Linux - Crontab
Crontab (CRON TABle) is a file which contains the schedule of cron entries to be run and at specified times
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.)
1crontab -e
230 18 * * * [command]
3
430 18 * * * rm /home/someuser/tmp/* > /home/someuser/cronlogs/clean_tmp_dir.log
5
6
7* * * * * command to be executed
8- - - - -
9| | | | |
10| | | | +----- day of week (0 - 6) (Sunday=0)
11| | | +------- month (1 - 12)
12| | +--------- day of month (1 - 31)
13| +----------- hour (0 - 23)
14+------------- min (0 - 59)
15
16min hour day/month month day/week Execution time
1730 0 1 1,6,12 * – 00:30 Hrs on 1st of Jan, June & Dec.
180 20 * 10 1-5 – 8.00 PM every weekday (Mon-Fri) only in Oct.
190 0 1,10,15 * * – midnight on 1st ,10th & 15th of month
205,10 0 10 * 1 – At 12.05,12.10 every Monday & on 10th of every month
Notes
Repeat pattern like /2 for every 2 minutes or /10 for every 10 minutes is not supported by all operating systems.
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.
130 18 * * * rm /home/someuser/tmp/*
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 .
130 18 * * * rm /home/someuser/tmp/* > /dev/null 2>&1
Generate log file
To collect the cron execution execution log in a file :
130 18 * * * rm /home/someuser/tmp/* > /home/someuser/cronlogs/clean_tmp_dir.log
run the drupal cron process every hour of every day
10 * * * * /usr/bin/wget -O - -q -t 1 http://localhost/cron.php
run this apache kludge every minute of every day
1* * * * * /var/www/devdaily.com/bin/check-apache.sh
generate links to new blog posts twice a day
15 10,22 * * * /var/www/devdaily.com/bin/mk-new-links.php
run the backup scripts at 4:30am
130 4 * * * /var/www/devdaily.com/bin/create-all-backups.sh
re-generate the blog "categories" list (four times a day)
15 0,4,10,16 * * * /var/www/devdaily.com/bin/create-cat-list.sh
reset the contact form just after midnight
15 0 * * * /var/www/devdaily.com/bin/resetContactForm.sh
rotate the ad banners every five minutes
10,20,40 * * * * /var/www/bin/ads/freshMint.sh
25,25,45 * * * * /var/www/bin/ads/greenTaffy.sh
310,30,50 * * * * /var/www/bin/ads/raspberry.sh
415,35,55 * * * * /var/www/bin/ads/robinsEgg.sh