=Weekly Timer

[Timer]
OnBootSec=15min
OnCalendar=weekly
Persistent=true
Unit=timer-weekly.target

[Install]
WantedBy=basic.target
/etc/systemd/system/timer-weekly.target
[Unit]
Description=Weekly Timer Target
StopWhenUnneeded=yes

Monthly events

/etc/systemd/system/timer-monthly.timer
[Unit]
Description=Monthly Timer

[Timer]
OnBootSec=45min
OnCalendar=monthly
Persistent=true
Unit=timer-monthly.target

[Install]
WantedBy=basic.target
/etc/systemd/system/timer-monthly.target
[Unit]
Description=Monthly Timer Target
StopWhenUnneeded=yes

Yearly events

/etc/systemd/system/timer-yearly.timer
[Unit]
Description=Yearly Timer

[Timer]
OnBootSec=90min
OnCalendar=yearly
Persistent=true
Unit=timer-yearly.target

[Install]
WantedBy=basic.target
/etc/systemd/system/timer-yearly.target
[Unit]
Description=Yearly Timer Target
StopWhenUnneeded=yes

Adding events

Adding events to these targets is as easy as dropping them into the correct wants folder. So if you wish for a particular job to take place daily, create a systemd service file and drop it into the relevant folder.

For example, if you wish to run foo.service daily (which runs program bar), you would create the following file:

/etc/systemd/system/timer-daily.target.wants/foo.service
[Unit]
Description=Starts program bar

[Service]
User=                                          # Add a user if you wish the service to be executes as a particular user, else delete this line
Type=                                          # Simple by default, change it if you know what you are doing, else delete this line
Nice=19
IOSchedulingClass=2
IOSchedulingPriority=7
ExecStart=/usr/bin/bar --option1 --option2     # More than one ExecStart can be used if required

Some important default Arch Linux cronjobs

If you wish to completely migrate away from cron to systemd, then it is prudent to have these services up and running for your system to remain workable.

logrotate

This service runs logrotate, which rotates, compresses and mails system logs. It relies upon package logrotate which by default provides a cron job for this task (/etc/cron.daily/updatedb). See man logrotate for details.

/etc/systemd/system/timer-daily.target.wants/logrotate.service
[Unit]
Description=Rotate Logs

[Service]
Nice=19
IOSchedulingClass=2
IOSchedulingPriority=7
ExecStart=/usr/bin/logrotate /etc/logrotate.conf

man-db

This service runs mandb, which creates or updates the manual page index caches. It relies upon package man-db which by default provides a cron job for this task (/etc/cron.daily/man-db). See man mandb for details.

/etc/systemd/system/timer-daily.target.wants/man-db-update.service
[Unit]
Description=Update man-db

[Service]
Nice=19
IOSchedulingClass=2
IOSchedulingPriority=7
ExecStart=/usr/bin/mandb --quiet

shadow

This service runs pwck and grpck, which together verify integrity of password and group files (/etc/{passwd,shadow,group,gshadow} ). Those tools are provided by the shadow package, which by default provides a cron job for this task (/etc/cron.daily/shadow). See man shadow, man pwck and man grpck for details.

/etc/systemd/system/timer-daily.target.wants/verify-shadow.service
[Unit]
Description=Verify integrity of password and group files

[Service]
Type=oneshot
ExecStart=/usr/bin/pwck -r
ExecStart=/usr/bin/grpck -r

Enable and start the timers

# systemctl enable timer-{hourly,daily,weekly,monthly,yearly}.timer 
# systemctl start timer-{hourly,daily,weekly,monthly,yearly}.timer

Starting events according to the calendar

If you wish to start a service according to a calendar event and not a monotonic interval (i.e. you wish to replace the functionality of crontab), you will need to create a new timer and link your service file to that. An example would be:

/etc/systemd/system/foo.timer
[Unit]
Description=foo timer

[Timer]
# To add a time of your choosing here, please refer to systemd.time manual page for the correct format
OnCalendar=Mon-Thu *-9-28 *:30:00
Persistent=true
Unit=foo.service

[Install]
WantedBy=basic.target

The service file may be created the same way as the events for monotonic clocks. However, take care to put them in the /etc/systemd/system/ folder.

Custom/example service files

Note: These example scripts assume that you have created the hourly, daily and weekly timers as described above.

Reflector

This service file may be used to update pacman's mirrorlist daily using the reflector script.

/etc/systemd/system/timer-daily.target.wants/reflector.service
[Unit]
Description=Update the mirrorlist

[Service]
Nice=19
IOSchedulingClass=2
IOSchedulingPriority=7
Type=oneshot
ExecStart=/usr/bin/reflector --protocol http --latest 30 --number 20 --sort rate --save /etc/pacman.d/mirrorlist
Note: Reflector's options should be tweaked according to users' criteria. Use case examples may be found in this forum post by Moderator/TU Xyne.

pkgstats

If you have pkgstats installed, this service will be necessary in order to send data back to the Arch servers.

Note: It should be noted that the user nobody, which is being used here, is already present on all Arch systems. It is common to use this user as one which runs nothing but daemons. However, if your run multiple daemons as nobody, then they can communicate with and debug each other, hence a security-minded Archer would be wise to run all potential security risks as separate users.
/etc/systemd/system/timer-weekly.target.wants/pkgstats.service
[Unit]
Description=Run pkgstats

[Service]
User=nobody
ExecStart=/usr/bin/pkgstats -q

Update mlocate database

This service runs updatedb, which updates the mlocate database. The package mlocate provides by default a cron job for this task (/etc/cron.daily/updatedb). See man 8 updatedb for details.

/etc/systemd/system/timer-daily.target.wants/mlocate-update.service
[Unit]
Description=Update mlocate database

[Service]
Nice=19
IOSchedulingClass=2
IOSchedulingPriority=7
ExecStart=/usr/bin/updatedb -f proc

The modprobed_db service

This service is of great use to people who compile their own kernels because it reduces compilation time by a significant amount. Refer to the Modprobed_db page for further details.

/etc/systemd/system/timer-daily.target.wants/modprobed_db.service
[Unit]
Description=Run modprobed_db

[Service]
User=enter user here
ExecStart=/usr/bin/modprobed_db store

Hosts-update service

Package hosts-updateAUR uses mvps blocklist to update /etc/hosts.

/etc/systemd/system/timer-daily.target.wants/hosts-update.service
[Unit]
Description=Update hosts file
After=network.target

[Service]
Nice=19
IOSchedulingClass=2
IOSchedulingPriority=7
ExecStart=/usr/bin/hosts-update

[Install]
WantedBy=timer-daily.target

See also