systemd/Timers

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

The factual accuracy of this article or section is disputed.

Reason: The method described below seems to be counterproductive in the words of Arch Linux developer Falconindy (a.k.a Dave Reisner); see Talk:Systemd/cron functionality#Counterproductive Suggestions for explanations and proposed corrections. (Discuss in Talk:Systemd/Timers)

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^AUR 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.

Note: There is also an AUR package systemd-crontab-generator^AUR which is intended to be a drop-in replacement for cron and anacron daemons. It works by parsing crontab files in known directories (like /etc/crontab and /var/spool/cron/) and generating timers and services in systemd. Though it is a beta product (while it works on several hosts owned by program's author), so it is not quite ready for production. Please read comments carefully before installation.

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]
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]
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]
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]
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.

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

/etc/systemd/system/foo.service

[Unit]
Description=Starts program bar
Wants=timer-daily.timer

[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

[Install]
WantedBy=timer-daily.target

Then, start and enable the service.

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.

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/modprobed_db.service

[Unit]
Description=Run modprobed_db
Wants=timer-daily.target

[Service]
Type=oneshot
RemainAfterExit=yes
User=enter user here
Nice=19
IOSchedulingClass=2
IOSchedulingPriority=7
ExecStart=/usr/bin/modprobed_db store

[Install]
WantedBy=timer-daily.target

Then, start and enable the service

Hosts-update service

Package hosts-update^AUR 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