Use cron when a task must run at a particular wall-clock time, such as a backup at 02:00 or a report at 06:30. Use anacron when the task must happen periodically but the exact minute is unimportant: if the VPS is powered off at the planned time, anacron runs the job after the system returns. Cron jobs that fall during downtime are skipped.
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This guide shows where root and user schedules belong, how to write each file, how to make jobs observable, and how to avoid duplicate execution on distributions that translate cron files into systemd timers.
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Cron and anacron solve different scheduling problems
Cron is a wall-clock scheduler. It checks the current date and time and starts a command when the five time fields in a schedule match. Red Hat’s Enterprise Linux System Administrator’s Guide summarizes the limitation precisely: “A cron job is only executed if the system is running on the scheduled time.” A VPS reboot, shutdown, or outage at that instant means that occurrence is lost.
Anacron is a periodic catch-up scheduler. It tracks whether a periodic job has completed and postpones it when the machine is unavailable. Red Hat describes the behavior this way: “If the system is not running at the scheduled time, an anacron job is postponed until the system is running.” A given anacron job runs no more than once per day, even if its period or other conditions might otherwise cause more frequent execution.
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| Concern | Cron | Anacron |
|---|---|---|
| Timing model | Specific minute, hour, date, and weekday combinations | Periodic interval with a startup delay |
| When the VPS is down | The occurrence is skipped | The job is deferred and runs after the system is available |
| Minimum repeat behavior | Can run every minute, subject to the schedule | At most once per day for each job |
| Typical uses | 02:00 backups, 06:30 reports, five-minute polling | Daily, weekly, or monthly maintenance where eventual completion matters |
| Configuration | /etc/cron.d/, /etc/crontab, or a user crontab |
/etc/anacrontab and the implementation’s supporting files |
| Privilege | Root files name a user; user crontabs run as their owner | The system anacrontab is normally edited as root and runs with the configured system context |
| Observability | Daemon status, mail, logs, and redirected output | Daemon or generated-timer logs, anacron state, mail, and redirected output |
Choose based on the failure you are willing to accept. Missing a precise 02:00 window points to cron. Missing an entire day of maintenance because the VPS was rebooting points to anacron or a persistent systemd timer.
Before creating a schedule
Identify the distribution and scheduler implementation
Package names and service names differ between Linux families. On Red Hat-based systems, the documented prerequisites install the cronie package, enable crond.service, and start it for the current session. Debian-family systems use their own package and service conventions, so follow the distribution’s package documentation rather than copying the Red Hat package command.
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Check the running services before adding entries:
systemctl status crond
systemctl status cronOne of those names may not exist on your VPS. The status command that succeeds identifies the service name used by that installation. If your distribution uses systemd-cron, also inspect the generated timers:
systemctl list-timers --allDo not assume that a file has been activated merely because it was saved. Confirm that one scheduler is reading it and that the service is enabled for future boots.
Decide which account should run the command
Write down the least-privileged account that can read the inputs and write the outputs. A backup that needs access to protected system directories may require root; an application export should usually run as the application account. The account determines file permissions, environment variables, network credentials, and the location of user-owned files.
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Set up a root cron job
For a system-wide job, create a file in /etc/cron.d/ (the preferred location in the Red Hat guide) or edit /etc/crontab. These formats have six scheduling columns: minute, hour, day of month, month, day of week, and username, followed by the command.
| Position | Allowed values | Meaning |
|---|---|---|
| Minute | 0-59 |
Minute within the hour |
| Hour | 0-23 |
Hour in the 24-hour clock |
| Day of month | 1-31 |
Calendar day |
| Month | 1-12 or names supported by the implementation |
Month of the year |
| Day of week | 0-7 or names supported by the implementation |
Weekday; implementations commonly treat 0 and 7 as Sunday |
| Username | An existing local account | User whose privileges execute the command |
A nightly root backup at 02:15 can be represented as:
15 2 * * * root /usr/local/sbin/backup.shSave it as a clearly named file such as /etc/cron.d/nightly-backup. The file’s command must use an absolute path. Keep one logical job per file when practical; that makes ownership, review, and removal safer.
Use the correct syntax for a user crontab
Run crontab -e while logged in as the intended account, or explicitly select that account through your administration procedure. A user crontab has only the five time fields and the command; it does not contain a username column:
15 2 * * * /home/app/bin/backup.shPutting root in this format shifts every field and produces an invalid or dangerous schedule. Conversely, omitting the username in /etc/cron.d/ can cause the file to be rejected or interpreted incorrectly.
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Common schedule expressions
| Purpose | Entry pattern |
|---|---|
| Every five minutes | */5 * * * * app /path/to/poll in a system file, or omit app in that user’s crontab |
| At 06:30 every day | 30 6 * * * app /path/to/report |
| At 02:00 every Sunday | 0 2 * * 0 app /path/to/weekly-task |
| On the first day of each month at 03:10 | 10 3 1 * * app /path/to/monthly-task |
Use the syntax accepted by your cron implementation for names and special shortcuts. Numeric fields are the most portable choice. Review schedules involving both day-of-month and day-of-week carefully; implementations can treat those two fields as an inclusive match rather than requiring both, so test the actual dates your command will run.
Make cron jobs reliable and observable
Define the execution environment
Cron is not an interactive shell session. Set a predictable shell and path in a system cron file when needed, and do not rely on aliases, interactive profile files, or a working directory:
SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
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15 2 * * * root /usr/local/sbin/backup.shUse the real mail destination in your environment; the example domain is only a placeholder. If a job needs application variables, define them explicitly or load a controlled environment inside the script. Use absolute paths for interpreters, utilities, input files, and output files.
Capture output and errors
Unobserved failures are the most common cron problem. Redirect standard output and standard error to a log that the job’s account can write:
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15 2 * * * root /usr/local/sbin/backup.sh >>/var/log/nightly-backup.log 2>&1Alternatively configure mail with MAILTO and ensure the VPS has a working local mail transport. Do not redirect everything to /dev/null until you have another way to detect failures.
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Check permissions and secrets
- Make the script executable, for example with
chmod 750 /usr/local/sbin/backup.sh, and verify that the scheduled account can traverse every parent directory. - Give the account write access to the destination but not broader access than required.
- Keep passwords, API keys, and private keys out of world-readable cron files and command lines. Store them in a protected configuration file or secret mechanism readable only by the job’s account.
- Run the command manually as the target account before scheduling it. This catches missing permissions and path assumptions without waiting for the next run.
Configure anacron for catch-up work
Edit /etc/anacrontab as root. Each job has four fields:
- Period: the interval in days.
- Delay: minutes to wait after anacron starts before launching the command.
- Job identifier: a stable name used to track the job.
- Command: the full command to execute.
This entry runs the script every three days, waiting 60 minutes after the scheduler becomes active:
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3 60 cron.daily /usr/local/bin/my-script.shThe delay is useful when many services start together after boot or when you want routine maintenance away from the initial recovery workload. Keep the identifier stable; changing it can make the scheduler treat the entry as a new job.
Special period tokens and portability
The Debian systemd-cron implementation documents tokens including @daily, @weekly, and @monthly. It also notes that traditional anacron accepts only @monthly and @yearly. If a schedule must move between distributions, use numeric day periods or verify the exact implementation first. Do not assume a token accepted by a generated systemd timer is accepted by a standalone anacron daemon.
Choose a delay and interval deliberately
Use anacron for work whose completion matters more than an exact timestamp: filesystem cleanup, certificate or cache maintenance, periodic reports, and other daily, weekly, or monthly tasks. A weekly anacron job has a guaranteed minimum six-day gap according to Debian’s documentation. That prevents repeated execution during a short outage-and-reboot cycle, but it also means an anacron entry is not a substitute for a five-minute poller or a precise deadline.
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What happens after a missed cron time
Cron does not queue an occurrence that happened while the VPS was off. If the machine was down at 02:00, a 0 2 * * * job waits until the next scheduled 02:00. If that lost run is unacceptable, redesign the task as an anacron period or as a persistent systemd timer, then make the script safe to run after a long delay.
What happens after a missed anacron period
Anacron records periodic completion and runs the pending job when the machine returns, subject to its configured delay. It is therefore appropriate for “at least once per period” maintenance, not for a guaranteed minute. The command should be idempotent: running it after a delayed boot should produce a correct result rather than corrupting partially completed work.
Debian systemd-cron caveats
On Debian systems using systemd-cron, cron files can be translated into systemd timers. Native systemd timers take precedence over cron jobs with the same name. Debian also warns that running systemd-cron alongside another cron daemon or anacron can execute scripts more than once. Pick one scheduling path for each job and disable the competing daemon or entry.
After editing /etc/anacrontab, inspect recent messages and generated timers:
journalctl -n
systemctl list-timers --allFor a persistent systemd timer, Debian notes that a timer may run at boot after downtime and then again at the next calendar boundary. That behavior differs from an anacron weekly job’s minimum six-day spacing, so choose the mechanism whose catch-up semantics match the task.
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- Validate the file location and syntax. Confirm whether the entry is in
/etc/cron.d/,/etc/crontab, a user crontab, or/etc/anacrontab. Count the fields and check the username column rules. - Check the service. Run
systemctl status crondor the distribution’s cron service name. Ensure it is enabled for boot and active now. - Check generated timers. On systemd-cron installations, run
systemctl list-timers --alland confirm the expected unit and next trigger. - Inspect logs. Use
journalctlfor service and timer messages. Review the job’s own log or mail output for the command’s exit status and error text. - Test as the target account. Execute the exact absolute command with the same account, environment, and working-directory assumptions. Do not test only as an interactive administrator.
- Test the downtime policy. For anacron or a persistent timer, verify behavior on a disposable VPS or maintenance window by stopping the service or rebooting, then checking that one catch-up run occurs. Do not induce an outage on a production machine merely to test scheduling.
Troubleshoot the common failure modes
The job never runs
- The daemon is stopped or the wrong service name was checked. Verify the active service with
systemctl status. - The file has the wrong number of fields, especially a missing username in
/etc/cron.d/or an extra username in a user crontab. - The command path is relative, the script is not executable, or its interpreter line points to a nonexistent path.
- The schedule uses a timezone different from the one you expected. Confirm the VPS timezone and express the schedule in that timezone, or use a scheduler configuration that explicitly supports the required timezone.
The job runs manually but fails under cron
- Set
PATHand other required variables explicitly. - Replace relative paths with absolute paths and choose a working directory inside the script.
- Check read, execute, and write permissions for the scheduled account.
- Read the redirected log and service journal; cron’s non-interactive shell will not show prompts or terminal-only output.
The job runs twice
- Look for the same command in a user crontab,
/etc/crontab, and/etc/cron.d/. - On Debian, check whether systemd-cron is translating the file while a separate cron daemon or anacron is also active.
- Check for a native systemd timer with the same purpose. Disable one scheduling path and retain a single owner.
- Add a lock or an in-progress check inside long-running scripts so an overlap cannot corrupt output while you correct the schedule.
A supposedly missed job does not catch up
That is expected for ordinary cron. Replace it with an anacron entry or a persistent systemd timer if eventual execution is required. Then confirm the implementation’s period, delay, state tracking, and generated timer behavior rather than assuming all “persistent” schedulers use the same rules.
A practical decision checklist
- Use cron for a precise wall-clock event: a report at 06:30, a backup window at 02:00, or polling every five minutes.
- Use anacron for daily, weekly, or monthly maintenance where one completed run per period is enough and a reboot should not lose the work.
- Use a systemd timer when your distribution already standardizes on native systemd units and you need its documented persistence and timer inspection tools.
- Put privileged system jobs in
/etc/cron.d/or/etc/crontabwith an explicit username; put account-specific jobs in that account’scrontab -ewithout a username. - Use absolute paths, explicit environment settings, protected secrets, and visible logs for every scheduler.
- Before production, verify the service, generated timer, permissions, timezone, and catch-up behavior on the actual VPS distribution.
The reliable design is the one whose downtime behavior matches the business requirement: cron preserves timing but can skip work, while anacron sacrifices the exact minute to preserve periodic completion.
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