Cron is a time-based job scheduler, it is configured it to run commands at given times or intervals. Each User has a cron table which defines what to execute and at what interval. crontab command is used to create, modify and view cron jobs.
| Directory | Time |
|---|---|
| /etc/cron.hourly | First minute of every hour |
| /etc/cron.daily | Between 3:05 AM to 10.55 PM each day |
| /etc/cron.weekly | Between 3:25 AM and 11:10 PM after 7 days since last execution |
| /etc/cron.monthly | Between 3:45 AM and 11:30 PM after a month since last execution |
All the sysadmin needs to do is to place a shell script or a link to an executable in one of the directories and it will automatically be run at the appropriate time.
With crontab you can run an script at every time you need without be dependend on the regular routines.
For an example file enter:
# cat /etc/crontab SHELL=/usr/bin/bash PATH=/usr/sbin:/usr/bin MAILTO=root # For details see man 4 crontabs # Example of job definition: # .---------------- minute (0 - 59) # | .------------- hour (0 - 23) # | | .---------- day of month (1 - 31) # | | | .------- month (1 - 12) OR jan,feb,mar,apr ... # | | | | .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat # | | | | | # * * * * * user-name command to be executed
Lets create an job that runs every 5 minutes. This opens the vi editor.
# crontab -u apache -e */5 * * * * sudo -u apache php /srv/www/vhosts/cloud.robkalmeijer.nl/httpsdocs/cron.php