Wednesday, February 13, 2008

How to use crontabs

Crontab Commands
__________
export EDITOR=vi ;to specify a editor to open crontab file.

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 only available on a few systems.)

Crontab syntax :-
A crontab file has five fields for specifying day , date and time followed by the command to be run at that interval.
* * * * * command to be executed
- - - - -
| | | | |
| | | | +----- day of week (0 - 6) (Sunday=0)
| | | +------- month (1 - 12)
| | +--------- day of month (1 - 31)
| +----------- hour (0 - 23)
+------------- min (0 - 59)

* in the value field above means all legal values as in braces for that column.
The value column can have a * or a list of elements separated by commas. An element is either a number in the ranges shown above or two numbers in the range separated by a hyphen (meaning an inclusive range).

Note: The specification of days can be made in two fields: month day and weekday. If both are specified in an entry, they are cumulative meaning both of the entries will get executed .

Crontab Example

A line in crontab file like below removes the tmp files from /home/someuser/tmp each day at 6:30 PM.

30 18 * * * rm /home/someuser/tmp/*

Below runs the script every 3 minutes.

*/3 * * * * /home/perl/update.pl

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

No comments:

Google