Learn about the basics of cron

This is Nakagawa from the System Solutions Department.
This time I will be writing about the basics of cron.

What is cron?

This is a daemon process for UNIX-based operating systems that executes commands at a registered time. It

automatically executes commands at the set time, such as "execute a command every day" or "execute a command every hour."

How to write

・The basic format is as shown above, from left to right: "minutes," "hours," "days," "months," "weeks," and "execution commands."

Try writing it yourself

* * * * * echo "test01" >> /var/www/html/CronTest_01.txt ## Run every minute

・Setting "*" will select all. In other words,
it is "Write test01 to /var/www/html/CronTest_01.txt every minute."
If you save this file and wait 3 minutes...

$ less /var/www/html/CronTest_01.txt

test01 test01 test01

In this way, the CronTest_01.txt file is created and three lines are written to it because cron was executed three times

10 * * * * echo "test02" >> [Execution command] ## Execute at 10 minutes past every hour

The command you set will be executed at 10:00

0 15 26 * * echo "test03" >> [Execution command] ## Execute at 15:00 on the 26th

The command will be executed at 15:00 on the 26th

10 20 * 1 * echo "test04" >> [Execution command] ## Execute at 20:10 in January

The command will be executed at 8:10 PM on January 1st

The week can be set to a number between 0 and 7, as follows:

0 3 * * 7 echo "test05" >> /var/www/html/CronTest_05.txt ## Run at 3:00 on Sunday

The command will be executed at 3:00 on Sunday

30-40 * * * * [Execution command]

In addition, if you write it as above, the command you set will be executed every minute between 30 and 40 minutes

Now you should be able to read when a cron command will be executed even if it was set up by someone other than yourself

summary

I often check cron settings during operations,
and for example, a possible increase in load can be caused by heavy processing set in cron.
If the load suddenly increases at a certain time, such as 00:00, and
check with the ps command and find that the cron process is running
, checking the cron settings may provide a clue to the solution.