Agus Priyadi and Mafia.Or.ID

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What is a cronjob, and how do I use it?

Posted by mafiaid on 10 August 2007

The cron daemon is a long running process that executes commands at specific dates and times. To schedule one-time only tasks with cron, use at or batch. For commands that need to be executed repeatedly (e.g. hourly, daily or weekly), use crontab, which has the following options:

crontab filename Install filename as your crontab file.

The crontab command creates a crontab file containing commands and how often cron should execute them. Each entry in a crontab file consists of six fields, specified in the following order:

    minute(s) hour(s) day(s) month(s) weekday(s) command(s)

The fields are separated by spaces or tabs. The first five are integer patterns and the sixth is the command to be executed. The following table briefly describes each of the fields.

Field Value Description

Each of the patterns from the first five fields may either be an asterisk (*) (meaning all legal 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). Note that the specification of days may be made by two fields (day of the month and day of the week). If both are specified as a list of elements, both are followed. For example:

    MAILTO=user@domain.com
    0 0 1,15 * 1 /big/dom/xdomain/cgi-bin/scriptname.cgi

The cron daemon would run the program scriptname.cgi in the cgi-bin directory on the first and fifteenth of each month, as well as on every Monday. To specify days by only one field, the other field should be set to *. For example:

    MAILTO=user@domain.com
    0 0 * * 1 /big/dom/xdomain/cgi-bin/scriptname.cgi

The program would then only run on Mondays.

If a cron job specified in your crontab entry produces any error messages when it runs, they will be reported to you via email.

You may create crontab files in notepad (being sure to upload them in ASCII) or you may create them within telnet by simply typing:

    mcedit cronfile.txt

For more information, consult the man pages. man pages are the directions and tutorials available to you right at the command line from within telnet. Type any of the following lines to open the relevant tutorials:

    man 5 crontab {enter}
    man 1 crontab {enter}
    man cron {enter}
    man at {enter}
    man batch {enter}
    man 1 cron {enter}

Note:
Your crontab file must end with a line feed – in other words, make sure to press [Enter] after the last line in the file.

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