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This article will give you a detailed explanation on how to use Systemctl management services. The editor thinks it is very practical, so I share it with you for reference. I hope you can get something after reading this article.

The Origin of systemctl

Before centos7, service was used to manage services. Although it is relatively simple to use, for each service, we need to artificially write scripts to control the start, stop, restart and so on of the service. Whether the service is controlled well or not depends entirely on the person who wrote the script, and some scripts will be very long, and there are many situations that need to be handled by ourselves.

For example, / etc/init.d/network is a script for managing network services. It has 250 lines, and the logic processing in the script is quite complex.

In order to solve the above problems, the systemctl service management tool is introduced into centos7, which uses fixed format scripts to control the start and stop of the service. Therefore, after the introduction of systemctl, the management of the service becomes very simple. It only needs one command to start, stop and restart the service.

How to use systemctl to manage services

Boot up

For software that supports systemd, it automatically adds a configuration file in the / usr/lib/systemd/system directory when installed

Let's take mysql as an example to illustrate how to set boot.

Systemctl enable mysqld

After executing the above command, add a symbolic link in the / etc/systemd/system/multi-user.target.wants directory that points to the mysqld.service file in / usr/lib/systemd/system/

After boot is set, the files in the / etc/systemd/system/ directory will not be executed until the next boot.

If you forget whether a service is set to boot, you can use the systemctl is-enabled service name to check

[root@ecs-centos-7 ~] # systemctl is-enabled mysqld enabled startup service

Execute the systemctl start service name to start the service. Let's take mysql as an example.

Systemctl start mysqld

After the service starts, you can execute the systemctl status service name to check whether the service started successfully

[root@ecs-centos-7] # systemctl status mysqld ● mysqld.service-MySQL Server Loaded: loaded (/ usr/lib/systemd/system/mysqld.service; enabled; vendor preset: disabled) Active: active (running) since five 2020-12-04 21:51:53 CST 3 months 9 days ago Docs: man:mysqld (8) http://dev.mysql.com/doc/refman/en/using-systemd.html Main PID: 1053 (mysqld) CGroup: / system.slice/mysqld.service └─ 1053 / usr/sbin/mysqld-- daemonize-- pid-file=/var/run/mysqld/mysqld.pid December 04 21:51:51 ecs-centos-7.4-64bit-20200212 systemd [1]: Starting MySQL Server... December 04 21:51:53 ecs-centos-7.4-64bit-20200212 systemd [1]: Started MySQL Server.

The meaning of each field in the above result

Loaded: location of the configuration file. Here is / usr/lib/systemd/system/mysqld.service Active: status, active (running) indicates startup, and inactive (dead) indicates closed Docs: server documentation Main PID: main process ID CGroup: all child processes in the process group

In addition to using the systemctl status service name, you can use the following command to view the status of the service

[root@ecs-centos-7 ~] # systemctl is-active mysqld active [root@ecs-centos-7 ~] # systemctl stop mysqld [root@ecs-centos-7 ~] # systemctl is-active mysqld inactive

In the above example, mysql is started at first, so executing the systemctl is-active mysqld command displays active, indicating that mysql is on

When the mysql service is turned off, the execution of the systemctl is-active mysqld command shows that inactive indicates that mysql is off

Out of Service

To stop a running service, execute the systemctl stop service name command

Systemctl stop mysqld

If you want to restart the service directly, execute the systemctl restart service name command

Configuration file for systemctl restart mysqld service

When looking at the status of mysqld above, we know that its configuration file is located in / usr/lib/systemd/system/mysqld.service, and other service configuration files are also located in this directory, but the specific file names are not the same.

The service is started and stopped according to its configuration. We use the editor or use the systemctl cat service name to view the configuration file of the service. Let's explain it with the configuration file of the sshd service.

[root@ecs-centos-7 ~] # systemctl cat sshd # / usr/lib/systemd/system/sshd.service [Unit] Description=OpenSSH server daemon Documentation=man:sshd (8) man:sshd_config (5) After=network.target sshd-keygen.service Wants=sshd-keygen.service [Service] Type=notify EnvironmentFile=/etc/sysconfig/sshd ExecStart=/usr/sbin/sshd-D $OPTIONS ExecReload=/bin/kill-HUP $MAINPID KillMode=process Restart=on-failure RestartSec=42s [Install] WantedBy=multi-user.target

Profile field description

Unit: start the dependency of sequential units

Description: service description, a paragraph describing the service Documentation: document location of the service After: the current service starts after the specified service, indicating that the sshd service needs to be started after the network and sshd-keygen services are started. If the sshd service needs to be started before the specified service starts, use the Before field Wants: service dependency, which is a weak dependency. What is indicated here is that there is a dependency between sshd and sshd-keygen, but there is a weak dependency between them, that is, if sshd-keygen fails to start, it will not affect sshd. If you want to indicate strong dependency shutdown, use the Requires field, that is, if sshd-keygen fails to start or exits, sshd must also exit.

Service: startup commands and parameters for the service

Type: startup type. The commonly used values are as follows-simple default, the process started in the ExecStart field is the main process-the forking ExecStart field will be started in fork () mode, the parent process will exit and the child process will become the main process-notify is similar to simple, and a notification signal will be sent after startup. Systemd then starts other services EnvironmentFile: the environment variable file for the service. The current configuration file can refer to the parameters in the environment variable file in the form of $KEY. In the example above, the environment variable file for sshd is / etc/sysconfig/sshd ExecStart: command to start the service execution ExecReload: command to restart the service execution KillMode: how to stop the service Possible values are-control-group default value, all processes in the current control group will be killed-process kills only the main process, mixed main process will receive SIGTERM signal, child process will receive SIGKILL signal-no process will be killed Just execute the stop command Restart of the service: for what reason the service exits before the service is restarted. Possible values are-always always restart regardless of the circumstances under which the service exits-when on-success exits normally-when on-failure exits very normally-on-abnormal is signaled to terminate and timeout Restart-on-abort restarts only when it receives a signal that has not been captured-on-watchdog exits when it times out, and only restarts. In most cases, just set it to on-failure: how many seconds do you have to wait before the server is restarted?

Install: how to install the configuration file, which defines how to boot up

The WantedBy field indicates the Target where the service resides. The Target here can be understood as a group of services.

The value of WantedBy is one or more Targe, and when the current Unit (unit) is activated (enable), the symbolic link will be placed in a subdirectory under the / etc/systemd/system directory with the suffix Target name + .marker.

For example, the WantedBy field value of sshd is multi-user.target, and after systemctl enable sshd is executed

It is equivalent to executing the command ln-s / usr/lib/systemd/system/sshd.service / etc/systemd/system/multi-user.target.wants/sshd.service

Correspondingly, when the systemctl disable sshd command is executed, the symbolic link between the two directories in the above figure will be removed, which means that the sshd service will not be started when powered on.

The suffix .service of the configuration file name sshd.service in the figure above indicates the type of Unit (unit). If omitted, the default suffix is .service, so sshd will be treated as sshd.service

Reload configuration

If you modify the configuration file of the service, you need to reload the configuration file, and then restart the service

[root@ecs-centos-7 ~] # systemctl daemon-reload [root@ecs-centos-7 ~] # systemctl restart mysqld rescue mode

When the system does not boot properly, you can put the system in rescue mode. Rescue mode provides a single user interface for fixing system problems. Execute the systemctl rescue command to enter rescue mode

[root@cghost21] # systemctl rescue PolicyKit daemon disconnected from the bus. We are no longer a registered authentication agent. Broadcast message from root@cghost21 on pts/1 (Tue 2021-03-10 20:47:51 CST): The system is going down to rescue mode NOW!

After entering rescue mode, other users who are currently logged in will receive a notification that the system will enter rescue mode.

As shown in the following example, the above root user executes the systemctl rescue command, and the test user receives a broadcast notification from the following system

[test@cghost21 ~] $Broadcast message from root@cghost21 on pts/1 (Tue 2021-03-10 20:47:51 CST): The system is going down to rescue mode NOW! This is the end of the article on "how to use Systemctl Management Services". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, please share it for more people to see.

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