How to write a crontab scheduled execution script in Linux

crontab scheduled execution script

No nonsense, go directly to what you want:

such as As shown in the box, this command means that the script file in this directory will be executed every ten minutes, that is, /home/alan/test1.sh

Ok. Now let’s talk about why the command is like this. Write.

1. What is crond and what is crontab

First of allcrond is a service used in the Linux system to regularly execute commands ##. After writing the command, the system will start the crondtask scheduling command. crond will regularly check whether there is work to be performed every minute. If the work is to be performed, it will automatically Do the job. crondService is like an alarm clock.

and

crontab is used to set the command to be executed periodically. This command reads the command from the standard input device and stores it in the "crontab" file in for subsequent reading and execution.

To put it bluntly, the

crond service needs to be implemented through crontab.

crontab command format

OptionsDescription##-u userfile-e-l-r-i2. Install crond

is used to set a scheduled task for a certain user.
file is the name of the command file. Indicates that the file is used as the task list file of crontab and loaded into crontab
Edit the scheduled task of a certain user. If the user is not specified, it means the current user
Display the scheduled tasks of a certain user. If no user is specified, it indicates the current user
Delete a user's scheduled task. If no user is specified, it means that the current user
is given when deleting a user's scheduled task. Confirmation prompt

1. First check whether crond is installed

yum list installed | grep "crond"

2. Install crond (centos 7)

• systemctl status crond

  : Check the crond service status.

• systemctl start crond

  : Start the crond service.

• systemctl stop crond

  : Close the crond service.

• systemctl restart crond

  : Restart the crond service.

• systemctl reload crond

  : Reload the crond service.

If it is centos 7 or below, write service crond status/start/.....

3. How to use crontab

As shown in the figure:

##Except for the # comment character, there are five in total from left to right Each * sign represents the minute, hour, day, month, and day of the week respectively.

is followed by the user who performs the task, followed by the executed command or script file (the script file needs to bring the

absolute path

directory, for example, mine is

/home/alan/test1.sh)The specific meaning and range of the asterisks (from left to right):

AsterixMeaningTime range##*The number in an hour Minute(minute)Hour of the dayDay of the month The month of the year (month)##*The week of the week Day (week) 0-7 (0 and 7 both represent Sunday) Besides *, there are other symbols with special meanings.

0-59		*
0-23		*
1-31		*
1-12

For example:

Symbol

MeaningDescription* (asterisk) means any time * 6 * * * : means to execute the command every minute at 6 o'clock every day , (comma) Separate by commas to indicate the value of this field 10 8,9,10, * * *: Indicates 8: every day: Execute the command again at 10, 9:10 and 10:10-(middle pole) represents the value range between two integers* 6 * * 1-2: Indicates the execution of the command at 6 o'clock every Monday to Tuesday / (forward slash) Indicates how often the execution interval is*/10 * * * *: Indicates that the command will be executed every ten minutes

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