This utility helps you build Cron expressions easily by choosing job scheduling scenarios. The crontab entries produced work with Vixie Cron, popular in many Linux distributions.
Every on
Every minutes between and on
At : on
Vixie Cron is configured by editing a "crontab" file that indicates the date and times to run given commands. Each user of the system can have their own crontab file and there is also a system-wide crontab file.
The system-wide file is usually located under the "/etc" directory and called "crontab". Changes to this file are checked and reloaded automatically but changes to user-specific crontab files wont get picked up unless you reload them with the crontab command.
The beginning of the /etc/crontab file usually has some environment variable settings. Below that each line in the file gives the information for a particular job to run. A typical crontab line looks something like this:
The user field ("some-user" above) is specific to the system-wide crontab file. This field isn't necessary for user-specific crontab files as commands are run as the owning user.
The command field ("some-command" above) may be a straight-forward shell command or an executable script. The path to use to locate the script can be given in the PATH environment variable in the crontab file or you can give absolute paths.
The bit that this utility is designed to assist with is the Cron expression. It consists of 5 fields that represent the following:
The values accepted for each field are: The minute field value must be 0-59, the hour field 0-23, the day of month field 1-31, the month field 1-12 and the day of week field 0-6 (Sunday is 0 but this can also be given as 7). The values can be given in a variety of formats:
*) character will match all possible values for the field: e.g. the Cron expression "* * * * *" will run the command every minute since this is the smallest representable time period.30 * * * *" will run the command whenever the minute is 30, i.e. once an hour at half-past the hour. * * 5 * *" will run every minute when it is the 5th day of the month.0,15,30,45 * * * *" will run the command whenever the minute is either 0, 15, 30 or 45. Another example, "0 1,2,3 * * *" will run the command between 1am and 3am (inclusive) but only when the minute is 0, i.e. on the hour. Lists can also contain ranges (see below).-): e.g. "0 1 1-5 * *" will run the command at 1am on the first, second, third, fourth and fifth days of the month.*/15 * * * *" will run the command every 15 minutes starting on the hour. i.e. this example is the same as the list example given above to run whenever the minute is 0, 15, 30 or 45.