Ruby on Rails: Crontab (Cron Job) Reference

1. Commands

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.)

2. Syntax
A crontab file has five fields for specifying day , date and time followed by the command to be run at that interval.

* * * * *  [ command statement 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).

3. 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.

4. Crontab Environment
cron invokes the command from the user’s HOME directory with the shell, (/usr/bin/sh).
cron supplies a default environment for every shell, defining:
HOME=user’s-home-directory
LOGNAME=user’s-login-id
PATH=/usr/bin:/usr/sbin:.
SHELL=/usr/bin/sh

Users who desire to have their .profile executed must explicitly do so in the crontab entry or in a script called by the entry.

5. 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

6. Generate log file
To collect the cron execution execution log in a file :

30 18 * * * rm /home/someuser/tmp/* > /home/someuser/cronlogs/clean_tmp_dir.log


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