Cron - Overview

Overview

Cron is driven by a crontab (cron table) file, a configuration file that specifies shell commands to run periodically on a given schedule. The crontab files are stored where the lists of jobs and other instructions to the cron daemon are kept. Users can have their own individual crontab files and often there is a system wide crontab file (usually in /etc or a subdirectory of /etc) which only system administrators can edit.

Each line of a crontab file represents a job and is composed of a CRON expression, followed by a shell command to execute. Some implementations of cron, such as that in the popular 4th BSD edition written by Paul Vixie and included in many Linux distributions, add a sixth field to the format: an account username that the specified job will be run by (subject to user existence and permissions). This is allowed only in the system crontabs, not in others which are each assigned to a single user to configure. The sixth field is also sometimes used for year instead of an account username; the nncron daemon for Windows does this. However vixie cron does not use the sixth column as a year and if used will treat the year as the command to run and fail.

For "day of the week" (field 5), both 0 and 7 are considered Sunday, though some versions of Unix such as AIX do not list "7" as acceptable in the man page. While normally the job is executed when the time/date specification fields all match the current time and date, there is one exception: if both "day of month" and "day of week" are restricted (not "*"), then either the "day of month" field (3) or the "day of week" field (5) must match the current day.