systemd

￡神魔★判官ぃ 2022-02-23 05:46 236阅读 0赞

# 1. 背景 #

首先，我们先看一下`/etc/init.d/README`内容:

You are looking for the traditional init scripts in /etc/rc.d/init.d,
    and they are gone?
    
    Here's an explanation on what's going on:
    
    You are running a systemd-based OS where traditional init scripts have
    been replaced by native systemd services files. Service files provide
    very similar functionality to init scripts. To make use of service
    files simply invoke "systemctl", which will output a list of all
    currently running services (and other units). Use "systemctl
    list-unit-files" to get a listing of all known unit files, including
    stopped, disabled and masked ones. Use "systemctl start
    foobar.service" and "systemctl stop foobar.service" to start or stop a
    service, respectively. For further details, please refer to
    systemctl(1).
    
    Note that traditional init scripts continue to function on a systemd
    system. An init script /etc/rc.d/init.d/foobar is implicitly mapped
    into a service unit foobar.service during system initialization.

根据上面的解释，目前RED HAT 7, Centos 7等都采用systemd方式来作为系统初始化工具.

# 2. 为何替换使用systemd #

*  `systemd`通过并行启动服务来加速系统启动时间；
 *  `systemd`使用`Cgroup`可以更好的管理程序，同时也能控制执行环境；
 *  `systemd`通过结合`journald`达到更好的日志机制，这种日志方式在Red Hat Enterprise Linux 7中引入；
 *  `systemd`可以处理系统配置动态调整；

# 3. systemd深入 #

`systemd`管理服务，这些服务的定义通过unit files

## unit file ##

Unit file存放在如下几个目录里面，`systemd`以如下的顺序来读取unit files  
/etc/systemd/system  
/run/systemd/system  
/usr/lib/systemd/system

同样，`systemd`可以针对用户上下文下设置unit files,路径如下:  
/etc/systemd/user  
/run/systemd/user  
/usr/lib/systemd/user

查看系统所有的unit files  
`systemd list-unit-files`

unit files 分为`service`和`target`两类

systemctl list-unit-files --type service
    systemctl list-unit-files --type target

## 1. 启动或暂时停用 ##

systemctl start serviceX
    systemctl stop serviceX

## 2. 设置服务自启动或者剔除自启动 ##

systemctl enable serviceX
    systemctl disable serviceX