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This article is to share with you the detailed introduction of init and systemd under linux. The editor thought it was very practical, so I shared it with you as a reference. Let's follow the editor and have a look.

Early init management mechanism

When centOS6, the service management mechanism of init is still adopted. I'll introduce it a little bit here, because there are some things that can be used in centOS7. Mainly look at the following characteristics:

Start, shut down and status check of the service

Start the service: / etc/init.d/nginx start

Restart the service: / etc/init.d/nginx restart

Shut down the service: / etc/init.d/nginx stop

Status View: / etc/init.d/nginx status

Service startup mode

Start independently: most services adopt this mode, such as mysqld, php-fpm, nginx, httpd and so on.

Started by super daemon hosting: the startup of these services is hosted by another service, and the service hosting these services is called super daemon. The common supuer daemon are inetd and xinetd.

Executive level

There are seven execution levels on linux, which are 0, 1, 2, 3, 4, 5, and 6, respectively. The commonly used ones are

1 single maintenance mode

3 plain text mode

5 graphic mode

The startup scripts for each execution level are linked to / etc/init.d/daemon through / etc/rc [0-6] .d / SNNdaemon.

Ll / etc/rc3.d/S55nginx lrwxrwxrwx 1 root root 15 Sep 21 11:30 / etc/rc3.d/S55nginx->.. / init.d/nginx

S means to start, and NN means numbers, which indicate the order in which the script is executed, and the smaller the number, the first to be executed. This order of execution can well manage the dependency relationships of services.

Boot self-booting service

Boot self-boot: chkconfig on deamon

Turn it off and start it on: chkconfig off deamon

Check whether the service has been started: chkconfig-- list daemon

Systemd management mechanism

Since centos7, the previous init management mechanism has been abandoned and systemd has been used instead. Let's take a look at how systemd management is different.

Parallel processing of all services to speed up the boot process.

Unlike init to start a service, systemd can start many services at the same time. Therefore, this will greatly speed up the boot and wait for the opportunity.

Address service dependency

For example, to start B service, you must start A service first. At this time, use systemd to start B service, it will automatically check for dependencies, and then start A service before starting B service.

Unit Typ

Compared with stand alone and super daemon, which have only two startup modes in init, systemd defines a unified service unit (unit), and unit is divided into: service, socket, target, path, snapshot, timer and other types.

Downwards compatible service scripts for init

The old / etc/inid.d/ service startup script can also be managed through systemd. Note: if you start the service startup script in the / etc/init.d directory manually, systemd cannot detect the health of the service.

# systemctl status mysqld ● mysqld.service-LSB: start and stop MySQL Loaded: loaded (/ etc/rc.d/init.d/mysqld; bad; vendor preset: disabled) Active: active (running) since Wed 2020-10-28 13:26:53 CST; 1 weeks 5 days ago. # / etc/init.d/mysqld restartShutting down MySQL.... [OK] Starting MySQL. [OK] [root@lijia ~] # systemctl status mysqld ● mysqld.service-LSB: start and stop MySQL Loaded: loaded (/ etc/rc.d/init.d/mysqld; bad; vendor preset: disabled) Active: active (exited) since Wed 2020-10-28 13:26:53 CST; 1 weeks 5 days ago.

With regard to init and systemd, we need to focus on systemd, and we also need to understand init.

Thank you for reading! This is the end of the detailed introduction of init and systemd under linux. I hope the above content can help you to some extent, so that you can learn more knowledge. If you think the article is good, you can share it and let more people see it.

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