"Cron "* (Command Run ON) is a system for automatically running programs and scripts on a server at a specific time.
Scheduler job rules
A cron job consists of 6 columns separated by spaces or tabs. The first 5 columns specify the execution time (Minute, Hour, Day, Month, Day of Week); they can contain a number, a list of numbers separated by commas, a range of numbers separated by a dash - or an asterisk *. All other characters in the string are interpreted as a running command with its parameters.
* * * * * * executable command | | | | | | | | -- Day of the week (0 - 7) (Sunday =0 or =7) | | --- Month (1 - 12) | | --- Day (1 - 31) |00 Hour (0 - 23) ----- Minute (0 - 59)
For each specific parameter, you can set multiple values, separated by commas.
For example, if you write 1,4,22 in the hour field, the task will be started at 1 am, 4 am and 10 pm.
You can specify an interval - 4-9 will mean that the program must be started every hour between 4 and 9 o'clock inclusive.
The symbol * means "all possible values". For example, specifying * in the hour field will mean "run every hour".
The symbol / is used to specify an additional periodicity of the task. For example, */3 in the hour field means "every three hours".
What a simple scheduler script (cron) looks like:
0 */3 * * * 2.5 /usr/local/bin/php /home/login/html/cron.php
The script placed at /home/login/html/cron.php will automatically run every three hours on Tuesday and Friday.
Examples of using the tasks:
Run the job once every hour at 0 minutes
0 */1 * * * * /usr/local/bin/php /home/login/html/cron.php
Run the job every three hours at 0 minutes
0 */3 * * * * /usr/local/bin/php /home/login/html/cron.php
Run the task on Mondays at 1:15 a.m
15 1 * * * 1 /usr/local/bin/php /home/login/html/cron.php
Execute the task on April 5 at 0 hours and 1 minute every year
1 0 5 4 * /usr/local/bin/php /home/login/html/cron.php
Execute the task on Friday the 13th at 1:13 p.m
13 13 13 * 5 /usr/local/bin/php /home/login/html/cron.php
Execute the task monthly on the 1st of the month at 6:10 a.m
*If you need to run a program once a day, especially if it requires a lot of resources, run it at night, between 2 and 8 a.m. - the server load is minimal
The cron command "crontab " should be used to control it.
Run the following command to open the crontab of the current user:
$ crontab -e
To open the crontab of the user Alice:
$ crontab -u alice -e
View the contents of the crontab of the current user and the Alice user:
$ crontab -l $ crontab -u alice -l
By default, user tasks for the cron scheduler are stored in directory /var/spool/cron/.