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 file

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: monthday and weekday. If both are specified in an entry, they are cumulative .

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/*

Changing the parameter values as below will cause this command to run at different time schedule below :

min hour day/month month day/week Execution time

30 0 1 1,6,12 * — 00:30 Hrs on 1st of Jan, June & Dec.

0 20 * 10 1-5 –8.00 PM every weekday (Mon-Fri) only in Oct.

0 0 1,10,15 * * — midnight on 1st ,10th & 15th of month

5,10 0 10 * 1 — At 12.05,12.10 every Monday & on 10th of every month

Note : If you inadvertently enter the crontab command with no argument(s), do not attempt to get out with Control-d. This removes all entries in your crontab file. Instead, exit with Control-c.

(taken from the crontab 5 manual pages)