29 Mar, 2023
On a unix-like operating system you have a command line tool to manage scheduled tasks called cron. This is similar to what Windows has, Task Scheduler. With cron you can schedule any command line command and also bash or shell scripts, and these tasks can run periodically at fixed times, dates or intervals. Cron jobs can be used for a general purpose like a repetitive task or performing system updates.
So cron is the name of the command but each command is stored in a cron table or crontab. These cron tables store each command either by user and also a system-wide file as well. Each file has a unique structure that sets up the schedule and the command to run as well.
15 14 1 * * echo hello worldSo what does that cron line actually do? Well, at 14:15 on day-of-month 1 run echo hello world. Okay, lets look at how to read that.
# ┌───────────── minute (0 - 59)
# │ ┌───────────── hour (0 - 23)
# │ │ ┌───────────── day of the month (1 - 31)
# │ │ │ ┌───────────── month (1 - 12)
# │ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday
# │ │ │ │ │ 7 is also Sunday on some systems)
# │ │ │ │ │
# │ │ │ │ │
# * * * * * <command to execute>You start with minute, hour, day (of the month), month, day (of the week) command/script. The values are either the appropriate numerical value for that segment or an asterisk denoting any value will work. So that is why we see on the 15th minute of the 14th hour of the 1st day of the month, and every month, no matter the day, echo hello world.
Cron can get complicated and add intersections and unions of the schedule. This is a little more advanced than my overview will cover.
To schedule a simple backup script to run each night at 8 PM you could do the following:
0 20 * * * /home/username/Desktop/backup.shWithin my backup.sh file, are the commands and normal backup scenario logic. This file I did make to be executable as well with chmod +x ./backup.sh.
I spoke of crontab earlier but this is the command line interface to basically vi into the cron jobs and edit them and see what is happening on your system.
usage: crontab [-u user] file
crontab [ -u user ] [ -i ] { -e | -l | -r }
(default operation is replace, per 1003.2)
-e (edit user's crontab)
-l (list user's crontab)
-r (delete user's crontab)
-i (prompt before deleting user's crontab)The two simple crontab options I use regularly are crontab -l to list the jobs, and crontab -e to edit the jobs.
Running cron is a system service you can check the status, start, stop, and restart it to look into any issues you may have.
On Ubuntu and other Debian-based systems:
service cron status
service cron stop
service cron startWhen creating cron jobs, it is always best to have a way to check that the schedule you entered actually makes sense. With the advanced cron jobs it gets real literal. Take the following, At 05:05 on day-of-month 4 and on Wednesday. So at 05:05 when the day of the month is the 4th and its a Wednesday do something... This can be complicated to find out why you job did not run when expected.
5 5 4 * 3 /home/username/Desktop/backup.shTo help with this, I found a site called crontab.guru, and it has helped to verify the scheduled I made makes sense, before waiting for it. They also have great examples as well.
There are also some additional character notations that help with adjusting schedules on additional intervals.
*/30 * * * * echo "this will happen every half hour"The folks at crontab.guru has a great tip I'd like to quote as well:
If the day-of-month or day-of-week part starts with a *, they form an intersection. Otherwise they form a union.
* * 3 * 1runs on the 3rd day of the month and on Monday (union), whereas* * */2 * 1runs on every second day of the month only if it's also a Monday (intersection)
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