For a nightly reboot at 03:00, add a root crontab entry that runs /usr/sbin/shutdown -r now. First confirm that path, check the server’s timezone, and schedule the job only inside an approved maintenance window because the command immediately terminates sessions and running workloads. Plain cron does not replay a reboot missed while the machine was powered off; use a persistent systemd timer when catch-up behavior is required.
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What the cron job does
Cron is a background scheduler. It reads each user’s crontab independently of whether that user is logged in, so a root-owned entry can reboot an Ubuntu server without an interactive shell. The reboot command itself is provided by Ubuntu’s shutdown utility. The -r option requests a reboot, and now means the operation starts immediately when cron launches it.
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A daily 03:00 schedule is expressed with five time fields followed by the command:
| Field | Value | Meaning |
|---|---|---|
| Minute | 0 |
At minute zero |
| Hour | 3 |
At 03:00 in the host’s configured timezone |
| Day of month | * |
Every day of the month |
| Month | * |
Every month |
| Day of week | * |
Every day of the week |
Cron does not translate that time to a business, customer, or administrator timezone. If the server is set to UTC, 03:00 means 03:00 UTC; if it is set to another zone, the job follows that local setting.
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Before you schedule a reboot
An automatic reboot is an operational change, not a harmless housekeeping task. Confirm that the workload can tolerate a restart and that every critical service is configured to start again afterward.
- Notify users and on-call staff of the recurring maintenance window.
- Drain or stop workloads that cannot be interrupted, such as active game servers, database writes, queues, or long-running deployment jobs.
- Check that remote access, networking, storage mounts, containers, and application services have boot-time recovery configured.
- Confirm that you have console or out-of-band access in case networking or a service fails after the reboot.
- Verify the timezone and clock before choosing the hour.
- Decide how you will verify the first scheduled run and where its output will be recorded.
Use timedatectl status and date to inspect the current timezone and clock. A daylight-saving change can alter the local clock time at which a recurring schedule runs, so document whether the maintenance window is defined in local time or UTC.
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Set up the nightly reboot in root’s crontab
1. Confirm the shutdown executable
Cron has a deliberately limited environment and may not search the same PATH as your interactive shell. Find the installed command first:
command -v shutdownUbuntu commonly returns /usr/sbin/shutdown. Use the path returned on this host rather than assuming a location. If the command is missing, stop and repair the system’s package or path configuration instead of installing an untested wrapper.
2. Edit the root crontab
Run:
sudo crontab -eThis edits root’s personal crontab. The unqualified command crontab -e edits the account that runs it; an entry placed in your ordinary user’s crontab will not have the intended root privilege and should not be used for a machine reboot.
If the editor asks which program to use, select one you can operate confidently. Add a comment that identifies the purpose and change owner, followed by the schedule:
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0 3 * * * /usr/sbin/shutdown -r nowReplace /usr/sbin/shutdown if command -v shutdown returned a different path. Do not add a username column to this five-field root crontab entry.
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3. Save and verify the installed entry
After saving and closing the editor, display root’s crontab:
sudo crontab -lCheck the minute, hour, spacing, command path, and any comment. A common mistake is saving the line in a personal crontab and then inspecting root’s crontab, which makes the expected entry appear to be missing.
4. Confirm that cron is running
On Ubuntu, the scheduler is normally provided by the cron service. Check its state with:
systemctl status cronIf the service is intentionally disabled, coordinate with the administrator before enabling it. A healthy crontab entry cannot run while its scheduler daemon is stopped.
Choose immediate or warned reboot behavior
shutdown -r now starts an immediate reboot and gives no five-minute grace period. The shutdown manual also supports a 24-hour clock time, a relative delay expressed as +m minutes, and now. To have the cron job announce a five-minute warning, use a timed command such as:
0 3 * * * /usr/sbin/shutdown -r +5 'Scheduled maintenance reboot'For a timed shutdown, Ubuntu documents that /run/nologin is created five minutes before the system goes down to prevent new logins. This can be useful for a controlled maintenance window, but it also changes the exact point at which users are locked out. Confirm the behavior on the installed Ubuntu release and test it away from production traffic. If a pending timed shutdown must be cancelled, consult that release’s shutdown manual for the supported cancellation command and verify that the cancellation succeeded before relying on it operationally.
Make the cron job diagnosable
Rebooting is intentionally disruptive, and a bare cron line can leave little evidence about what happened. Redirect standard output and errors to a root-owned log file:
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0 3 * * * /usr/sbin/shutdown -r now >>/var/log/cron-reboot.log 2>&1The redirection records messages emitted before the reboot. Ensure that the destination is writable by root and that log rotation is considered if the command or a wrapper ever becomes noisy. Cron may also mail job output if local mail delivery is configured, but a file and the system journal are easier to check on a small server.
Keep the command itself simple. If you add a wrapper script for notifications, drains, or health checks, use absolute paths inside that script and return a failure status when a prerequisite cannot be met. A wrapper can stop the reboot when a workload was not drained, whereas a direct shutdown line cannot make that decision.
Test safely before the first unattended run
There is no harmless dry run for a command whose purpose is to reboot the machine. Test only during an approved maintenance window and with an operator able to reconnect afterward.
- Confirm the exact command path and the final crontab line with
sudo crontab -l. - Check active sessions and workloads. Notify anyone who could lose work.
- Run the exact reboot command manually only when an immediate restart is acceptable. For a less abrupt test, use a short timed delay such as
sudo /usr/sbin/shutdown -r +5 'Cron reboot test', then follow the installed release’s documented cancellation procedure if you decide not to continue. - After the host returns, verify remote access, mounts, networking, containers, databases, and application services.
- Check the reboot history with
last rebootand inspect scheduler logs for the execution time.
Do not test by changing a production schedule to an arbitrary minute and forgetting to restore it. A temporary test entry can fire during a busy period and be difficult to distinguish from the permanent job.
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Use several independent checks after the first scheduled window:
- Reboot history:
last rebootshows recent boot records and helps correlate the restart with the expected time. - Cron messages: inspect the cron service journal, for example
journalctl -u cron --since 'today 02:50'. Depending on the Ubuntu release and logging configuration, cron messages may also be present in/var/log/syslog. - Command output: if you used redirection, review
/var/log/cron-reboot.logafter the host is back. - Service health: check the status of every workload that must return after boot, not just the operating system.
A journal entry proving that cron started the command is not the same as proof that all applications recovered. Treat the reboot as complete only after the services and external health checks are normal.
Root crontab or /etc/cron.d?
Both mechanisms can run a root-level reboot, but their formats and administration models differ.
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| Location | Entry format | Best fit | Important detail |
|---|---|---|---|
| Root’s crontab | Five time fields, then command | A single administrator-managed schedule | Edit with sudo crontab -e; no username field |
/etc/cron.d |
Five time fields, username, then command | A configuration-managed or package-owned file | Use root as the username and follow the directory’s filename and permission rules |
/etc/crontab |
Five time fields, username, then command | System-wide cron configuration | Changes affect a central system file and should be managed carefully |
An equivalent /etc/cron.d line is:
0 3 * * * root /usr/sbin/shutdown -r nowDo not copy that line unchanged into root’s personal crontab; the extra root would be interpreted as part of the command and the job would fail. For one manually maintained reboot, root’s crontab is usually the least surprising choice. Use /etc/cron.d when your configuration-management process owns a dedicated file and can enforce its format and permissions.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsUnderstand missed runs and the @reboot trap
Regular cron schedules are not persistent timers. If the server is powered off or the cron daemon is unavailable at 03:00, cron does not queue that occurrence and run it automatically when the machine returns. Anacron also does not provide the persistent, dependency-aware behavior that a systemd timer can provide for this use case.
@reboot is a different cron feature: it runs a job once when cron starts after boot. It does not mean “run the daily reboot when the server comes back,” and adding both an @reboot entry and a 03:00 reboot can create an unintended reboot loop or extra restart.
If the requirement is “reboot at the normal time, but perform the missed reboot once after the machine returns,” use a persistent systemd timer and define that behavior explicitly.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When a systemd timer is a better design
Choose a systemd timer instead of cron when you need any of the following:
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- Explicit ordering or dependencies on services and mounts.
- Journal-native status and logs for a separately managed unit.
- A schedule that is easier to inspect with systemd tooling.
- A dedicated service unit that can be changed without editing a shared crontab.
A basic systemd-native design uses one service to invoke the same reboot command and one timer to schedule it.
Create the reboot service
Create /etc/systemd/system/scheduled-reboot.service with:
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[Unit]
Description=Scheduled maintenance reboot
[Service]
Type=oneshot
ExecStart=/usr/sbin/shutdown -r nowThe absolute path avoids the limited environment issue that also affects cron. The service has no Install section because the timer, not the service, is enabled.
Create the timer
Create /etc/systemd/system/scheduled-reboot.timer with:
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[Unit]
Description=Run the maintenance reboot every day at 03:00
[Timer]
OnCalendar=*-*-* 03:00:00
Persistent=true
Unit=scheduled-reboot.service
[Install]
WantedBy=timers.targetOnCalendar=*-*-* 03:00:00 expresses a daily local-calendar time. Persistent=true tells systemd to trigger the service when the timer becomes active after a missed time, subject to systemd’s timer semantics. That catch-up behavior is the principal difference from a plain cron entry and should be approved before deployment; it can cause a reboot soon after an unexpectedly long outage.
Load and inspect the timer
Run:
sudo systemctl daemon-reload
sudo systemctl enable --now scheduled-reboot.timer
systemctl list-timers scheduled-reboot.timerThe list should show the next trigger and the last trigger, if one has occurred. Use systemctl status scheduled-reboot.timer and journalctl -u scheduled-reboot.service when diagnosing a run. If you need a business-timezone schedule rather than the host’s local timezone, specify the timezone in the calendar expression according to the installed systemd version and test the resulting next trigger before enabling it.
Troubleshoot a reboot that did not occur
The entry is not running at all
- Run
sudo crontab -land confirm that the line is in root’s crontab. - Check
systemctl status cronand the cron journal. - Verify that the file was saved without an accidental leading character, wrapped line, or malformed field.
- Use the absolute path returned by
command -v shutdown.
The reboot occurred at the wrong clock time
- Run
timedatectl statusand compare the configured zone with the timezone used when the maintenance window was approved. - Remember that cron follows the host’s configured timezone; it does not convert 03:00 to another region.
- Check for daylight-saving transitions if the schedule is defined in local civil time.
The command works interactively but fails from cron
Cron supplies a smaller environment than an interactive shell. Replace relative command names with absolute paths, and put any required environment variables in the crontab or, preferably, in a controlled wrapper script. Capture standard output and errors so that permission or path errors survive long enough to inspect.
The server was down at the scheduled time
That is expected with plain cron: the missed occurrence is not replayed. Decide whether an immediate post-boot reboot is safe; if the requirement is automatic catch-up, migrate the schedule to a tested persistent systemd timer.
The machine rebooted but the application stayed offline
Inspect service enablement and dependencies separately from cron. Check service status, boot logs, mounted filesystems, network readiness, container orchestration, and application health checks. A successful shutdown invocation only proves that the operating system was asked to restart.
Quick Recap
Production checklist
- Privilege: the entry is in root’s crontab or has the correct
rootfield in/etc/cron.d. - Path: the reboot command uses the verified absolute executable path.
- Time: the server timezone and daylight-saving policy match the approved window.
- Impact: users are warned, workloads are drained, and interruption is acceptable.
- Recovery: critical services and mounts are configured to return at boot.
- Evidence: cron or systemd logs, reboot history, and an optional redirected log are available.
- Downtime policy: you have decided whether a missed run should be skipped or caught up by a persistent timer.
- Rollback: the operator knows how to remove the crontab line or disable the timer if the reboot causes an incident.
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