Cron Scheduling

Time-based scheduling service in Unix-like computer operating systems.

# Use the hash sign to prefix a comment
# +---------------- minute (0 - 59)
# |  +------------- hour (0 - 23)
# |  |  +---------- day of month (1 - 31)
# |  |  |  +------- month (1 - 12)
# |  |  |  |  +---- day of week (0 - 7) (Sunday=0 or 7)
# |  |  |  |  |
# *  *  *  *  *  command to be executed

minute This controls what minute of the hour the command will run on,
and is between '0' and '59'
hour This controls what hour the command will run on, and is specified in the 24 hour clock, values must be between 0 and 23 (0 is midnight)
dom This is the Day of Month, that you want the command run on, e.g. to run a command on the 19th of each month, the dom would be 19.
month This is the month a specified command will run on, it may be specified numerically (0-12), or as the name of the month (e.g. May)
dow This is the Day of Week that you want a command to be run on, it can also be numeric (0-7) or as the name of the day (e.g. sun).
user This is the user who runs the command.
cmd This is the command that you want run. This field may contain multiple words or spaces.

  • The comma (',') operator specifies a list of values, for example: "1,3,4,7,8"
  • The dash ('-') operator specifies a range of values, for example: "1-6", which is equivalent to "1,2,3,4,5,6"
  • The asterisk ('*') operator specifies all possible values for a field. For example, an asterisk in the hour time field would be equivalent to 'every hour'.
    Disable Email
    By default cron jobs sends a email to the user account executing the cronjob. If this is not needed put the following command at the end of the cron job line:

    >/dev/null 2>&1

    Change where the email notification goes? Add the following to the file:

    MAILTO=<email address>

    Want to suppress the email completely? Add the following to the end of the cron command:

    >/dev/null 2>&1

    http://www.unixgeeks.org/security/newbie/unix/cron-1.html
    http://argray.com/unixfaq/cron.shtml

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