RHEL7 -- systemd

systemd 在RHEL7中，进程ID 1属于systemd这个新的进程。（代替之前版本中的init）

systemd提供了以下新功能：

·并行化功能，可以提高系统的启动速度

·按需启动守护进程，而不需要单独的服务

·自动服务依赖关系管理，可以防止长时间超时，例如在网络不可用时不启动网络服务

·利用linux控制组一起追踪相关进程

有了systmed以后，基于shell的服务脚本基本仅用于几个传统服务。因此，带有shell变量的配置文件将被取代，如/etc/sysconfig中可以找到的配置文件。 仍然在使用的配置文件作为systemd环境文件被纳入，并作为NAME=VALUE对进行读取。他们不再以shell脚本的形式提供。

systemctl和systemd单元

systemctl命令管理各种类型systemd对象，他们称为单元。可以通过systemctl -t help命令显示可用单元类型的列表

# systemctl -t help
Available unit types:
service
socket
target
device
mount
automount
snapshot
timer
swap
path
slice
scope

-服务单元具有.service扩展名，代表系统服务

-套接字单元具有.socket扩展名，代表进程间通信套接字

-路径单元具有.path扩展名，用于将服务的激活推迟到特定文件系统更改发生之后。

查看服务状态：

使用systemctl status name.type查看服务状态（取代了旧版中的service NAME status命令 ）

[root@rhce7 ~]# systemctl status network.service
network.service - LSB: Bring up/down networking
   Loaded: loaded (/etc/rc.d/init.d/network)
   Active: active (exited) since Thu -- :: CST; 1h 22min ago
  Process:  ExecStart=/etc/rc.d/init.d/network start (code=exited, status=/SUCCESS)

Jul  :: rhce7.example.com systemd[]: Starting LSB: Bring up/down networking...
Jul  :: rhce7.example.com network[]: Bringing up loopback interface:  Could not load file '/etc/sysconfig/networ...fg-lo'
Jul  :: rhce7.example.com network[]: Could not load file '/etc/sysconfig/network-scripts/ifcfg-lo'
Jul  :: rhce7.example.com network[]: Could not load file '/etc/sysconfig/network-scripts/ifcfg-lo'
Jul  :: rhce7.example.com network[]: Could not load file '/etc/sysconfig/network-scripts/ifcfg-lo'
Jul  :: rhce7.example.com network[]: [  OK  ]
Jul  :: rhce7.example.com network[]: Bringing up interface Profile_1:  [  OK  ]
Jul  :: rhce7.example.com network[]: Bringing up interface enp0s17:  [  OK  ]
Jul  :: rhce7.example.com network[]: Bringing up interface enp0s8:  [  OK  ]
Jul  :: rhce7.example.com systemd[]: Started LSB: Bring up/down networking.
Hint: Some lines were ellipsized, use -l to show in full.
[root@rhce7 ~]# 

如果没有指定type，即未提供单元类型，则systemctl将显示服务单元的状态（如果存在）

[root@rhce7 ~]# systemctl status session-
session-.service
   Loaded: not-found (Reason: No such file or directory)
   Active: inactive (dead)

[root@rhce7 ~]# systemctl status session-.scope
session-.scope - Session  of user root
   Loaded: loaded (/run/systemd/system/session-.scope; static)
  Drop-In: /run/systemd/system/session-.scope.d
           └─-After-systemd-logind\x2eservice.conf, -After-systemd-user-sessions\x2eservice.conf, -Description.conf, -SendSIGHUP.conf, -Slice.conf
   Active: active (running) since Thu -- :: CST; 1h 5min ago
   CGroup: /user.slice/user-.slice/session-.scope
           ├─ sshd: root@pts/
           ├─ -bash
           └─ systemctl status session-.scope

Jul  :: rhce7.example.com systemd[]: Starting Session  of user root.
Jul  :: rhce7.example.com systemd[]: Started Session  of user root.
Jul  :: rhce7.example.com sshd[]: pam_unix(sshd:session): session opened for user root by (uid=)
[root@rhce7 ~]# systemctl status network
network.service - LSB: Bring up/down networking
   Loaded: loaded (/etc/rc.d/init.d/network)
   Active: active (exited) since Thu -- :: CST; 1h 25min ago
  Process:  ExecStart=/etc/rc.d/init.d/network start (code=exited, status=/SUCCESS)

Jul  :: rhce7.example.com systemd[]: Starting LSB: Bring up/down networking...
Jul  :: rhce7.example.com network[]: Bringing up loopback interface:  Could not load file '/etc/sysconfig/networ...fg-lo'
Jul  :: rhce7.example.com network[]: Could not load file '/etc/sysconfig/network-scripts/ifcfg-lo'
Jul  :: rhce7.example.com network[]: Could not load file '/etc/sysconfig/network-scripts/ifcfg-lo'
Jul  :: rhce7.example.com network[]: Could not load file '/etc/sysconfig/network-scripts/ifcfg-lo'
Jul  :: rhce7.example.com network[]: [  OK  ]
Jul  :: rhce7.example.com network[]: Bringing up interface Profile_1:  [  OK  ]
Jul  :: rhce7.example.com network[]: Bringing up interface enp0s17:  [  OK  ]
Jul  :: rhce7.example.com network[]: Bringing up interface enp0s8:  [  OK  ]
Jul  :: rhce7.example.com systemd[]: Started LSB: Bring up/down networking.
Hint: Some lines were ellipsized, use -l to show in full.
[root@rhce7 ~]# 

服务状态表示的关键字说明

关键字	说明
loaded	单元配置文件已经处理
active（running）	正在通过一个或多个持续进程运行
active（exited）	已成功文成一次性配置
active（waiting）	运行中，正在等待事件
inactive	不在运行
enabled	开机自启动
disabled	开机不会自启动
static	无法启用，但可以由某一启动的单元自动启动