How to Write crontab, with a Cheat Sheet: The Five Fields, Symbols, Common Examples and Commands

A crontab line is “five fields plus a command”

crontab is the configuration file that tells a program called cron when to run what. Each line is one scheduled job: five time values separated by spaces at the start, followed by the command you want to run.

For example, “30 3 * * * /home/user/backup.sh” means “minute 30, hour 3, any day of month, any month, any day of week”, so backup.sh runs at 3:30 every day. The fields go from smaller units to larger ones, except that the day of week comes last. Swapping hour and minute (writing “9 30” for 9:30) is a very common mistake: that reads as “minute 9 of hour 30”, and since 30 isn’t allowed in the hour field, it’s an error.

Symbols: learn * , - / and you can read most expressions

Besides numbers, cron expressions use four symbols. They mean the same thing in every field, so once you know these four you can read almost any expression.

/ means “count in steps of n from the first value in the field”. “*/15” starts at 0 and steps by 15, giving minutes 0, 15, 30 and 45; “5-59/15” gives 5, 20, 35 and 50. It isn’t a timer that fires 15 minutes after the last run; it simply means “run at those minute values”. Keeping that in mind will help you avoid the pitfall in the next section but one.

There are also shorthand forms you can use instead of the five fields: @daily (every day at 0:00), @hourly (minute 0 of every hour), @weekly (Sunday 0:00), @monthly (the 1st at 0:00), @yearly (January 1 at 0:00) and @reboot (once at startup). They’re easy to read, but the time is fixed at 0:00, so watch out for them colliding with other midnight jobs.

A crontab cheat sheet you can copy

Here are frequently used schedules. Times are in the server’s time zone. The command part is left out, so add the command you want to run after the expression.

Some schedules, such as “the last day of every month”, “the second Tuesday of every month” or “the last Friday”, can’t be written exactly with crontab’s five fields alone. They need the command to check the date, which is covered in detail in cron’s day-of-month/weekday OR rule and how to write nth weekdays and month-end.

*/7 isn’t every 7 minutes: the step-value pitfall

“*/n” means “count from 0 in steps of n and stop when you pass the field’s maximum”. In the minute field it counts within 0–59, so if n doesn’t divide 60 evenly, the interval breaks at minute 0 of each hour.

Numbers that divide 60 or 24 evenly keep the interval regular: 1, 2, 3, 4, 5, 6, 10, 12, 15, 20 and 30 for minutes, and 1, 2, 3, 4, 6, 8 and 12 for hours. If you really need an even interval with any other number, run cron every minute or every hour and have the script check how much time has passed since the last run before deciding whether to do the work.

Using crontab -e, -l and -r, and how not to wipe your settings

You don’t open the crontab file directly; you edit it with the crontab command. Each user has their own crontab, and the command acts on the crontab of whoever runs it.

1. 1

   Check the current settings with crontab -l

   It prints every registered line. If nothing is registered you’ll see “no crontab for username”. Before editing, it’s wise to take a backup with crontab -l > ~/crontab.bak.

2. 2

   Edit with crontab -e

   An editor opens; write one job per line, then save and quit. Changes take effect as soon as you save, with no need to restart cron. If the syntax is wrong, you’ll get an error on saving and be asked whether to edit again.

3. 3

   End the last line with a newline

   Some cron implementations ignore the last line if it has no trailing newline. Make a habit of pressing Enter after the final line before saving.

4. 4

   Use sudo crontab -u for root or other users

   Specify the target user with -u, as in sudo crontab -u www-data -l. Which user you register a job under affects file permissions and the environment variables available.

Note that /etc/crontab and files under /etc/cron.d/ use a slightly different format: there’s an extra “user to run as” field between the five time fields and the command (e.g. 30 3 * * * root /usr/local/bin/backup.sh). If you copy a line in user-crontab format, the command name is read as the user name and nothing runs.

Four things to watch when writing the command

Even with all five time fields correct, the way the command is written often leads to “it works when I run it by hand but not from cron”. That’s because cron runs commands in a minimal environment, different from the one you get when you log in.

• Use absolute paths for commands and files. cron’s PATH is roughly just /usr/bin:/bin, so commands in places like /usr/local/bin won’t be found
• Escape %. In a crontab command, % is treated as a newline, so date +%Y%m%d must be written date +\%Y\%m\%d
• Keep the output in a log. Appending “>> /var/log/backup.log 2>&1” records output, including errors, by adding to the file
• The shell is /bin/sh. If you use bash-only syntax ([[ ]], arrays and so on), put it in a bash script that starts with #!/bin/bash

Putting this together, a job that “writes a report to a dated file name at 9:00 on weekdays” looks like “0 9 * * 1-5 /usr/local/bin/report.sh > /home/user/report-$(date +\%Y\%m\%d).txt 2>&1”. If it still won’t run, the checklist for when cron isn’t running goes through how to investigate, step by step.

The easiest way to check an expression does what you mean is to paste the crontab line straight into the cron expression checker. It translates it into plain language such as “weekdays (Mon–Fri) at 9:00” and lists the next 10 run times, so mix-ups in the weekday or time stand out immediately. It also warns you if you forgot to escape % in the command.

FAQ

How do I run something every 5 minutes in crontab?

Write “*/5 * * * * command”. It runs at minutes 0, 5, 10…55 of every hour: 12 times an hour, 288 times a day. To limit it to 9:00–18:59 on weekdays, narrow the hour and weekday fields, as in “*/5 9-18 * * 1-5”.

Which number is Monday in the crontab weekday field?

0 is Sunday, 1 is Monday, 2 is Tuesday…6 is Saturday, and 7 is also treated as Sunday. Weekdays (Mon–Fri) are 1-5 and weekends are 0,6. You can also use three-letter English names such as MON or FRI. Note that AWS EventBridge is off by one, with 1 as Sunday.

Do I need to restart cron after editing crontab?

If you edit with crontab -e, changes apply automatically when you save, so no restart is needed. If you change the server’s time zone, cron may keep running on the old time zone, so restart the cron (or crond) service.

Can I recover settings deleted with crontab -r?

crontab -r deletes the settings without confirmation, and there’s no undo. If you have a backup (a file with saved crontab -l output), you can restore it with crontab filename. Without a backup, you’ll have to rewrite it using the execution history in cron’s logs (syslog and so on) as clues to which commands ran when.

Why doesn’t “*/7” run every 7 minutes?

*/7 means “count from minute 0 in steps of 7 up to 59”, so it runs at minutes 0, 7, 14…56. After 56 comes minute 0 of the next hour, not 63, so that one gap is only 4 minutes. Using a number that divides 60 evenly, such as 5, 10, 15, 20 or 30, keeps the interval even.