Cron schedules live in two places: users' personal tables and system files. The line inside them looks almost the same.
cron is a service that checks the clock against the schedules once a minute and starts the jobs that match. A schedule line is five time fields and a command:
| Field | Values |
|---|---|
| minute | 0–59 |
| hour | 0–23 |
| day of month | 1–31 |
| month | 1–12 |
| day of week | 0–7, 0 and 7 are Sunday |
* in a field means any value, */10 every tenth, 1-5 a range, 1,15 a list. 15 2 * * * is every day at 02:15, * * * * * every minute.
A personal table is the user's crontab: crontab -l prints it, crontab -e opens it in an editor (the first time it asks which one: 1 is nano). Its commands run as the table's owner. In the system files /etc/crontab and /etc/cron.d/* there is one more field between the time and the command: the name of the user to run as.
This line from /etc/cron.d was copied as is into a personal table with crontab -e. What happens at 02:15?
15 2 * * * student /usr/local/bin/backup.sh-
crontabchecks the time format and treats the word after it as the start of the command. A line with five fields and a command is valid to it. -
In a personal table the command follows the five fields directly. cron runs
student /usr/local/bin/backup.sh, and the shell looks for a program calledstudent.
The user field exists in /etc/crontab and /etc/cron.d, while in a personal table the command starts in its place. cron throws the error away with the output, and the run shows up in sudo journalctl -u cron.
Key points
-
A cron line is five time fields and a command;
*means "any". -
A personal table is
crontab -landcrontab -e; its commands run as its owner. -
In
/etc/crontaband/etc/cron.dthe time is followed by a user name.