Different ways of writing crontab syntax

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Mattias Geniar, April 26, 2020

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Here’s what I’ve been doing for most of my life, when it comes to writing crontab entries:

0 */2 * 1 3 /run/this/on/wednesday.sh

The above is the arcane way of saying: run this job every 2 hours on every Wednesday in January.

But I only just recently learned you can replace the weekday and month with a human-readable form, if your cron version is recent enough.

0 */2 * JAN WED /run/this/on/wednesday.sh

It doesn’t help the 0 */2 ... syntax, but it’s a lot easier to see WED means Wednesday and JAN means January!

What else can you do in a crontab?

While some of these may be obvious if you’ve used crontab for a while, it is good to know the syntax allows for pretty flexible entries.

Ranges

This example shows you how to run a command ever hour between 8AM and 3PM, including those hours themselves.

0 8-15 * * * /script.sh

Lists

An alternate approach is to list the digits comma-separated.

0 8,9,10,11,12,13,14,15 * * * /script.sh

This would make sense if there’s a single hour you don’t want to run a command, you can simply leave it out of your comma-separated list.

Step values

This is the part of the crontab syntax that confuses most:

0 */2 * * * /script.sh

The */2 part can be read as: every 2 hours.

You can also pass a range with a step value:

0 0-12/2 * * * /script.sh

This one means: run this every 2 hours for all hours between 0 and 12.

Month and week names

Like I mentioned at the top, you can use the first 3 letters of a week or month to specify the entry.

0 0 * AUG SUN /script.sh

The above would run every Sunday in August at midnight.

Gotchas and booby traps

What does this Syntax do?

30 4 1,15 * 5 /script.sh

This would run a command on the 1st and 15th of each month, as well as on every Friday. The 1,15 in the day section and the 5 in the weekday are interpreted as an “either may match”, not a “should match both”.

Special shortcuts

Cron has some special shortcuts available to you, if you don’t want to write the arcane syntax every time.

string         meaning
------         -------
@reboot        Run once, at startup.
@yearly        Run once a year, "0 0 1 1 *".
@annually      (same as @yearly)
@monthly       Run once a month, "0 0 1 * *".
@weekly        Run once a week, "0 0 * * 0".
@daily         Run once a day, "0 0 * * *".
@midnight      (same as @daily)
@hourly        Run once an hour, "0 * * * *".

This means you can do this:

@daily /script.sh

instead of:

0 0 * * * /script.sh

At least @daily is more readable.

Any more tricks?

What other crontab magic am I missing here?



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