How to Write Cron Expressions | Hostvento Knowledge Base

What cron is

Cron is the time-based job scheduler of Linux and other Unix-like systems. You tell it when to run a command and it does it for you, again and again. The time part is called a cron expression.

The five fields

* * * * * command_to_execute
- - - - -
| | | | |
| | | | +----- Day of the week (0 - 7) (Sunday is 0 or 7)
| | | +------- Month (1 - 12)
| | +--------- Day of the month (1 - 31)
| +----------- Hour (0 - 23)
+------------- Minute (0 - 59)

Some tools (for example Quartz and some cloud schedulers) use six or seven fields. They add seconds at the start and sometimes a year at the end. Standard Linux cron has five.

Field	Range	Example
Minute	0-59	`0` is the start of the hour
Hour	0-23	`14` is 2 PM
Day of month	1-31	`15` is the 15th
Month	1-12	`7` is July
Day of week	0-7	`1` is Monday (0 and 7 are Sunday)

Special characters

Character	Meaning	Example
`*`	Every value	`* * * * *` runs every minute
`,`	A list of values	`1,15 * * * *` runs at minute 1 and 15
`-`	A range	`1-5 * * * *` runs at minutes 1 to 5
`/`	A step	`/5 * * *` runs every 5 minutes
`?`	No specific value (Quartz style)	`0 0 1 * ?`
`L`	Last (Quartz style)	`L` in day of month is the last day
`W`	Nearest weekday (Quartz style)	`15W` is the weekday nearest the 15th

?, L and W are not part of standard Linux cron.

Common examples

Every minute

* * * * * /path/to/script.sh

Every hour, at minute 0

0 * * * * /path/to/script.sh

Every day at midnight

0 0 * * * /path/to/script.sh

Every Sunday at midnight

0 0 * * 0 /path/to/script.sh

On the 1st of every month at midnight

0 0 1 * * /path/to/script.sh

Every 15 minutes

*/15 * * * * /path/to/script.sh

Weekdays at 6:30 AM

30 6 * * 1-5 /path/to/script.sh

Add the job

Open your crontab and add the line:

crontab -e

See what is scheduled with crontab -l.

Tip: Use full paths in cron jobs, and send the output to a log file, for example >> /var/log/myjob.log 2>&1. Cron runs with a small environment.

Quick recap

Five fields: minute, hour, day of month, month, day of week.
* is every, , is a list, - is a range, / is a step.
Test your expression before you rely on it.