crontab -e

As user root, run

#crontab -e
0 0 1,15 * 1 /mydir/myprogram

#crontab -l

The cron daemon would run the program myprogram in the mydir directory on the first and fifteenth of each month, as well as on every Monday.

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For commands that need to be executed repeatedly (e.g., hourly, daily, or weekly), you can use the crontab command. The crontab command creates a crontab file containing commands and instructions for the cron daemon to execute.

You can use the crontab command with the following options:

crontab -a filename
Install filename as your crontab file. On many systems, this command is executed simply as crontab filename (i.e., without the -a option).

crontab -e
Edit your crontab file, or create one if it doesn't already exist.

crontab -l
Display your crontab file.

crontab -r
Remove your crontab file.

crontab -v
Display the last time you edited your crontab file. (This option is available on only a few systems.)

crontab -u user
Used in conjunction with other options, this option allows you to modify or view the crontab file of user. When available, only administrators can use this option.

Each entry in a crontab file consists of six fields, specifying 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 execute. The following table briefly describes each of the fields:

Field Value Description minute 0-59 The exact minute that the command sequence executeshour 0-23 The hour of the day that the command sequence executesday 1-31 The day of the month that the command sequence executesmonth 1-12 The month of the year that the command sequence executesweekday 0-6 The day of the week that the command sequence executes (Sunday = 0, Monday = 1, Tuesday = 2, and so forth)command Special The complete sequence of commands to execute.

The command string must conform to Bourne shell syntax. Commands, executables (such as scripts), or combinations are acceptable.

Each of the patterns from the first five fields may be either * (an asterisk), meaning all legal values, or a list of elements separated by commas. An element is either a number or an inclusive range, indicated by two numbers separated by a minus sign (e.g., 10-12). You can specify days with two fields: day of the month and day of the week. If you specify both of them as a list of elements, cron will observe both of them, for example:

0 0 1,15 * 1 /mydir/myprogram

The cron daemon would run the program myprogram in the mydir 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:

0 0 * * 1 /mydir/myprogram

In the above example, the program would run only on Mondays.