Learn about the basics of cron

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

What is cron?

A UNIX-based OS daemon process that executes instructions at a registered time.
"Run XX command every day" "Run XX command every hour" The
command will be automatically executed at the time you set.

How to write

-Basically, as shown above, from left to right: "minute", "hour", "day", "month", "week", and "execution command".

actually write it

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

・If you set "*", all will be selected.In other words,
it will be "the process of writing test01 to /var/www/html/CronTest_01.txt every minute".
Save this file and after 3 minutes...

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

test01 test01 test01

In this way, the CronTest_01.txt file is created and 3 lines are written because cron was executed 3 times.

10 * * * * echo "test02" >> [Execution command] ## Execute at the 10th minute of 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 is executed even though I set it to 15:00 on the 26th.

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

The command is executed even though I set it to 20:10 in January.

The week can be set as a number from 0 to 7, as shown below.

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

The command will be executed when set to 3:00 on Sunday.

30-40 * * * * [Execution command]

In addition, if you write as above... The command set in 1 minute increments will be executed from 30 minutes to 40 minutes.

I think now you can also read when cron is executed even if it is set by someone other than yourself.

summary

I often check cron settings during operation,
for example, if the load is increasing, it may be due to heavy processing set to cron.
If the load suddenly increases at a timing like 〇〇:00, or
if the cron process is running when you check with the ps command,
looking at the cron settings will give you a clue to the solution. Maybe.