To schedule a command on a Linux VPS, write five time fields followed by the command: minute hour day-of-month month day-of-week command. For example, */5 * * * * /usr/local/bin/backup.sh runs the script every five minutes. This guide explains every field, the operators that combine values, the important day-of-month/day-of-week rule, special strings such as @reboot, and the commands and safeguards you need to run jobs reliably.
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The five-field crontab format
A per-user crontab entry has six logical parts: five schedule fields and the command to execute. Cron evaluates the fields using the VPS’s cron implementation, then starts the command when the current date and time match.
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| Field | Allowed values | Meaning | Example |
|---|---|---|---|
| Minute | 0–59 | Minute within the hour | 15 |
| Hour | 0–23 | Hour in 24-hour time | 22 |
| Day of month | 1–31 | Calendar day | 1,15 |
| Month | 1–12 or jan–dec |
Month of the year | jan-mar |
| Day of week | 0–7 or names such as mon |
Weekday; Sunday is both 0 and 7 | mon-fri |
Separate fields with spaces or tabs. Put the command, including its arguments and redirections, after the fifth field. A basic daily job at 10:00 is:
0 10 * * * /bin/sh /opt/test.shThe asterisks mean “every permitted value” for that field. In this example, every day of every month and every weekday is allowed, so the only restriction is 10:00.
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How each schedule operator works
Asterisk: every value
* matches every value in its field. Use it when that part of the calendar should not restrict the job. The expression 0 * * * * runs at minute zero of every hour.
Comma: a list of values
A comma combines individual values without creating the values between them. 20,40 * * * * runs at minutes 20 and 40 of every hour. Lists also work with weekdays and months, such as mon,wed,fri or jan,jul.
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Hyphen: an inclusive range
A hyphen selects a continuous range. 1-10 means values 1 through 10. Names can be used where supported: mon-wed selects Monday through Wednesday and jan-mar selects January through March.
Slash: a step interval
A slash selects every specified number within a starting set. */30 in the minute field means minute 0 and minute 30 of each hour, or every 30 minutes. */5 * * * * therefore runs at five-minute boundaries: 00, 05, 10, and so on. A stepped range such as 1-10/2 selects 1, 3, 5, 7, and 9.
You can combine operators. 0 9-17 * * mon-fri runs at 09:00 through 17:00, Monday to Friday. The command is still started once for each matching minute; cron does not queue missed invocations by default.
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The day-of-month and day-of-week rule
The two day fields have a behavior that causes many unexpected runs. When both day of month and day of week are restricted (neither is *), cron runs the job when either field matches, not only when both match.
For example:
30 4 1,15 * 5 /usr/local/bin/report.shThis runs at 04:30 on the 1st and 15th of the month, and at 04:30 every Friday. It does not require the 1st or 15th to be a Friday. This OR behavior is specified by common Debian and Ubuntu cron implementations; exact edge behavior can vary slightly among cron implementations, so check the man page on a different distribution before relying on unusual expressions.
If one of the fields is *, the remaining restricted field supplies the calendar condition. Thus 0 8 * * 1-5 means weekdays at 08:00, while 0 8 1 * * means the first day of every month at 08:00.
Special strings for common schedules
Eight predefined strings replace all five time fields:
| String | Equivalent schedule | Use |
|---|---|---|
@reboot |
Not a calendar equivalent | Run once when the cron service starts the user’s crontab after system startup |
@yearly or @annually |
0 0 1 1 * |
Midnight on January 1 |
@monthly |
0 0 1 * * |
Midnight on the first day of each month |
@weekly |
0 0 * * 0 |
Midnight at the start of Sunday |
@daily or @midnight |
0 0 * * * |
Midnight every day |
@hourly |
0 * * * * |
At minute zero of every hour |
Use @reboot for a one-time startup action, but do not assume every dependency is ready. The Ubuntu crontab documentation warns that startup, for this purpose, can occur before some system daemons or facilities have started. If the command needs a network mount, database, or another service, add your own readiness check or use a service manager with dependency support.
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Editing and managing a VPS crontab
Edit the current user’s jobs
- Connect to the VPS with the account that should own and run the job.
- Open that account’s crontab with
crontab -e. On the first run, the system may ask you to choose an editor. - Add one complete entry per line, save the file, and exit the editor. The cron service normally notices the updated crontab automatically.
- Confirm the installed entries with
crontab -l.
Use the account that has the required file permissions. A job installed for your login account is not the same as one installed for root; its environment, home directory, permissions, and available files differ.
List, remove, or administer another account
crontab -llists the current user’s entries.crontab -rremoves the current user’s entire crontab. Treat it as destructive; usecrontab -land save a copy before running it.- With sufficient privileges,
crontab -u username -e,crontab -u username -l, andcrontab -u username -redit, list, or remove another user’s crontab.
Do not manually edit files under the system’s spool directory. The crontab command applies the correct ownership and syntax checks.
Useful, complete examples
| Goal | Entry | What it does |
|---|---|---|
| Every five minutes | */5 * * * * /bin/sh /opt/test.sh |
Runs at each five-minute boundary |
| Every five minutes, discard normal output | */5 * * * * /bin/sh /opt/test.sh > /dev/null |
Suppresses standard output; errors still use standard error |
| Every day at 10:00 | 0 10 * * * /bin/sh /opt/test.sh |
Runs once daily at 10 AM |
| Hourly from 10:00 through 18:00 | 0 10-18 * * * /bin/sh /opt/test.sh |
Runs on the hour during the inclusive hour range |
| Sundays at 22:00 | 0 22 * * 0 /usr/bin/apt-get update |
Runs at 22:00 when the weekday is Sunday |
| First and fifteenth, plus every Friday | 30 4 1,15 * 5 /usr/local/bin/report.sh |
Uses the OR rule for the two day fields |
| At startup | @reboot /usr/local/bin/startup-check.sh |
Runs when cron processes the user’s crontab after startup |
Use an absolute path for the interpreter, script, and important utilities. Cron jobs commonly have a smaller PATH than an interactive shell, so a command that works after SSH login can fail when run by cron. Make scripts executable when invoking them directly, or explicitly call the interpreter as in the examples.
Environment, comments, and output
Comments and blank lines
Blank lines and leading spaces or tabs are ignored. A line whose first non-space character is # is a comment, which is useful for documenting the purpose, owner, and expected schedule.
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Variables and the execution environment
Set variables on their own lines using NAME=value or NAME = value; spaces around the equals sign are optional in implementations that follow the documented crontab format. A common setting is:
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The documented defaults include SHELL=/bin/sh, with LOGNAME and HOME taken from the crontab owner’s passwd entry. Do not assume your interactive shell’s aliases, functions, profile files, virtual environment, or custom PATH will be loaded. Source the required environment explicitly in a wrapper script, and use quoted paths when they contain spaces.
Capture standard output and errors
For jobs that need diagnosis, append both streams to a log:
*/5 * * * * /usr/local/bin/check.sh >> /var/log/check.log 2>&1>> appends instead of replacing the file, and 2>&1 sends standard error to the same destination as standard output. Ensure the job’s user can write the log, and rotate it so a repeatedly failing job cannot fill the VPS disk. Redirect to /dev/null only when losing output is intentional.
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User crontabs versus system-wide entries
A user crontab contains five time fields followed immediately by the command. A system-wide file such as /etc/crontab adds a user field between day of week and command:
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Diagnosing jobs that do not run
Validate the schedule and account
- Run
crontab -las the intended user and verify that the line is present. - Count the five schedule fields before the command. A user crontab must not contain the system-wide
USERfield. - Check whether the day-of-month/day-of-week OR rule is producing extra runs or whether a missing restriction is preventing the intended run.
- Confirm the VPS timezone and the cron daemon’s timezone assumptions before judging a timestamp. Schedule against the server’s clock, not your laptop’s clock.
Test the command outside cron
Run the exact command as the crontab owner, using the same absolute paths and working directory assumptions. Replace relative paths with absolute ones. If the script expects a working directory, change to it inside the script rather than relying on the directory from which you normally launch it.
Inspect permissions and logs
- Confirm the script is readable and executable, and that every parent directory can be traversed by the job’s user.
- Check that files, sockets, network mounts, and credentials used by the script are available at the scheduled time.
- Read the VPS’s cron or system log for syntax errors and launch failures. Log locations differ by distribution and logging configuration.
- Add temporary output redirection to a writable file, reproduce the problem with a short interval, then remove the temporary diagnostic entry.
Prevent overlapping runs
If a command can run longer than its interval, a new invocation may start before the previous one finishes. Design the script to be idempotent and use an appropriate locking mechanism when overlap could corrupt data or consume excessive resources. Remove stale locks safely after confirming that no valid process is running.
When a systemd timer is a better fit
Cron is compact and available on virtually every Linux VPS, making it appropriate for straightforward calendar schedules. Systemd timers are a commonly used alternative when you need explicit service dependencies, richer status reporting, or persistent handling of jobs that were missed while the machine was powered off. The exact timer behavior depends on the distribution and unit configuration. Choose one mechanism per job, document it, and avoid creating duplicate cron and timer entries that perform the same action.
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A safe deployment checklist
- Write the schedule in plain language, including the server timezone.
- Convert it to five fields and verify each field against its allowed range.
- Decide whether the job belongs to a user crontab or a system-wide file.
- Use absolute paths, an explicit interpreter, and a known working directory.
- Add a dedicated log or deliberate mail setting before going to production.
- Install with
crontab -e, then verify withcrontab -l. - Trigger a short-interval test and inspect the output and cron logs.
- Review overlap, permissions, disk growth, and startup dependencies.
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