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Use cron when a task must run at a particular wall-clock time; use anacron when it must run periodically even if the VPS was powered off at the nominal time. Cron skips a scheduled run when the machine is down. Anacron records periodic completion and runs the task after the system returns, with no more than one anacron run per day. The sections below show where each schedule belongs, how root and user permissions differ, how to survive reboots, and how to verify that jobs actually ran.
Cron and anacron solve different scheduling problems
Both services launch commands automatically, but their timing models are not interchangeable.
Cron is a wall-clock scheduler
Cron evaluates a schedule against the current date and time. A line such as 15 2 * * * root /usr/local/sbin/backup.sh means the command is due at 02:15 every day. If the VPS is stopped at 02:15, that occurrence is missed; cron does not normally replay it when the machine starts. Red Hat’s administrator guide summarizes this behavior: “A cron job is only executed if the system is running on the scheduled time.”
Choose cron when the minute matters: a database backup before a morning report, a poll that should run every five minutes, or a report that must be generated at 06:30. It is also the straightforward choice for high-frequency schedules and for jobs whose downstream systems expect a particular timestamp.
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Anacron is a periodic catch-up scheduler
Anacron tracks whether a periodic job has completed. If the VPS is off at the nominal time, it postpones the job until the system is running. Red Hat describes it this way: “If the system is not running at the scheduled time, an anacron job is postponed until the system is running.” An anacron job can run at most once per day, so it is intended for daily, weekly, or monthly maintenance rather than five-minute polling.
Anacron is appropriate for housekeeping where eventual execution is more important than an exact minute: rotating archives, refreshing indexes, pruning temporary files, or producing a periodic report after a reboot. It provides persistence across downtime, but not a guarantee that a weekly task will run at the same clock time each week.
Choose the scheduler before writing a line
| Requirement | Cron | Anacron |
|---|---|---|
| Timing precision | Precise wall-clock fields for minute, hour, date and weekday. | Periodic interval with a configurable startup delay; exact clock time is not the primary promise. |
| VPS down at the scheduled moment | The occurrence is skipped. | The job is postponed and can run when the VPS is available. |
| Shortest practical repeat interval | Suitable for schedules such as every five minutes. | Day-based periods; no more than one run per day. |
| Typical configuration | /etc/cron.d/, /etc/crontab, or a user’s crontab. |
/etc/anacrontab. |
| Identity and privileges | System files name a user; a user crontab runs as its owner. | The command is started by the anacron service context; arrange privileges explicitly, commonly by invoking a root-owned script from the system anacrontab. |
| Best fit | Backups at 02:00, reports at 06:30, and frequent polling. | Daily, weekly, or monthly maintenance that must catch up after downtime. |
Do not use anacron as a substitute for a time-critical backup window, and do not use cron alone when skipping a maintenance cycle would leave the VPS in an undesirable state. For a task that needs both an exact time and catch-up semantics, split the requirement: use a precise scheduler for the normal run and add a separately designed recovery check that detects missed work.
Prepare the VPS and identify its scheduler implementation
Confirm the operating system and service names
Package names and service names vary by distribution. Red Hat-based systems use the cronie package and the crond.service unit in the documented setup. Debian-family systems use their distribution’s cron package and may integrate cron files with systemd timers. Before editing schedules, identify which implementation is installed and whether another scheduler is already managing the same task.
- Check the distribution’s package documentation and install its supported cron implementation if it is absent.
- On a Red Hat-based VPS, enable the daemon for future boots and start it now:
sudo systemctl enable --now crond.service. - On systems where the unit is named
cron, use that unit name instead, for examplesudo systemctl status cron. - Record whether anacron, systemd-cron, or native systemd timers are also present. Two schedulers pointed at the same script can create duplicate executions.
Make the script safe to run unattended
Cron starts commands with a restricted, non-interactive environment. Use absolute paths for interpreters and utilities, set any required PATH or other variables in the schedule or wrapper script, and never depend on an interactive shell profile. Make the script executable, ensure its account can read every input and write every destination, and keep credentials in protected files rather than embedding them in a world-readable schedule.
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Decide where output goes before enabling the job. Redirect standard output and errors to a log, or configure the system’s mail destination for cron output. A silent failure is much harder to diagnose than a failed command with a timestamp and exit status in a log.
Create a root cron job
Use a file in /etc/cron.d/ for a separately managed system job; the Red Hat guide presents this as the preferred location. Editing /etc/crontab is also valid when you intentionally manage the central file.
Understand the system-cron fields
System entries contain five time fields, a username, and the command:
minute hour day-of-month month day-of-week username commandThe five time fields are minute (0–59), hour (0–23), day of month (1–31), month (1–12), and day of week (the implementation’s documented weekday values). An asterisk means every allowed value. A comma lists alternatives, a hyphen expresses a range, and a slash expresses a step. Check the local cron manual for implementation-specific details before relying on unusual expressions.
Install and protect the file
- Create a descriptively named file, such as
/etc/cron.d/nightly-backup, as root. - Put one complete entry in the file. This example runs the script at 02:15 daily as root:
15 2 * * * root /usr/local/sbin/backup.sh >> /var/log/backup.log 2>&1. - Use an absolute script path. If the script calls other programs, use absolute paths there too or set
PATHexplicitly inside the script. - Set ownership and permissions so unprivileged users cannot alter a root-executed command. Follow the permissions accepted by your distribution’s cron daemon; reject writable-by-everyone schedule files.
- Run the script manually as the scheduled account before waiting for the first window:
sudo -u root /usr/local/sbin/backup.sh. Confirm that the destination is writable and that the expected log output appears.
A root cron entry is powerful: a compromised or accidentally modified script can affect the entire VPS. Keep the script in a root-controlled directory, validate input, and grant narrower privileges when the task does not require root.
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Create a per-user cron job
For an application account or another non-root user, edit that account’s crontab with crontab -e. The command runs with the owner’s privileges and the line omits the username field.
15 2 * * * /home/app/bin/backup.sh >> /home/app/log/backup.log 2>&1- Switch to the intended account, or invoke the editor for it with the distribution-supported administrative command.
- Run
crontab -eand add the five time fields followed directly by the command. - Save and exit; the crontab tool validates and installs the file.
- List the installed entries with
crontab -las the same user. - Test the script in that account’s environment. A command that works in your SSH session may fail under cron because its working directory,
PATH, home directory, or credentials differ.
Do not add a sixth username field to a user crontab. Conversely, omitting the username in /etc/cron.d/ produces an invalid or misinterpreted system entry.
Use anacron for jobs that must catch up
Edit /etc/anacrontab as root. Each job has four fields:
period delay-in-minutes job-identifier commandThe period is expressed in days, the delay defers execution after anacron becomes active, the identifier names the job for tracking, and the final field is the command. Red Hat’s example is:
3 60 cron.daily /usr/local/bin/my-script.shThis means the command runs every three days, after a 60-minute delay once it is due. The identifier should be unique and descriptive because anacron uses it to record completion.
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Install an anacron entry safely
- Back up the existing file and open it with root privileges:
sudoedit /etc/anacrontab. - Add the period, delay, identifier, and an absolute command path on one line.
- Ensure the command is executable and that its user can access every file it touches. Put privileged work in a root-owned wrapper rather than granting broad permissions to data files.
- Choose a delay that avoids competing with boot-time services and other maintenance. The delay is in minutes, not seconds.
- After saving, inspect the service logs and generated timers on systems that integrate anacron through systemd.
Some Debian systemd-cron implementations document special period tokens such as @daily, @weekly, and @monthly. Traditional anacron accepts only @monthly and @yearly, so do not copy those tokens between distributions without checking the local implementation. Numeric day periods are the most portable form.
Understand reboot and downtime behavior
What happens to a missed cron run
Suppose a cron entry is scheduled for 02:00 and the VPS is powered off from 01:50 until 08:00. Cron was not running at 02:00, so that occurrence is lost. The next occurrence is the following day at 02:00. If the task is a backup or billing export, design an explicit recovery mechanism rather than assuming cron will replay it.
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What happens to a missed anacron period
Anacron records periodic completion and checks it when the system is available. If a daily, weekly, or monthly job became due while the VPS was off, anacron postpones it and starts it after the configured delay. It still limits the job to one run per day; a VPS that was down for several weeks does not receive a burst of one run for every missed day.
For weekly work, Debian documentation notes a guaranteed minimum six-day gap between anacron runs. This differs from some persistent systemd timers, which may run once at boot and again at the next calendar boundary. Choose the persistence model deliberately when migrating a job.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Account for Debian systemd-cron and native timers
On Debian systems using systemd-cron, cron files can be translated into systemd timer units. A native systemd timer with the same name takes precedence over the generated cron job. This means an entry can appear correct in a cron file yet be controlled by a timer you did not inspect.
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Do not run systemd-cron alongside another cron daemon or a separate anacron instance for the same schedules. Debian warns that doing so can execute scripts more than once. Before enabling a second implementation, map each task to exactly one owner.
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- List active and generated timers with
systemctl list-timers. - Inspect the cron or systemd journal with
journalctlwhen a generated timer should have fired. - After editing
/etc/anacrontab, check recent journal entries withjournalctl -nand then reviewsystemctl list-timersto confirm timer generation. - Search for a same-name native timer before assuming the cron definition is authoritative.
Verify a schedule before trusting it
Check the daemon
Start with the service status. On Red Hat-based systems use systemctl status crond; on distributions that call the service cron, substitute that unit name. A disabled or failed daemon means no schedule will run, regardless of how correct the line looks.
Check the installed entry
- For a user schedule, run
crontab -las the intended account. - For a system schedule, inspect the exact file in
/etc/cron.d/or/etc/crontaband verify the username column. - For anacron, inspect
/etc/anacrontab, its job identifier, and the period and delay fields. - For systemd integration, compare
systemctl list-timerswith the schedule you expect.
Check execution, not just configuration
Use a temporary test command that writes a timestamp to a user-writable log, or run the production script manually as the target account. Then inspect the cron or system journal and the script’s redirected output. Confirm the VPS timezone: a 02:00 schedule follows the host’s configured clock, not necessarily the timezone on your laptop.
Diagnose the common failure modes
| Symptom | Likely cause | Correction |
|---|---|---|
| No jobs run at all | The cron service is stopped, disabled, or the wrong unit name was checked. | Enable and start the distribution’s daemon, then inspect its status and journal. |
| Manual execution works, scheduled execution fails | Relative paths, missing PATH, a different home directory, or unavailable credentials. |
Use absolute paths and set required environment variables explicitly; test as the scheduled user. |
| A root job does not run | Missing or misplaced username field, unsafe file permissions, or a non-executable script. | Use the seven-field system format, protect the file, and verify the executable bit. |
| A task runs twice | Two cron daemons, anacron plus systemd-cron, or a native timer and generated cron timer own the same command. | Choose one scheduler, disable the duplicate, and confirm with service status and timer listings. |
| An anacron job never appears after editing | Unsupported period syntax or failed timer generation. | Use a numeric day period for portability and inspect journalctl -n and systemctl list-timers. |
| Output is missing | No redirection or mail destination, or the log directory is not writable. | Redirect both output streams to a protected log and verify permissions. |
Operational patterns for reliable VPS schedules
Use a wrapper for complex work
Put environment setup, locking, validation, and logging in a version-controlled wrapper script. The cron line then contains one stable absolute path. The wrapper can exit on an unsafe condition before touching production data and can return a meaningful non-zero status for monitoring.
Prevent overlapping runs
A five-minute cron job can still be running when the next invocation arrives. If overlap would corrupt output or overload the VPS, implement a lock in the script or use the locking facility documented by your distribution. Anacron’s one-run-per-day rule does not protect against a manually started copy or a second scheduler.
Separate schedule ownership
Document whether each task belongs to a user crontab, /etc/cron.d/, anacron, or a systemd timer. Give the file and script names a matching identifier, and remove obsolete entries when migrating. Clear ownership is the simplest defense against duplicate execution.
Plan for missed and failed work
Anacron handles machine downtime, not every failure. A job that starts and exits with an error may still be considered attempted by the scheduler. Have the script log start and completion, return failure statuses, and alert through the monitoring method used on your VPS. For cron jobs where a missed window matters, record the intended work item and run a separate reconciliation check at startup or on the next schedule.
A practical selection checklist
- Choose cron if the task must run at a defined minute or more often than once daily.
- Choose anacron if daily, weekly, or monthly completion matters more than the exact clock time and the VPS may be offline.
- Use a system file with a username for root or other centrally managed jobs; use
crontab -ewithout a username for per-user work. - Use absolute paths, explicit environment variables, protected secrets, and writable log destinations.
- Verify the daemon, installed entries, service journal, and timer list before declaring the schedule operational.
- Ensure only one scheduler owns a given command, especially on Debian systems using systemd-cron.
Final guidance
A dependable VPS schedule is a combination of the right timing model, the right execution identity, and observable results. Cron gives precise wall-clock execution but intentionally drops runs during downtime. Anacron trades minute-level precision for persistent periodic execution after the machine returns. Once you match that trade-off to the task, install the entry in the correct file, test it under the real account, and verify the service and logs, routine maintenance can run predictably without an interactive SSH session.
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