Use cron for a simple recurring command when a familiar crontab entry and compatibility with your target Unix-like environments are the priority. On a system running systemd, choose a systemd timer when the job needs service-manager integration, boot- or service-relative triggers, inspectable unit state, or a one-time catch-up after downtime. Neither choice guarantees execution at an exact instant; check the installed scheduler and version before relying on a particular feature.
How cron and systemd timers schedule work
Cron puts a schedule and command together in a crontab. Its familiar format has five time-and-date fields—minute, hour, day of month, month, and day of week—followed by the command. A system crontab may also include a username. The cited Linux crontab(5) implementation checks entries every minute; behavior can differ among cron implementations. Linux crontab(5) reference.
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A systemd timer is a unit that activates a service unit. The timer defines when activation should happen; the service defines the command and its service behavior. OnCalendar= handles wall-clock schedules, while monotonic settings can trigger work after boot or after the service manager starts. Timers can combine calendar and monotonic expressions. systemd.timer documentation.
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Which should you use for scheduled tasks?
| Need | Cron | systemd timer |
|---|---|---|
| Simple recurring calendar job | A good fit when a five-field schedule and command are enough. | Also works, but requires a timer unit and a service unit. |
| Compatibility beyond systemd hosts | A reasonable choice if the target provides the cron implementation your team expects; verify it on each platform. | Requires a systemd host and familiarity with unit files. |
| Triggers tied to boot or manager start | Not expressed by the standard five-field schedule alone. | Use monotonic settings such as OnBootSec= or OnStartupSec=. |
| Service integration and inspection | Commands are compact in a crontab; arrange logging, dependencies, and lifecycle behavior for the local setup. | The timer activates a service that can be managed and inspected with systemd tools. |
| Catch-up after the timer was inactive | Do not assume a universal missed-run policy from the word “cron”; check the installed implementation and surrounding tools. | Persistent=true can cause one catch-up activation for missed calendar events. |
For a straightforward recurring command, cron’s compactness and familiarity often win. Prefer a systemd timer when the job is part of a systemd-managed service, needs a boot-relative trigger, or benefits from timer-specific controls. These are conditional choices, not a claim that one scheduler is universally better.
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Where schedule expressions differ
Cron day-of-month and day-of-week rules
In the cited Linux crontab reference, the minute, hour, and month fields must match, and at least one of day-of-month or day-of-week must match when both are restricted. A schedule that specifies both day fields may therefore behave differently than a reader expects. Check the installed cron manual before translating it.
The same reference says a scheduled local time that does not exist during a daylight-saving clock change is skipped, while a repeated local time may run twice. It also documents CRON_TZ for a crontab-specific timezone; log timestamps remain in the daemon’s local timezone. These are details of that documented implementation, not guarantees for every cron variant. Linux crontab(5) reference.
systemd calendar expressions
OnCalendar= uses systemd calendar syntax rather than cron’s five fields. Multiple calendar expressions can be specified, and calendar expressions can be combined with monotonic triggers. Calendar timers depend on the realtime clock being set correctly; systemd orders them after time-sync.target. Validate expressions on the target machine instead of assuming a cron string can be copied unchanged. systemd.timer documentation.
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Will a systemd timer run if my computer was off?
Not while the computer is off. For a calendar timer, Persistent=true records the last trigger time and causes the service to activate when the timer starts if at least one scheduled event elapsed while the timer was inactive. It is false by default and applies only to OnCalendar= timers. If several events were missed, systemd triggers the service once; it does not replay every occurrence.
If the computer was suspended rather than powered off, missed calendar events likewise result in a single activation after resume. WakeSystem=true can wake a suspended system only where the hardware supports timer wake-up and the necessary privileges are available. systemd.timer documentation.
How precise are cron and systemd timers?
Neither scheduler should be treated as an exact real-time execution guarantee. The cited cron implementation checks entries every minute. In current upstream systemd documentation, AccuracySec= defaults to one minute and permits the timer expiry within a coalescing window; actual process start can also be affected by timer slack and service startup time. Linux crontab(5) reference; systemd.timer documentation.
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Systemd distinguishes coalescing wakeups from spreading load. AccuracySec= allows nearby timer events to be grouped; RandomizedDelaySec= adds a random delay to spread otherwise similar events. The manual recommends reducing AccuracySec= when the intent is to spread events across the delay window. Cron is not necessarily without randomization: the cited crontab manual documents RANDOM_DELAY, whose scaling factor is selected when the daemon starts and remains constant while it runs. Check the installed cron version before depending on it.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesCurrent upstream systemd documentation also describes FixedRandomDelay= (version 247) and RandomizedOffsetSec= (version 258). The latter provides a stable offset for calendar events across manager restarts. Availability depends on the systemd version installed, so consult the local systemd.timer(5) manual before using either option. systemd.timer documentation; systemd.timer(5).
Do systemd timers run after logout?
A system-wide timer is managed by the system manager. A user timer runs under that user’s systemd user manager, so its behavior depends on whether that manager remains active. In the SLES 16.0 setup documented by SUSE, a user timer runs only while the user is logged in unless lingering is enabled. Enable lingering with loginctl enable-linger USER to have the user’s manager start at boot and remain active after logout. Confirm the behavior on your distribution.
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Do not assume a user service inherits variables set in .profile or .bashrc. SUSE’s guide says those shell startup variables are not inherited in its documented setup; define required environment variables explicitly for the service. SUSE: Working with systemd Timers.
Example: run a service after boot and on weekday mornings
This SUSE example runs a script five minutes after boot and at 10:00 Monday through Friday. The service contains the command; the timer contains both triggers.
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[Unit]
Description=Hello World script
[Service]
ExecStart=/usr/local/bin/helloworld.sh# /etc/systemd/system/helloworld.timer
[Unit]
Description=Run helloworld.service 5min after boot and at 10am every Mon-Fri
[Timer]
OnBootSec=5min
OnCalendar=Mon..Fri *-*-* 10:00:00
Unit=helloworld.service
[Install]
WantedBy=multi-user.targetValidate this syntax against the systemd version on the target host before deploying. The SUSE guide recommends systemd-analyze verify for unit files and systemd-analyze calendar for calendar expressions. Use systemctl list-timers to inspect timers and journalctl to review timer and service logs. SUSE: Working with systemd Timers.
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What to check when migrating from cron
Migration involves more than translating a schedule string. Build and validate the timer and service separately, then verify the command’s environment and output handling.
- Write down the intended schedule semantics. Check cron’s day-of-month/day-of-week behavior, timezone, daylight-saving behavior, and any implementation-specific settings before translating the expression.
- Create a service unit. Put the command in
ExecStart=and explicitly configure environment variables that were previously supplied by the cron environment or shell startup files. - Create a timer unit. Use
OnCalendar=for wall-clock schedules and monotonic settings for boot- or manager-start-relative triggers. AddPersistent=trueonly if one catch-up activation after downtime is appropriate. - Decide how output and notifications should work. Cron’s
MAILTObehavior does not automatically become an equivalent systemd notification; configure logging or notification handling for the new service. - Validate and observe. Run
systemd-analyze verifyon the unit files andsystemd-analyze calendaron the schedule, then inspect timer state and service logs after activation.
These migration checks follow SUSE’s SLES 16.0 guide; confirm exact commands and behavior for the distribution and systemd version you use. SUSE: Working with systemd Timers.
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