CronTab Quick Reference

What is Cron Tab ?
cron is a unix utility that allows tasks to be automatically run in the background at regular intervals by the cron daemon often termed as cron jobs.
Crontab (CRON TABle) is a file which contains the schedule of cron entries to be run and at specified times.

Restriction
You can execute crontab if your name appears in the file
/usr/lib/cron/cron.allow. If that file does not exist, you can use
crontab if your name does not appear in the file
/usr/lib/cron/cron.deny.
If only cron.deny exists and is empty, all users can use crontab. If neither file exists, only the root user can use crontab. The allow/deny files consist of one user name per line.

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
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 (1 - 7) (monday = 1)
| | | +------- month (1 - 12)
| | +--------- day of month (1 - 31)
| +----------- hour (0 - 23)
+------------- min (0 - 59)

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.

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

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

Examples :
Cron set For Database BackUp

5 21 * * * mysqldump -u root -pclarion --opt specto | gzip > /backup1/amodd/spec
to_`date +%Y%m%d%H%M`.gz

Cron Set for run PHP File ..
* 0,6,12,18 * * * /usr/bin/php -q /var/www/html/vikrant/goskype/expertblog/blogsadmin/runcron.php > /dev/null