How to use cron job in Linux system

Source: Internet
Author: User

Cron is a Daemon,cron job for Linux that is a task that Cron is scheduled to perform. Cron uses a special configuration file, the crontab file, to set the execution time or frequency of a command or script. Crontab has previously been introduced to the basics of use, no longer detailed.
Re-explain the command format for the next crontab

#┌─────────────min (0-59)
#│┌──────────────hour (0-23)
#││┌───────────────day of month (1-31)
#│││┌────────────────month (1-12)
#││││┌─────────────────day of Week (0-6) (0 to 6 are Sunday to Saturday, or with names; 7 is Sunday, the same as 0)
# * * * * command to execute

About the first five parameters date and time expression:
Denotes any: * number means "arbitrary" (first-last).
Specifies a number that represents the specified time.
Specify a segment that represents the number contained in start-end: For example, 3-6, which means 3,4,5,6
Specify the list: for example, "1,2,3,4″," 0-4,8-12″
Specify "Step size": 8-14/2 indicates 8,10,12,14

And look at the following crontab expression

0 0 5-20/5 2 2/path/to/command

From February 5 to 20th, every 5 days plus all Tuesday of February, the visible month and week parameters have specified values (non *), they are or.

In addition, some special predefined expressions for crontab parameters are:

@yearly, @annually Run once a year at midnight of January 1 (0 0 1 1 *)
@monthly Run Once a month, at midnight of the the the month (0 0 1 * *)
@weekly Run once a week at midnight of Sunday (0 0 * * 0)
@daily Run once a day at midnight (0 0 * * *)
@hourly Run at the beginning of every hour (0 * * * * *)
@reboot Run once at startup

You can also execute more than one command, separated by;

* * * * * * */PATH/TO/COMMAND-1; /path/to/command-2

If there is a dependency between the commands, you can use the logical operator &&, as follows if the first command fails, the second command does not execute.

* * * * * */path/to/command-1 &&/path/to/command-2

Contab can set redirect output

* * * * * */path/to/php/path/to/the/command >>/var/log/cron.log

You can also prevent cron from sending messages

* * * * * * */path/to/php/path/to/the/command >/dev/null 2>&1

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