Simply put, cron is a basic utility available on Unix-based systems. It enables users to schedule tasks to run periodically at a specified date/time. And it's naturally a great tool for automating lots of process runs, which otherwise would require human intervention.
Cron runs as a daemon process. This means it only needs to be started once and it will keep running in the background. This process makes use of crontab to read the entries of the schedules and kicks off the tasks.
Over time, the cron expression format became widely adopted, and many other programs and libraries make use of it.
2. Working With Crontab
A cron schedule is a simple text file located under /var/spool/cron/crontabs on Linux systems. We cannot edit the crontab files directly, so we need to access it using the crontab command.
To open crontab file, we need to fire this command:
Each line in crontab is an entry with an expression and a command to run:
* * * * * /usr/local/ispconfig/server/server.sh
This entry runs the mentioned script every single minute.
3. Cron Expression
Let's understand the cron expression.
It consists of five fields:
<minute> <hour> <day-of-month> <month> <day-of-week> <command>
3.1. Special Characters in Expression
- * (all) specifies that event should happen for every time unit. For example, “*” in the <minute> field means “for every minute.”
- ? (any) is utilized in the <day-of-month> and <day-of -week> fields to denote the arbitrary value and thus neglect the field value. For example, if we want to fire a script at “5th of every month” irrespective of what day of the week falls on that date, we specify a “?” in the <day-of-week> field.
- – (range) determines the value range. For example, “10-11” in the <hour> field means “10th and 11th hours.”
- , (values) specifies multiple values. For example, “MON, WED, FRI“ in <day-of-week> field means on the days “Monday, Wednesday and Friday.”
- / (increments) specifies the incremental values. For example, a “5/15” in the <minute> field means at “5, 20, 35 and 50 minutes of an hour.”
- L (last) has different meanings when used in various fields. For example, if it's applied in the <day-of-month> field, it means last day of the month, i.e. “31st of January” and so on as per the calendar month. It can be used with an offset value, like “L-3”, which denotes the “third to last day of the calendar month.” In <day-of-week>, it specifies the “last day of a week.” It can also be used with another value in <day-of-week>, like “6L”, which denotes the “last Friday.”
- W (weekday) determines the weekday (Monday to Friday) nearest to a given day of the month. For example, if we specify “10W” in the <day-of-month> field, it means the “weekday near to 10th of that month.” So if “10th” is a Saturday, the job will be triggered on “9th,” and if “10th” is a Sunday, it will trigger on “11th.” If we specify “1W” in <day-of-month> and if “1st” is Saturday, the job will be triggered on “3rd,” which is Monday, and it will not jump back to the previous month.
- # specifies the “N-th” occurrence of a weekday of the month, for example, “third Friday of the month” can be indicated as “6#3”.
3.2. Cron Expression Examples
Let's see some examples of cron expressions by using the fields and special characters combinations:
At 12:00 p.m. (noon) every day:
0 12 * * ?
Every five minutes starting at 1 p.m. and ending at 1:55 p.m. and then starting at 6 p.m. and ending at 6:55 p.m., every day:
0/5 13,18 * * ?
Every minute starting at 1 p.m. and ending at 1:05 p.m., every day:
0-5 13 * * ?
At 1:15 p.m. and 1:45 p.m. every Tuesday in the month of June:
15,45 13 ? 6 Tue
At 9:30 a.m. every Monday, Tuesday, Wednesday, Thursday and Friday:
30 9 ? * MON-FRI
At 9:30 a.m. on the 15th day of every month:
30 9 15 * ?
At 6 p.m. on the last day of every month:
0 18 L * ?
At 6 p.m. on the third to last day of every month:
0 18 L-3 * ?
At 10:30 a.m. on the last Thursday of every month:
30 10 ? * 5L
At 10 a.m. on the third Monday of every month:
0 10 ? * 2#3
At 12 midnight on every day for five days starting on the 10th day of the month:
0 0 10/5 * ?
4. Cron Special Strings
In addition to the fields specified in the cron expression, there's also support for some special, predefined values that we can use instead of the fields:
- @reboot – run once at the startup
- @yearly or @annualy – run once a year
- @monthly – run once a month
- @weekly – run once a week
- @daily or @midnight – run once a day
- @hourly – run hourly
In this quick article, we've explored cron jobs and crontab.
We've also seen a number of expression examples we can use in our daily work or simply infer other expressions from.