A cron schedule has five fields—in order, minute, hour, day of month, month, and day of week—followed by the command to run. Read each field as a set of matching calendar values, not as a general elapsed-time timer. The examples below use the documented Linux cron syntax; other cron implementations may differ.
How to read the five cron fields
A standard user crontab entry follows this pattern:
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minute hour day-of-month month day-of-week commandFor example:
15 14 1 * * /path/to/monthly-taskIn the documented Linux implementation, this runs at 14:15 on the first day of each month, using the cron daemon’s applicable local time context. The five fields use these ranges:
| Field | Position | Linux range | Meaning |
|---|---|---|---|
| Minute | 1 | 0–59 | Minute within the hour |
| Hour | 2 | 0–23 | Hour of the day |
| Day of month | 3 | 1–31 | Calendar day |
| Month | 4 | 1–12 | Calendar month |
| Day of week | 5 | 0–7 | Weekday; 0 and 7 both represent Sunday in this Linux implementation |
POSIX defines weekday values as 0–6, with 0 for Sunday, so the Linux 7-for-Sunday extension should not be assumed to work everywhere. See the Linux crontab(5) manual and POSIX crontab specification.
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User crontab versus system crontab
A user crontab contains the five time fields and command. A system crontab, such as /etc/crontab or a file in /etc/cron.d, inserts a username after the time fields:
minute hour day-of-month month day-of-week username commandDo not copy a system-crontab line into a user crontab unchanged: the extra username would be read as part of the command, not as a user field.
What cron operators mean
The operators choose values within an individual field:
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| Syntax | Meaning | Example use |
|---|---|---|
* |
Every allowed value in that field | * in the month field means every month |
5 |
One value | 5 in the minute field means minute 5 |
1,15 |
A list of values | Days 1 and 15 of the month |
8-11 |
An inclusive range | Hours 8, 9, 10, and 11 |
*/2 |
Every second value across that field’s range | Every other hour when used in the hour field |
The documented Linux implementation also accepts month and weekday names. Check the target host’s manual if you need syntax that may vary among cron implementations.
Cron schedule examples
| Schedule | What it runs |
|---|---|
0 22 * * 1-5 /path/to/weekday-task |
At 22:00 Monday through Friday |
5 0 * * * /path/to/daily-task |
At 00:05 every day |
0 */2 * * * /path/to/every-other-hour-task |
At the start of every other hour |
30 4 1,15 * 5 /path/to-example |
At 04:30 on the 1st and 15th of each month, and on Fridays |
In the last example, both day-of-month and day-of-week are restricted. The documented Linux implementation treats those two fields with OR-style matching: a match in either field is enough. It does not mean “only when the date is the 1st or 15th and it is also Friday.”
Why */5 is not always an elapsed interval
A step is evaluated within its field; it does not necessarily measure time elapsed since a previous run. For example, */35 in the minute field runs at minute 0 and minute 35 of every hour. The gap from 35 to the next hour’s 0 is 25 minutes, so it is not a repeating 35-minute elapsed interval.
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Use cron for calendar patterns such as “at these minutes of each hour” or “at this time each weekday.” If a task must recur after a fixed elapsed interval even across calendar boundaries, use a timer or application-level scheduling method suited to that requirement.
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In the documented Linux implementation, cron runs commands through /bin/sh unless the crontab changes SHELL. The command portion ends at a newline or an unescaped percent sign (%). An unescaped percent becomes a newline, and text after the first percent is sent to the command’s standard input.
- Use absolute paths for executables and files where appropriate; the job’s environment may not match your interactive shell.
- Decide intentionally where standard output and errors should go, or configure mail handling with settings such as
MAILTO. - Put comments on their own lines. A trailing inline comment is treated as part of the command.
- End the crontab file with a newline.
- Review settings such as
SHELL,HOME, andMAILTO; their values can affect execution and output handling.
Environment details depend on the cron implementation and host configuration. The crontab(5) manual documents the Linux behavior described here.
Account for timezone and daylight-saving changes
A cron expression is interpreted in the daemon’s applicable time context, so identify the timezone that matters for the task. The Linux crontab manual says a scheduled local time that does not exist during the spring clock change does not match; a local time repeated during the autumn change can match twice. The cron daemon manual also describes special behavior for certain clock changes, including catch-up or duplicate avoidance for some jobs. These details are implementation- and schedule-dependent; do not assume every cron daemon handles clock changes identically. See the crontab(5) and cron(8) manuals.
For jobs with financial, operational, or user-visible effects, decide how a missed, repeated, or delayed run should be handled. A schedule alone does not define safe behavior for those cases.
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Validate a schedule and deploy carefully
- Check the expression syntax. On the documented Linux crontab utility, run
crontab -Tto test syntax before installation. - Check the intended calendar interpretation. On systemd systems,
systemd-analyze calendarcan validate and normalize a calendar expression. - Test the command separately. Syntax checks do not prove that the command succeeds, has the required credentials, or behaves safely if runs overlap.
- Inspect output and logs after deployment. Confirm the job ran at the intended time and that its result is correct.
These checks answer different questions: a valid schedule can still launch a failing command. See the relevant crontab(5) and systemd.time(7) documentation.
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When a systemd timer may fit better
On a systemd host, a .timer unit activates a corresponding unit, commonly a service. OnCalendar= expresses wall-clock calendar events; monotonic directives such as OnBootSec= and OnUnitActiveSec= express timing relative to boot or unit activation. Calendar and monotonic directives can be combined. Timer behavior also includes configured accuracy and handling for suspend and resume; consult the manuals for the systemd version installed on the host. See systemd.timer(5) and systemd.time(7).
Choose based on the host and the job’s operational needs rather than assuming one scheduler is universally better:
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- Host and supervision: Does the system use systemd, and do service-unit dependencies or supervision matter?
- Timing model: Is this a wall-clock event, such as 02:00 on weekdays, or elapsed time since boot or a prior activation?
- Calendar requirements: Does the schedule need explicit timezone handling, systemd calendar syntax, or catch-up behavior?
- Failure and downtime behavior: What should happen after suspension, downtime, clock correction, or a long-running previous activation?
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