systemd/cron functionality

Summary
Covers how to replace cron by systemd.
Related
systemd
systemd/User
systemd/Services
systemd FAQ
cron

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.

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 ot be done properly (eg IOSchedulingPriority, Nice or JobTimeoutSec)
These jobs can be made to depend on other systemd units if required

While you do lose support for anacron; systemd timer, target and service files can fill up for the rest of cron very easily.

Hourly, daily and weekly 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-cron 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}.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
OnUnitActiveSec=1h
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
OnUnitActiveSec=1d
Unit=timer-daily.target

[Install]
WantedBy=basic.target

/etc/systemd/system/timer-daily.target

[Unit]
Description=Daily Timer Target
StopWhenUnneeded=yes

Weekly events

/etc/systemd/system/timer-weekly.timer

[Unit]
Description=Weekly Timer

[Timer]
OnBootSec=15min
OnUnitActiveSec=1w
Unit=timer-weekly.target

[Install]
WantedBy=basic.target

/etc/systemd/system/timer-weekly.target

[Unit]
Description=Weekly 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

/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

Update man-db

/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

Update mlocate database

/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

Verify integrity of password and group files

/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}.timer && systemctl start timer-{hourly,daily,weekly}.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]
OnCalendar=Mon-Thu *-9-28 *:30:00 # To add a time of your choosing here, please refer to systemd.time manual page for the correct format
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.

The pkgstats service

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

Tip: It should be noted that the user nobody, which is being used here, is already present on all Arch systems. The use of this user is recommended for all services which can function in a completely unprivileged environment.

/etc/systemd/system/timer-weekly.target.wants/pkgstats.service

[Unit]
Description=Run pkgstats

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

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

The reflector service

This service file may be used to upgrade the mirrorlist daily using the Reflector program.

/etc/systemd/system/timer-daily.target.wants/reflector.service

[Unit]
Description=Update the mirrorlist

[Service]
Nice=19
IOSchedulingClass=2
IOSchedulingPriority=7
ExecStart=/usr/bin/reflector --protocol http --latest 30 --number 20 --sort rate --save /etc/pacman.d/mirrorlist

The man-db service

This service runs mandb, which updates the manual page index caches. See man 8 mandb for details. The package man-db provides a cron job for this task (file /etc/cron.daily/man-db).

/etc/systemd/system/timer-daily.target.wants/man-db.service

[Unit]
Description=Update manual page index caches

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

The shadow service

This service verifies integrity of password and group files (/etc/{passwd,shadow,group,gshadow} ). The package shadow provides a cron job for this task (file /etc/cron.daily/shadow).

/etc/systemd/system/timer-daily.target.wants/shadow.service

[Unit]
Description=Verify integrity of password and group files

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