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Today, I will talk to you about how to execute commands and scripts on linux on a regular basis. Many people may not know much about it. In order to make you understand better, the editor has summarized the following for you. I hope you can get something from this article.

Human beings cut time and imagine a straight line moving forward forever. there is nothing on this straight line, but human beings have made dense marks on this line according to the length of time (unit) (century-year-month-day-hour-minute-second-nanosecond …...) In this way, every moment in history can be taken out separately, and every moment in the future can be planned separately. More importantly, each individual moment can be compared with adjacent similar moments controlled by larger units, for example, 12:00 tonight and 12:00 tomorrow night, similarly, both times are 12:00. The difference is that today and tomorrow, they are adjacent larger units. Because of this, we can tell the computer to repeat a task at the same time separated by larger units.

Under linux, if you want to perform a task at some point in the future, and at every such moment in the future, for example, if you want to restart the router at 12:00 every day in the future, most distributions come with a daemon cron to do the job.

1. The relationship between anacron and CronBronTab

Cron is a daemon that comes with most linux distributions to repeatedly run certain tasks that are set to run for a certain time, such as monthly, weekly, daily, or even every minute. Tasks executed with cron are suitable for machines running 24 hours a day. Tasks executed by cron are executed at a set time. When the machine is shut down and the time for task execution is missed, cron tasks cannot be expected to be executed.

Crontab (short for cron table) can refer to either the list files that cron uses to perform specific tasks on a regular basis, or the commands used to create, delete, or view the crontab files of the current user (or the specified user).

Anacron is not a daemon, but can be seen as a supplement to the cron daemon. Anacron is an independent linux program that is started and run by the cron daemon or other boot scripts, and can periodically execute a task every day, every week, and every month (the smallest unit is days). Suitable for machines that may shut down frequently, when the machine restarts the anacron program, anacron checks whether the anacron task is executed in the appropriate cycle, and if not, it only executes the task once after the delay set by anacron, regardless of how many cycles the task missed. For example, if you set a weekly task to back up files, but your computer is turned off for four weeks because you are away on vacation, when you get home and turn it on, anacron will back up files only once after a certain delay. Depending on the distribution, how the cron daemon runs anacron varies.

2.crontab command, crontab file syntax

The default crontab file is / etc/crontab, as well as the files in the / etc/cron.d/ directory, and some programs will put their own crontab files in the / etc/cron.d/ directory. Root permissions are required to modify files in the / etc/crontab and / etc/cron.d/ directories. The cron daemon examines the files in the / etc/crontab and / etc/cron.d/ directory, decides whether or not to execute the task based on the execution time set by the cron task in these files, and if the current time is the same as the execution time set by the cron task.

Each user's own crontab file is placed in the / var/spool/cron directory, which is empty by default and can be created using the crontab command. The cron daemon examines the files in the / var/spool/cron directory and decides whether to execute the task based on the execution time set by the cron task in these files, or if the current time is the same as the execution time set by the cron task.

When the configuration file of cron changes, there is no need to reset the cron daemon. The cron daemon checks for changes to the configuration file.

Before you create your own crontab, first set the environment variable EDITOR,cron process to determine which editor to use to edit the crontab file. Add a line to .bashrc or .profile in the home directory

You can also use your favorite terminal editor EDITOR=vim; export EDITOR

2.1.crontab command parameters and usage examples

123456usage: crontab [- u user] file crontab [- u user] [- I] {- e |-l |-r}-e (edit user's crontab)-l (list user's crontab)-r (delete user's crontab)-I (prompt before deleting user's crontab)

Create and edit the crontab of the current user

Crontab-e

List the crontab of the current user

Crontab-l

Delete the crontab of user linus

Crontab-u linus-I-r

Detailed explanation of 2.2.crontab file syntax and its examples

When editing the current user's crontab file with crontab-e, write the following first.

1234567891011121314 crontab-eSHELL=/bin/bashMAILTO=root@example.comPATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin# For details see man 4 crontabs# Example of job definition:#.-minute (0-59) # |.-hour (0-23) # | |.-day of month ( 1-31) # |.-month (1-12) OR jan Feb,mar,apr. # |.-day of week (0-6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat# | # * user-name command to be executed

The first three lines of code in this file set the default environment. The cron daemon does not provide any environment. The SHELL variable sets the shell,MAILTO variable when the cron task (commands and scripts) runs sets the mailbox to be sent by the cron task execution result, and the PATH sets the command under which directory to find the cron task. The comments section explains the composition of a cron task. A cron task is a single line, and as many cron tasks as you want to set, write as many lines. A cron task consists of seven parts, from left to right:

Minutes (0-59)

Hours (0-23)

Days (1-31)

Month (1-12): or you can use the first three letters of the English word of month, such as jan,feb,mar,apr.

Week (0-6): you can use 0 or 7 on Sunday, or you can use the first three letters of the week's English word, such as sun,mon,tue,wed,thu,fri,sat.

User name (can be omitted)

Directory of commands or scripts to be executed

The first five parts should pay attention to the meaning of special symbols:

If you want to match all the values in the range, use "*"

To match some special values, use ",", such as 2, 4, 7, to match 2, 4 and 7.

The two values are concatenated to represent a range, which matches all values in the range, including values on both sides of the "-". For example, 4-7 matches from 4 to 7.

To express that a task is executed at regular intervals, use "/", for example, "* / 10" in the minute section means to run every 10 minutes, and "10-22 impulse 2" in the hour section means to run every two hours between 10:00 and 10:00 in the evening. Note: the task will run only when the value on the left of "/" can be divided by the value on the right of "/".

Example 1:

Executed every morning.

0 0 * root command

Example 2:

Every Sunday at five o'clock in the morning.

0 5 * * sun root command

Example 3:

Orders are executed every 10 minutes starting at 10:00 on the first 10 days of each month.

* / 10 22 1-10 * * root command

Example 4:

Orders are executed at the 10th, 20th and 30th minutes of each hour from Monday to Friday

10, 10, 20, 30 * 1-5 root command

Example 5:

From 10:00 to 10:00 in the evening, the command runs every five minutes in every even-numbered hour (e.g. 10:00, 12:00)

* / 5 10-22 Compact 2 * root command

Special and simple writing method:

@ hourly stands for 0 * and runs every hour

@ daily stands for 0 *, which is run every morning.

@ weekly stands for 0 0 * * 0, which runs every Sunday morning.

@ monthly stands for 0 01 * *, which is run in the early hours of the first day of every month.

@ yearly stands for 0 0 1 1 *, which is run at the first minute of each year

@ reboot is executed once after restart

Example:

Run it once every morning.

@ daily command

You can put scripts that need to be run every hour into the / etc/cron.hourly directory, and the cron daemon will run every hour.

2.3. Restrict users who can use cron

In the / etc/ directory, there may be cron.allow and cron.deny files by default, or you can create them without them. Cron.allow files contain usernames that can be used with cron, and cron.deny files contain usernames that cannot be used with cron. There is a line for each user name in the two files, and spaces are not allowed.

Root users can use cron in any case.

If cron.allow exists, only the user names listed in this file can use cron, and cron.deny is ignored.

If cron.allow does not exist, the user name listed in the cron.deny file cannot use cron.

3. Perform periodic tasks with anacron

Anacron programs can perform tasks periodically, but the exact execution time is uncertain. Can perform tasks periodically every day, every week and every month. When the configuration file of anacron changes, the next time anacron runs, it will check for changes in the configuration file. The configuration file of anacron is / etc/anacrontab, and root permission is required for editing. Default is as follows:

12345678910111 See anacron (8) and anacrontab (5) for details.SHELL=/bin/shPATH=/sbin:/bin:/usr/sbin:/usr/binMAILTO=root# the maximal random delay added to the base delay of the jobsRANDOM_DELAY=45# the jobs will be started during the following hours onlySTART_HOURS_RANGE=3-22#period in days delay in minutes job-identifier command1 5 cron.daily nice run-parts / etc/cron.daily7 25 cron.weekly nice run-parts / etc/cron.weekly@monthly 45 cron.monthly nice run-parts / etc/cron.monthly

You can see that the first three lines set the default environment, the RANDOM_DELAY variable sets the maximum delayed execution time, and the START_HOURS_RANGE variable sets the time range for anacron task execution, which defaults between 3: 00 and 22:00 every day. The last three lines set up three default anacron tasks, which are executed daily, weekly, and monthly.

3.1.anacron file syntax

Looking at the first three default anacron tasks, you can see that an anacron task is divided into four parts, from left to right:

Cycle (days): set the frequency of task execution in days. Write 1 for daily execution and 3 for 3 days. You can use the special symbol "@", @ daily for daily, @ weekly for weekly, and @ monthly for monthly.

Delay time: set the delay time of a task in minutes, such as write 5, then when anacron starts, anacron waits for 5 minutes to execute the task. The delay time is set so that when the machine starts, it will not be overloaded by performing many anacron tasks.

Task identification: the purpose is to identify messages, log files, and perform special operations.

Command to be executed

Of the three default anacron tasks in / etc/anacrontab, the nice command is used to adjust the priority of subsequent commands, and the run-parts command is used to execute all scripts in the set directory, that is, these three tasks execute scripts in the / etc/cron.daily,/etc/cron.weekly,/etc/cron.monthly directory on a daily, weekly, and monthly basis. So instead of adding anacrontab tasks to / etc/anacrontab, just put the scripts in the appropriate directory, and anacron will execute them periodically.

How 3.2.anacron works

Since anacron is not a daemon, how does it perform tasks periodically? Under centos7, cron runs / etc/cron.d/0hourly, and in the / etc/cron.d/0hourly file, there is a cron task that looks like this:

01 * root run-parts / etc/cron.hourly

This cron task runs all the scripts in the / etc/cron.hourly directory, including a script named 0anacron, which runs anacron,anacron at the right time, checks / etc/anacrontab, and runs the scripts in the / etc/cron.daily,/etc/cron.weekly,/etc/cron.monthly directory at the right time.

After reading the above, do you have any further understanding of how to execute commands and scripts on linux on a regular basis? If you want to know more knowledge or related content, please follow the industry information channel, thank you for your support.

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