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 a daily, weekly, or monthly task must eventually run even if the VPS was powered off at its scheduled time. Cron skips a missed instant; anacron records periodic completion and catches up after the machine returns.
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This guide shows where root and user entries belong, how to configure both schedulers, how Debian’s systemd integration changes troubleshooting, and how to verify that jobs run only once with the intended permissions.
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Cron is a wall-clock scheduler. It evaluates entries at their specified minute, hour, date, and weekday while the operating system is running. If the VPS is stopped at that exact time, the cron invocation is missed; it is not replayed automatically when the server boots. The Red Hat Enterprise Linux System Administrator’s Guide summarizes this behavior: “A cron job is only executed if the system is running on the scheduled time.”
Anacron is a periodic catch-up scheduler. It tracks when each job last completed and runs it after the machine is available again. 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 traditional anacron job can run at most once per day, so it is intended for daily-or-longer maintenance rather than minute-level polling.
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| Decision point | Cron | Anacron |
|---|---|---|
| Timing model | Exact wall-clock fields (minute, hour, date, weekday) | Elapsed periodic interval with a configurable startup delay |
| When the VPS is down | The scheduled run is skipped | The job is postponed and runs after the VPS returns |
| Typical shortest interval | One minute in a standard five-field entry; practical intervals depend on workload | One day for traditional anacron jobs |
| Best uses | 02:00 backups, 06:30 reports, every-five-minute polling | Daily log rotation, weekly cleanup, monthly maintenance |
| Primary configuration | /etc/cron.d/, /etc/crontab, or a user’s crontab |
/etc/anacrontab |
| Privilege context | Root files name a user; user crontabs run as their owner | The system anacrontab is normally edited and run as root |
| Observability | Daemon status, mail, redirected logs, and system journal | Daemon or generated-timer logs, timestamps, mail, and redirected logs |
Choose based on what failure means. A report that is useless after its intended time belongs in cron. A cleanup task that is still useful later belongs in anacron.
Choose the scheduler for the task
Use cron for time-sensitive work
Put a job in cron when its start time is part of the requirement. Examples include taking a database snapshot shortly before a maintenance window, polling an endpoint every five minutes, or generating a morning report at 06:30. Cron gives you the precise calendar expression, but you must accept that downtime at the scheduled instant means no run.
Use anacron for maintenance that may catch up
Anacron is appropriate for jobs that need to happen once per period, not at a particular minute. Daily package-cache cleanup, weekly archival, and monthly accounting are common examples. If the VPS is rebooting or stopped during the nominal period, anacron waits until it is available and then applies the configured delay.
When neither behavior is sufficient
If a job must run at a precise time and also be recovered after downtime, do not silently substitute one scheduler for the other. Design an explicit recovery step, such as checking the last successful run from the job itself, and then decide whether cron plus that check or a native systemd timer better matches the requirement. On Debian systems, persistent systemd timers have behavior that differs from traditional anacron; inspect the actual timers before enabling overlapping mechanisms.
Prepare the VPS before creating a schedule
- Identify the distribution and service name. Red Hat-family systems commonly use the
croniepackage andcrond.service. Debian-family systems use distribution-specific cron packages and commonly expose a service namedcron. Follow the package documentation for the installed release rather than copying a package command from another distribution. - Check that the scheduler is installed and running. On a system using the Red Hat service name, run:
systemctl status crondIf your distribution uses another name, substitute it. A service can be installed yet disabled, or enabled for boot but stopped in the current session.
- Confirm the clock and timezone. Cron evaluates the host’s local time. Check it with:
timedatectlVerify the timezone, synchronization state, and daylight-saving expectations before choosing a calendar time.
- Prepare the command as the account that will run it. Confirm that the script exists, is executable, can read its inputs, and can write its output. Test it manually in that account before placing it in a scheduler.
- Decide where output will go. Cron can mail job output, but unattended VPS work is easier to diagnose when standard output and errors are redirected to a known log. Ensure the selected account can create or append to that file.
Understand cron’s two configuration forms
Cron has a five-field schedule, but system-wide files add a username field. Confusing these formats is a common cause of jobs that never run or run as the wrong account.
| Location or command | Fields | Execution identity |
|---|---|---|
/etc/crontab |
Minute, hour, day of month, month, day of week, username, command | The username field selects the account |
/etc/cron.d/<name> |
Minute, hour, day of month, month, day of week, username, command | The username field selects the account |
crontab -e |
Minute, hour, day of month, month, day of week, command | The account that owns the crontab |
The day-of-month and day-of-week fields have the usual cron calendar semantics provided by the installed implementation. Use an explicit, readable expression and test it rather than relying on a complicated combination of both fields.
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Root job in /etc/cron.d/
For a service-wide task, create a descriptively named file in /etc/cron.d/. The Red Hat guide prefers this approach for a separately managed job because it avoids editing the distribution’s main file.
15 2 * * * root /usr/local/sbin/backup.sh >> /var/log/backup.log 2>&1This means minute 15 of hour 2 every day, running as root. The command uses absolute paths and sends both output streams to a log. Keep the file owned by root and readable by the scheduler, and make the script executable:
chmod 750 /usr/local/sbin/backup.shBefore relying on the schedule, run the script directly as root and verify the resulting files, permissions, and log entries.
Root job in /etc/crontab
You can place the same seven-field entry in /etc/crontab:
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User job with crontab -e
Log in as the intended account and run:
crontab -eAdd only five schedule fields followed by the command; do not add a username:
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15 2 * * * /home/app/bin/backup.sh >> /home/app/logs/backup.log 2>&1The job runs with that user’s privileges. It cannot read root-only inputs or write directories owned by another account. This is usually the safer choice for application data because a script compromise does not automatically grant root access.
Make cron jobs reliable and observable
Use an explicit environment
Cron starts jobs with a limited environment. Commands that work in an interactive SSH shell can fail because a binary is not on PATH, a required variable is absent, or the working directory is different. Call programs by absolute path and set required variables in the script or crontab. For example:
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
15 2 * * * root /usr/local/sbin/backup.sh >> /var/log/backup.log 2>&1Use the shell path supported by your distribution and script. If the script expects a particular directory, change to it explicitly inside the script instead of assuming an interactive working directory.
Capture both success and failure signals
Redirect standard output and standard error to a log or configure a mail destination for cron output. A silent job is difficult to distinguish from a successful one. Include a timestamped success message in the script’s own log when that helps operations, and arrange log rotation so an unattended task cannot fill the filesystem.
Match permissions to the execution user
Check every input, output directory, configuration file, and secret against the account that runs the job. Protect credentials from world-readable cron files and scripts. Test the exact command as the target user, not only as root; a root test can hide permission and environment problems that the scheduled account will encounter.
Configure anacron for catch-up execution
Edit /etc/anacrontab as root. Each job has four fields:
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- Period: the interval in days.
- Delay: minutes to wait after anacron decides the job is due.
- Job identifier: a stable name used to record the job’s completion.
- Command: the program to execute.
Red Hat’s documented format is:
3 60 cron.daily /usr/local/bin/my-script.shThis asks anacron to run the script every three days, waiting 60 minutes after it becomes due. Use an identifier that remains unchanged; changing it can make the scheduler treat the work as a new job.
Choose a delay deliberately
The delay prevents every deferred task from starting immediately at boot. Stagger heavy jobs so they do not compete with application startup, backups, or database recovery. The delay is measured in minutes and applies after anacron determines that the period has elapsed.
Use portable period syntax
The Debian anacrontab documentation for its systemd-cron implementation describes special tokens such as @daily, @weekly, and @monthly. It also notes that traditional anacron accepts only @monthly and @yearly. If the file must work across distributions or implementations, use numeric day periods and verify the installed manual before relying on a token.
Understand periodic limits
Traditional anacron runs a job no more than once per day. A weekly job therefore does not become a burst of seven executions after a week offline; it is one execution when due. Debian also documents a guaranteed minimum six-day gap for weekly anacron jobs. This is different from some persistent systemd timers, which can run at boot and again at the next calendar boundary.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Debian systemd-cron and duplicate execution
Some Debian configurations use systemd-cron to translate cron files into systemd timers. In that arrangement, inspect the generated timers rather than assuming that only a traditional cron daemon is involved. A native systemd timer with the same name takes precedence over the translated cron job.
Do not run systemd-cron alongside another cron daemon or anacron unless you have deliberately verified the interaction. Debian warns that doing so can execute the same scripts more than once. After editing /etc/anacrontab, inspect recent journal messages and generated timers:
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journalctl -n
systemctl list-timersLook for the expected unit, next activation time, and most recent activation. If a native timer and a cron-derived timer both target the same script, disable one scheduling path before production use.
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Verify a schedule before trusting it
Check the service state
For a Red Hat-style service, use:
systemctl status crondReview whether it is loaded, enabled, and active. On another distribution, query that system’s cron service name. A disabled service will not start after reboot even if an entry is syntactically correct.
Inspect timers and journal output
On a Debian system using systemd integration, run:
systemctl list-timers
journalctl -nThese commands show whether cron or anacron files were translated, when a timer last fired, and whether the service reported an error. For ordinary cron installations, inspect the configured log destination and the system journal supplied by the distribution.
Test the command outside the scheduler
Execute the exact absolute command as its intended user. Confirm the exit status, output files, ownership, and log message. This isolates script defects from scheduler defects. If the command succeeds manually but fails on schedule, compare the environment, working directory, timezone, and permissions.
Troubleshoot common failure modes
| Symptom | Likely cause | What to check |
|---|---|---|
| No execution at all | Daemon is stopped or entry is in the wrong format | Service status, file location, five versus seven fields, and journal messages |
| Runs as the wrong account | A username was added to a user crontab, or omitted from a system file | Use crontab -e without a username; include one in /etc/cron.d/ and /etc/crontab |
| “Command not found” or missing files | Interactive shell environment is not present | Absolute paths, explicit PATH, required variables, and working directory |
| Permission denied | The scheduled user cannot read inputs or write outputs | File ownership, mode bits, parent-directory permissions, and secret access |
| Expected run after reboot never appears | Cron was used for a missed wall-clock instant | Use anacron or a deliberately configured persistent timer for catch-up behavior |
| Job starts twice | Duplicate entries or multiple scheduling daemons | Search both system and user crontabs; inspect systemctl list-timers; avoid overlapping systemd-cron, cron, and anacron services |
| Job runs at an unexpected local time | Timezone or daylight-saving assumption is wrong | timedatectl, the VPS timezone, and the schedule’s intended region |
| Anacron runs later than expected | Configured startup delay or a period that has not elapsed | Period and delay fields, job identifier, last-completion record, and journal output |
Practical schedule patterns
Nightly backup at a fixed time
When the backup must begin at 02:15, use a root cron entry:
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Polling every five minutes
A time-sensitive polling command can use:
*/5 * * * * app /usr/local/bin/poll-endpointThis seven-field form belongs in /etc/cron.d/ or /etc/crontab. In the account’s own crontab -e, remove app because the owner is implicit. Polling is a cron use case; anacron cannot provide five-minute cadence.
Maintenance every three days with catch-up
In /etc/anacrontab:
3 60 maintenance /usr/local/sbin/maintenance.shThe job becomes due every three days, then waits up to the configured 60-minute delay before execution. If the VPS was offline when the period elapsed, anacron runs it after startup rather than discarding that period.
Quick Recap
Operational checklist
- Choose cron for an exact wall-clock time; choose anacron for periodic work that must catch up.
- Confirm the installed distribution, scheduler package, service name, timezone, and clock synchronization.
- Put root entries in
/etc/cron.d/or/etc/crontabwith a username field. - Put per-user entries in
crontab -ewithout a username field. - Use absolute command paths and define required environment variables.
- Make scripts executable and verify read/write permissions as the scheduled account.
- Redirect output or configure mail so failures are visible.
- Use numeric anacron periods when portability matters; verify implementation-specific
@tokens. - On Debian systemd-cron systems, inspect
journalctlandsystemctl list-timers. - Remove duplicate definitions and avoid running overlapping cron, anacron, and systemd-cron schedulers.
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