The cron command has officially been deprecated in Mac OS X, but still functions if called upon. cron starts a process that executes commands at specified dates and times. Regularly scheduled commands can be specified according to instructions found in the crontab files in the directory /var/spool/cron/crontabs. Users can submit their own crontab files via the crontab command. Crontab copies the specified file or standard input if no file is specified, into a directory that holds all users’ crontabs. crontab options:
- The -e option edits a copy of the current users’ crontab file or creates an empty file to edit if crontab does not exist.
- The-r option removes a user’s crontab from the crontab directory.
- The -l options lists the crontab file for the invoking user.
A crontab file consists of lines of six fields each.The fields are separated by spaces or tabs. The first five are integers that specify the following (in order):
- minute (0-59)
- hour (0-23)
- day of the month (1-31)
- month of the year (1-12)
- day of the week (0-6 with 0=Sunday)
Each of these patterns may be either an asterisk (meaning all valid values) or a list of elements separated by commas. An element is either a number or two numbers separated by a minus sign ( meaning an inclusive range). Notice the time is in 24 hour format, 0 is midnight and 13 is one in the afternoon. The * wildcard can be used to run on every instance of a given object.
The sixth field of a line in a crontab file is a string to be executed by the shell at the specified times by the first fife fields. A percent character in this field (unless escaped by ) is translated to a newline character. Only the first line (up to a % or end of line) of the command field is executed by the shell. The other lines are made available to the command as standard input.
Any line beginning with a # is a comment and is ignored.
