layout: post
title: Systemd
date: 2018-01-06
tags: [“Linux”,”系统的启动和内核的管理”]
一、Systemd简介
1、Systemd是一个新兴的系统启动和服务器守护进程管理器，负责在系统启动或运行时，激活系统资源，服务器进程和其他进程。systemd由红帽公司开发，用以代替Upstart，”系统的第一个进程init”。
2、Systemd的新特性
系统引导时实现服务并行启动
按需启动守护进程
自动化的服务依赖关系管理
同时采用socket式与D-Bus总线式激活服务 （socket与服务程序分离，由systemd来监控，代替了xinetd）
系统状态快照
二、核心概念：Unit
所谓Unit（单元），就是systemd的管理对象，以下是各种unit类型。
Service：用来定义系统服务
Target：用来模拟实现系统运行级别（systemd中实际已经没有了运行级别这么一个说法）
Device：用来定义内核是别的设备
Mount：用来定义文件系统的挂载点
Socket：用于表示进程间通信用的socket文件，也可以在系统启动时，延迟启动服务，系统按需启动（systemd的新特性，后面会做介绍）
Snapshot：管理系统快照
Swap：用于表示swap设备
Automount：文件系统的自动挂载点
Path：用于定义文件系统中的一个文件或目录使用，常用于文件系统变化时，延迟激活服务，如：spool目录
配置文件：
/usr/lib/systemd/system:每个服务最主要的启动脚本设置，类似于之前的/etc/init.d/
/run/systemd/system：系统执行过程中所产生的服务脚本，比上面目录优先运行
/etc/systemd/system：管理员建立的执行脚本，类似于/etc/rc.d/rcN.d/Sxx类的功能，比上面目录优先运行（实际就是/usr/lib/systemd/system下的各个文件的软连接）
三、使用Systemd来管理服务（这里只列出单独的命令，后面会演示）
1、服务的常规操作
[table id=4 /]
2、服务的查看
注意：一下的unit代表service’target’device….
[table id=5 /]
服务状态
systemctl list-unit-files —type service —all显示状态
loaded:Unit配置文件已处理
active(running):一次或多次持续处理的运行
active(exited):成功完成一次性的配置
active(waiting):运行中，等待一个事件
inactive:不运行
enabled:开机启动
disabled:开机不启动
static:开机不启动，但可被另一个启用的服务激活
systemctl status services 查看的服务，当服务状态有问题的时候，可以使用journalctl -xe 来查看问题日志，journal可以为我们提供部分服务的解决方案。
3、运行级别（systemd的运行级别不再是0-6了，或者说systemd没有运行级别，而是target，我们完全可以设定适合我们的target）
[table id=6 /]
4、演示
常规服务的操作：
- [root@newhostname /]# systemctl start httpd #当命令成功时不会提示
- [root@newhostname /]# systemctl status httpd #查看服务状态
- ● httpd.service - The Apache HTTP Server
- Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
- Active: active (running) since 六 2018-01-06 00:43:58 CST; 9s ago #Active：active（running）表示激活的
- Docs: man:httpd(8)
- man:apachectl(8)
- Process: 3535 ExecStop=/bin/kill -WINCH ${MAINPID} (code=exited, status=1/FAILURE)
- Process: 3237 ExecReload=/usr/sbin/httpd $OPTIONS -k graceful (code=exited, status=0/SUCCESS)
- Main PID: 3568 (httpd)
- Status: "Total requests: 0; Current requests/sec: 0; Current traffic: 0 B/sec"
- CGroup: /system.slice/httpd.service
- ├─3568 /usr/sbin/httpd -DFOREGROUND
- ├─3571 /usr/sbin/httpd -DFOREGROUND
- ├─3572 /usr/sbin/httpd -DFOREGROUND
- ├─3573 /usr/sbin/httpd -DFOREGROUND
- ├─3574 /usr/sbin/httpd -DFOREGROUND
- └─3575 /usr/sbin/httpd -DFOREGROUND
- [root@newhostname /]# systemctl stop httpd #同start，命令成功不糊提示
- [root@newhostname /]# systemctl status httpd
- ● httpd.service - The Apache HTTP Server
- Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
- Active: inactive (dead) #inactive目标服务未启动
- Docs: man:httpd(8)
- man:apachectl(8)
- 1月 06 00:43:35 newhostname kill[3535]: kill: cannot find process ""
- 1月 06 00:43:35 newhostname systemd[1]: httpd.service: control process exited, code=exited status=1
- 1月 06 00:43:35 newhostname systemd[1]: Failed to start The Apache HTTP Server.
- 1月 06 00:43:35 newhostname systemd[1]: Unit httpd.service entered failed state.
- 1月 06 00:43:35 newhostname systemd[1]: httpd.service failed.
- 1月 06 00:43:58 newhostname systemd[1]: Starting The Apache HTTP Server...
- 1月 06 00:43:58 newhostname httpd[3568]: AH00558: httpd: Could not reliably determine the server's fully qualified domain name, using fe80::20...s message
- 1月 06 00:43:58 newhostname systemd[1]: Started The Apache HTTP Server.
- 1月 06 00:44:21 newhostname systemd[1]: Stopping The Apache HTTP Server...
- 1月 06 00:44:22 newhostname systemd[1]: Stopped The Apache HTTP Server.
- Hint: Some lines were ellipsized, use -l to show in full.
- 我们现在把selinux打开，再启动一次httpd服务（因为我之前已经把http端口的端口改了，selinux内并没有添加相应的策略，所以启动服务肯定会失败）
- [root@newhostname /]# setenforce 1
- [root@newhostname /]# systemctl start httpd
- Job for httpd.service failed because the control process exited with error code. See "systemctl status httpd.service" and "journalctl -xe" for details.
- [root@newhostname /]# systemctl status httpd #我们查看一下启动失败后的状态信息
- ● httpd.service - The Apache HTTP Server
- Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
- Active: failed (Result: exit-code) since 六 2018-01-06 00:50:51 CST; 17s ago #failed为启动失败状态
- Docs: man:httpd(8)
- man:apachectl(8)
- Process: 3616 ExecStop=/bin/kill -WINCH ${MAINPID} (code=exited, status=1/FAILURE)
- Process: 3615 ExecStart=/usr/sbin/httpd $OPTIONS -DFOREGROUND (code=exited, status=1/FAILURE)
- Main PID: 3615 (code=exited, status=1/FAILURE)
- 1月 06 00:50:51 newhostname httpd[3615]: (13)Permission denied: AH00072: make_sock: could not bind to address [::]:3
- 1月 06 00:50:51 newhostname httpd[3615]: (13)Permission denied: AH00072: make_sock: could not bind to address 0.0.0.0:3
- 1月 06 00:50:51 newhostname httpd[3615]: no listening sockets available, shutting down
- 1月 06 00:50:51 newhostname httpd[3615]: AH00015: Unable to open logs
- 1月 06 00:50:51 newhostname systemd[1]: httpd.service: main process exited, code=exited, status=1/FAILURE
- 1月 06 00:50:51 newhostname kill[3616]: kill: cannot find process ""
- 1月 06 00:50:51 newhostname systemd[1]: httpd.service: control process exited, code=exited status=1
- 1月 06 00:50:51 newhostname systemd[1]: Failed to start The Apache HTTP Server.
- 1月 06 00:50:51 newhostname systemd[1]: Unit httpd.service entered failed state.
- 1月 06 00:50:51 newhostname systemd[1]: httpd.service failed.
- 现在我们使用"journalctl -xe"查看详细信息。
- [root@newhostname /]# journalctl -xe
- 1月 06 00:50:51 newhostname dbus-daemon[808]: dbus[808]: [system] Successfully activated service 'org.fedoraproject.Setroubleshootd'
- 1月 06 00:50:52 newhostname setroubleshoot[3622]: SELinux is preventing /usr/sbin/httpd from name_bind access on the tcp_socket port 3. For complete SELinu
- 1月 06 00:50:52 newhostname python[3622]: SELinux is preventing /usr/sbin/httpd from name_bind access on the tcp_socket port 3.
- ***** Plugin bind_ports (99.5 confidence) suggests ************************
- If you want to allow /usr/sbin/httpd to bind to network port 3
- Then you need to modify the port type.
- Do
- # semanage port -a -t PORT_TYPE -p tcp 3
- where PORT_TYPE is one of the following: http_cache_port_t, http_port_t, jboss_management_port_t, jboss_mess
- ***** Plugin catchall (1.49 confidence) suggests **************************
- If you believe that httpd should be allowed name_bind access on the port 3 tcp_socket by default.
- Then you should report this as a bug.
- You can generate a local policy module to allow this access.
- Do
- allow this access for now by executing:
- # ausearch -c 'httpd' --raw ' audit2allow -M my-httpd
- # semodule -i my-httpd.pp
- 1月 06 00:50:52 newhostname setroubleshoot[3622]: SELinux is preventing /usr/sbin/httpd from name_bind access on the tcp_socket port 3. For complete SELinu
- 1月 06 00:50:52 newhostname python[3622]: SELinux is preventing /usr/sbin/httpd from name_bind access on the tcp_socket port 3.
- ***** Plugin bind_ports (99.5 confidence) suggests ************************
- If you want to allow /usr/sbin/httpd to bind to network port 3
- Then you need to modify the port type.
- Do
- # semanage port -a -t PORT_TYPE -p tcp 3
- where PORT_TYPE is one of the following: http_cache_port_t, http_port_t, jboss_management_port_t, jboss_mess
- ***** Plugin catchall (1.49 confidence) suggests **************************
- If you believe that httpd should be allowed name_bind access on the port 3 tcp_socket by default.
- Then you should report this as a bug.
- You can generate a local policy module to allow this access.
- Do
- allow this access for now by executing:
- # ausearch -c 'httpd' --raw ' audit2allow -M my-httpd
- # semodule -i my-httpd.pp
- journal 直接告诉了我们，是selinux出了问题，可以通过"semanage port -a -t PORT_TYPE -p tcp 3"来解决问题
- 关于服务的常规操作，其他的几个操作就不再演示了，能理解意思就行
服务的查看：
- [root@newhostname /]# systemctl --failed -t service #查看失败的服务
- UNIT LOAD ACTIVE SUB DESCRIPTION
- ● httpd.service loaded failed failed The Apache HTTP Server
- LOAD = Reflects whether the unit definition was properly loaded.
- ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
- SUB = The low-level unit activation state, values depend on unit type.
- 1 loaded units listed. Pass --all to see loaded but inactive units, too.
- To show all installed unit files use 'systemctl list-unit-files'.
- [root@newhostname /]# systemctl --failed #查看所有启动失败的unit
- UNIT LOAD ACTIVE SUB DESCRIPTION
- ● httpd.service loaded failed failed The Apache HTTP Server
- LOAD = Reflects whether the unit definition was properly loaded.
- ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
- SUB = The low-level unit activation state, values depend on unit type.
- 1 loaded units listed. Pass --all to see loaded but inactive units, too.
- To show all installed unit files use 'systemctl list-unit-files'.
- [root@newhostname /]# systemctl stop httpd
- [root@newhostname /]# systemctl is-active httpd #查看服务状态
- unknown
- [root@newhostname /]# echo $? #值不为0都是未启动
- 3
- [root@newhostname /]# setenforce 0
- [root@newhostname /]# systemctl start httpd
- [root@newhostname /]# systemctl is-active httpd
- active
- [root@newhostname /]# echo $? #值为0表示服务正在运行
- 0
演示运行级别： 首先，我们先说默认的target，设置方法:systemctl set-default name.target,这里我们就已设置图形界面为例
- [root@newhostname /]# systemctl list-units -t service #如果不加 -t service 默认会打印出所有的unit单元
- UNIT LOAD ACTIVE SUB DESCRIPTION #这里我只粘贴了少量的一部分，因为service的unit太多了
- abrt-ccpp.service loaded active exited Install ABRT coredump hook
- abrt-oops.service loaded active running ABRT kernel log watcher
- abrt-xorg.service loaded active running ABRT Xorg log watcher
- abrtd.service loaded active running ABRT Automated Bug Reporting Tool
- alsa-state.service loaded active running Manage Sound Card State (restore and store)
- atd.service loaded active running Job spooling tools
- auditd.service loaded active running Security Auditing Service
- blk-availability.service loaded active exited Availability of block devices
- bluetooth.service loaded active running Bluetooth service
- chronyd.service loaded active running NTP client/server
- crond.service loaded active running Command Scheduler
- cups.service loaded active running CUPS Printing Service
- dbus.service loaded active running D-Bus System Message Bus
- [root@newhostname /]# systemctl list-unit-files #打印所有的unit状态
- UNIT FILE STATE
- proc-sys-fs-binfmt_misc.automount static
- dev-hugepages.mount static
- dev-mqueue.mount static
- proc-fs-nfsd.mount static
- proc-sys-fs-binfmt_misc.mount static
- sys-fs-fuse-connections.mount static
- sys-kernel-config.mount static
- sys-kernel-debug.mount static
- tmp.mount disabled
- var-lib-nfs-rpc_pipefs.mount static
- brandbot.path disabled
- cups.path enabled
- systemd-ask-password-console.path static
- systemd-ask-password-plymouth.path static
- systemd-ask-password-wall.path static
- session-25.scope static
- session-27.scope static
- abrt-ccpp.service enabled
- abrt-oops.service enabled
- abrt-pstoreoops.service disabled
- abrt-vmcore.service enabled
- abrt-xorg.service enabled
- abrtd.service enabled
- accounts-daemon.service enabled
- [root@newhostname /]# systemctl list-unit-files -t service #打印unit为service的服务状态
- UNIT FILE STATE
- abrt-ccpp.service enabled
- abrt-oops.service enabled
- abrt-pstoreoops.service disabled
- abrt-vmcore.service enabled
- abrt-xorg.service enabled
- abrtd.service enabled
- accounts-daemon.service enabled
- alsa-restore.service static
- alsa-state.service static
- alsa-store.service static
- anaconda-direct.service static
- anaconda-nm-config.service static
- anaconda-noshell.service static
- anaconda-pre.service static
- anaconda-shell@.service static
- anaconda-sshd.service static
- anaconda-tmux@.service static
- anaconda.service static
- arp-ethers.service disabled
- atd.service enabled
- auditd.service enabled
- auth-rpcgss-module.service static
- autofs.service disabled
- autovt@.service enabled
- blk-availability.service disabled
- bluetooth.service enabled
- brandbot.service static
- canberra-system-bootup.service disabled
- canberra-system-shutdown-reboot.service disabled
- canberra-system-shutdown.service disabled
- certmonger.service disabled
- cgconfig.service disabled
- cgdcbxd.service disabled
- cgred.service disabled
- chrony-dnssrv@.service static
- chrony-wait.service disabled
- chronyd.service enabled
- colord.service static
- configure-printer@.service static
- console-getty.service disabled
- console-shell.service disabled
- container-getty@.service static
### 四、服务Unit文件 所有的systemd单元都存放在了"/usr/lib/systemd/system/"这个目录下，不管是target还是service还是socket都在这里。 1、我们查看一个service的unit，来看看它的结构，就以httpd的为例吧
- [root@newhostname system]# systemctl set-default graphical.target #这是图形界面为启动机器之后的默认target
- Removed symlink /etc/systemd/system/default.target.
- Created symlink from /etc/systemd/system/default.target to /usr/lib/systemd/system/graphical.target. /etc/systemd/system/default.target
- 它执行的动作，很简单，把"/etc/systemd/system/default.target"这个软连接删除了，建立新的target到/etc/systemd/system/default.target
- 现在我们在当前的target切换到其他的target中(当前是字符界面，我们切换到图形界面，因为我没有装图形，所以不会真正的切过去）
- [root@newhostname default.target.wants]# systemctl isolate graphical.target
- [root@newhostname default.target.wants]#
- 执行成功了？为什么没报错，这个问题后面再讲，马上就讲，请继续看
- [root@newhostname multi-user.target.wants]# cat /usr/lib/systemd/system/httpd.service
- [Unit]
- Description=The Apache HTTP Server
- After=network.target remote-fs.target nss-lookup.target
- Documentation=man:httpd(8)
- Documentation=man:apachectl(8)
- [Service]
- Type=notify
- EnvironmentFile=/etc/sysconfig/httpd
- ExecStart=/usr/sbin/httpd $OPTIONS -DFOREGROUND
- ExecReload=/usr/sbin/httpd $OPTIONS -k graceful
- ExecStop=/bin/kill -WINCH ${MAINPID}
- # We want systemd to give httpd some time to finish gracefully, but still want
- # it to kill httpd after TimeoutStopSec if something went wrong during the
- # graceful stop. Normally, Systemd sends SIGTERM signal right after the
- # ExecStop, which would kill httpd. We are sending useless SIGCONT here to give
- # httpd time to finish.
- KillSignal=SIGCONT
- PrivateTmp=true
- [Install]
- WantedBy=multi-user.target
- [root@newhostname multi-user.target.wants]#
可以看出service的unit是由3大块组成，即[Unit]、[Service]、[Install]
[Unit]：定义与Unit类型无关的通用选项；用于提供unit的描述信息、unit行为及依赖关系等；Unit段的常用选项：
Description：描述信息
After：定义unit的启动次序，表示当前unit应该晚于哪些unit启动，其功能与Before相反
Requires：依赖到的其它units，强依赖，被依赖的units无法激活时，当前unit也无法激活
Wants：依赖到的其它units，弱依赖
Conflicts：定义units间的冲突关系
[Service]：与特定类型相关的专用选项；Service段的常用选项：
EnvironmentFile：环境配置文件
ExecStart：指明启动unit要运行命令或脚本的绝对路径
ExecStartPre： ExecStart前运行
ExecStartPost： ExecStart后运行
ExecStop：指明停止unit要运行的命令或脚本
Restart：当设定Restart=1 时，则当次daemon服务意外终止后，会再次自动启动此服务
KillSignal：执行systemctl kill service 时，发送的信号
Type：定义影响ExecStart及相关参数的功能的unit进程启动类型
• simple：默认值，这个daemon主要由ExecStart接的指令串来启动，启动后常驻于内存中
• forking：由ExecStart启动的程序透过spawns延伸出其他子程序来作为此daemon的主要服务。原生父程序在启动结束后就会终止
• oneshot：与simple类似，不过这个程序在工作完毕后就结束了，不会常驻在内存中
• dbus：与simple类似，但这个daemon必须要在取得一个D-Bus的名称后，才会继续运作.因此通常也要同时设定BusNname= 才行
• notify：在启动完成后会发送一个通知消息。还需要配合NotifyAccess 来让 Systemd 接收消息
• idle：与simple类似，要执行这个daemon必须要所有的工作都顺利执行完毕后才会执行。这类的daemon通常是开机到最后才执行即可的服务
[Install]： 定义由”systemctl enable”以及”systemctl disable”命令在实现服务启用或禁用时用到的一些选项；Install段的常用选项：
Alias：别名，可使用systemctl command Alias.service
RequiredBy：被哪些units所依赖，强依赖
WantedBy：被哪些units所依赖，弱依赖
Also：安装本服务的时候还要安装别的相关服务
那么我们现在编写一个服务脚本
2、socket Unit Systemd引用了socket这种unit，从而替代了以前系统的超级守护进程xinetd，之前以来xinetd的服务，systemd可以完全接手，下面我们就以telnet服务为例
- [root@newhostname system]# cat <
bak.service #编写简单脚本 [Unit] Description=backup /etc Requires=atd.service [Service] Type=simple ExecStart=/usr/bin/echo "ok" [Install] WantedBy=multi-user.target [root@newhostname system]# systemctl start bak.service [root@newhostname system]# systemctl status bak.service ● bak.service - backup /etc Loaded: loaded (/usr/lib/systemd/system/bak.service; disabled; vendor preset: disabled) Active: inactive (dead) 1月 06 15:17:26 newhostname echo[2832]: ok 1月 06 15:17:28 newhostname systemd[1]: Started backup /etc. 1月 06 15:17:28 newhostname systemd[1]: Starting backup /etc... 1月 06 15:17:28 newhostname echo[2840]: ok 1月 06 15:17:29 newhostname systemd[1]: Started backup /etc. 1月 06 15:17:29 newhostname systemd[1]: Starting backup /etc... 1月 06 15:17:29 newhostname echo[2848]: ok 1月 06 15:17:29 newhostname systemd[1]: Started backup /etc. 1月 06 15:17:29 newhostname systemd[1]: Starting backup /etc... 1月 06 15:17:29 newhostname echo[2856]: ok
其实telent服务平时是不启动的，当有数据接收时才会触发telnet服务启动，而telnet服务也是一个unit单元，命名为telnet@.service,我们可以再在/usr/lib/systemd/system/下查看一下
- 首先我们启动telnet服务(telnet是一个socket单元，在systemd中）
- [root@newhostname /]# systemctl start telnet.socket
- [root@newhostname /]#
- 我们来查看一下链接
- [root@newhostname /]# ss -antp ' grep 23
- LISTEN 0 128 :::23 :::* users:(("systemd",pid=1,fd=29))
- 我们可以看到数据的最后一列，telent端口是由systemd来管理的
- [root@newhostname /]# cat //usr/lib/systemd/system/telnet@.service
- [Unit]
- Description=Telnet Server
- After=local-fs.target
- [Service]
- ExecStart=-/usr/sbin/in.telnetd
- StandardInput=socket
带有@的服务是不能够被systemctl start直接启动的，其由systemd直接来监管。
2、target单元
在redhat7之前第一个进程是init，自redhat7开始使用systemd作为所有进程的父进程，而之前所使用的运行级别，到这里则变成了target概念，那么什么是tareget，target就是一组服务或某特殊功能的一个指针，我们就先已graphical.target为例
- [root@newhostname system]# ll ' grep graphical.target
- lrwxrwxrwx. 1 root root 16 11月 7 16:12 default.target -> graphical.target
- -rw-r--r--. 1 root root 558 8月 5 14:38 graphical.target
- drwxr-xr-x. 2 root root 50 11月 7 16:12 graphical.target.wants
- lrwxrwxrwx. 1 root root 16 11月 7 16:12 runlevel5.target -> graphical.target
- default.target默认的启动目标，联想一下/etc/systemd/system/default.target 很容易理解
- runlevel5.target 这个其实也很好理解，为了兼容以前的版本么，之前的 init 5实际指向了 graphical.target
- 在这里我们真正要说的是这两个
- -rw-r--r--. 1 root root 558 8月 5 14:38 graphical.target
- drwxr-xr-x. 2 root root 50 11月 7 16:12 graphical.target.wants
- 我们查看一下graphical.target这个文件
- [root@newhostname system]# cat graphical.target
- # This file is part of systemd.
- #
- # systemd is free software; you can redistribute it and/or modify it
- # under the terms of the GNU Lesser General Public License as published by
- # the Free Software Foundation; either version 2.1 of the License, or
- # (at your option) any later version.
- [Unit]
- Description=Graphical Interface
- Documentation=man:systemd.special(7)
- Requires=multi-user.target
- Wants=display-manager.service
- Conflicts=rescue.service rescue.target
- After=multi-user.target rescue.service rescue.target display-manager.service
- AllowIsolate=yes
这个是不是已经很眼熟了，好像service的 Unit，其实他们的选项配置基本一样，下面我们来细细的分析一下，此[Unit]的各个标签的含义
Description 这个是描述
Requires 这个是强依赖，”看来图形界面也是以来字符界面的”
Wants display-manager,很显然是图形界面管理器么，但这是个弱依赖，这也就是为什么，
之前我们使用systemctl isolate graphical.target 不会报错的原因
Conflicts 定义了与之冲突的目标，这里定义的是 rescue救援模式
After 表示需要在multi-user.target rescue.service rescue.target display-manager.service这些单元启动之后启动
AllowIsolate 这个是一个beloon值，也是和service_unit区分的地方，它代表是否可以切换目标
*.target.wants的意义：表示切换了运行目标想要操作的unit，这个是弱依赖
我们接着看graphical.target.wants这个目录下都有些什么
`systemd-update-utmp-runlevel.service` 服务 用于在 SysV 运行级发生变化时，更新 utmp 与 wtmp 文件， 以及记录审计日志。 结合上面的信息，我们得出一个结论，那么就是切换到图形界面，实际上是在原来字符界面的基础上又启动了一个display-manager.service,但它是一个弱依赖，在我的机器上并没有安装图形，但是systemd-update-utmp-runlevel.service又真实运行了，那么我们查看一下一下现在的运行目标
- [root@newhostname system]# cd graphical.target.wants/
- [root@newhostname graphical.target.wants]# ls
- systemd-update-utmp-runlevel.service
- 只有这个一个服务
- [root@newhostname system]# systemctl get-default
- multi-user.target #现在的运行目标
- [root@newhostname system]# runlevel #为了展示兼容以前
- N 3
- [root@newhostname system]# systemctl isolate graphical.target #切换成图形
- [root@newhostname system]# runlevel
- 3 5
- 实际上我们并没有启动图形，因为没有安装，但是切换操作影响了systemd-update-utmp-runlevel.service这个服务，所以显示的运行级别出现了变化
在Systemd中兼容了之前的init，但是像runlevel这样的命令，其实都是即将废弃的命令，我们可以通过 systemctl list-units —type=target 来查看运行目标
- [root@newhostname system]# systemctl list-units --type=target
- UNIT LOAD ACTIVE SUB DESCRIPTION
- basic.target loaded active active Basic System
- cryptsetup.target loaded active active Encrypted Volumes
- getty.target loaded active active Login Prompts
- graphical.target loaded active active Graphical Interface
- local-fs-pre.target loaded active active Local File Systems (Pre)
- local-fs.target loaded active active Local File Systems
- multi-user.target loaded active active Multi-User System
- network-online.target loaded active active Network is Online
- network.target loaded active active Network
- nfs-client.target loaded active active NFS client services
- nss-user-lookup.target loaded active active User and Group Name Lookups
- paths.target loaded active active Paths
- remote-fs-pre.target loaded active active Remote File Systems (Pre)
- remote-fs.target loaded active active Remote File Systems
- slices.target loaded active active Slices
- sockets.target loaded active active Sockets
- swap.target loaded active active Swap
- sysinit.target loaded active active System Initialization
- timers.target loaded active active Timers
- LOAD = Reflects whether the unit definition was properly loaded.
- ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
- SUB = The low-level unit activation state, values depend on unit type.
- 19 loaded units listed. Pass --all to see loaded but inactive units, too.
- To show all installed unit files use 'systemctl list-unit-files'.
我们一级一级向上看，我们接下来看一下multi-user.target
- [root@newhostname system]# cat multi-user.target
- # This file is part of systemd.
- #
- # systemd is free software; you can redistribute it and/or modify it
- # under the terms of the GNU Lesser General Public License as published by
- # the Free Software Foundation; either version 2.1 of the License, or
- # (at your option) any later version.
- [Unit]
- Description=Multi-User System
- Documentation=man:systemd.special(7)
- Requires=basic.target
- Conflicts=rescue.service rescue.target
- After=basic.target rescue.service rescue.target
- AllowIsolate=yes
- 最主要的还是这几行
- Requires=basic.target #强依赖
- Conflicts=rescue.service rescue.target #与之冲突的目标
- After=basic.target rescue.service rescue.target #在这些服务启动之后才会启动
- AllowIsolate=yes #表示可以切换
我们查看multi-user.target.wants
- [root@newhostname system]# cd multi-user.target.wants/
- [root@newhostname multi-user.target.wants]# ll
- 总用量 0
- lrwxrwxrwx. 1 root root 16 11月 7 16:12 brandbot.path -> ../brandbot.path
- lrwxrwxrwx. 1 root root 15 11月 7 16:12 dbus.service -> ../dbus.service
- lrwxrwxrwx. 1 root root 15 11月 7 16:12 getty.target -> ../getty.target
- lrwxrwxrwx. 1 root root 24 11月 7 16:12 plymouth-quit.service -> ../plymouth-quit.service
- lrwxrwxrwx. 1 root root 29 11月 7 16:12 plymouth-quit-wait.service -> ../plymouth-quit-wait.service
- lrwxrwxrwx. 1 root root 33 11月 7 16:12 systemd-ask-password-wall.path -> ../systemd-ask-password-wall.path
- lrwxrwxrwx. 1 root root 25 11月 7 16:12 systemd-logind.service -> ../systemd-logind.service
- lrwxrwxrwx. 1 root root 39 11月 7 16:12 systemd-update-utmp-runlevel.service -> ../systemd-update-utmp-runlevel.service
- lrwxrwxrwx. 1 root root 32 11月 7 16:12 systemd-user-sessions.service -> ../systemd-user-sessions.service
- 切换运行目标到multi-user会启动这些unit
我们再向上看，basic.target
- [root@newhostname multi-user.target.wants]# cd ..
- [root@newhostname system]# cat basic.target
- # This file is part of systemd.
- #
- # systemd is free software; you can redistribute it and/or modify it
- # under the terms of the GNU Lesser General Public License as published by
- # the Free Software Foundation; either version 2.1 of the License, or
- # (at your option) any later version.
- [Unit]
- Description=Basic System
- Documentation=man:systemd.special(7)
- Requires=sysinit.target
- After=sysinit.target
- Wants=sockets.target timers.target paths.target slices.target
- After=sockets.target paths.target slices.target
- basic.target强制依赖sysinit.target
- 再启动basic.target时，希望启动sockets.target timers.target paths.target slices.target（不会影响运行）
- 最后一行的After表示启动sockets.target paths.target slices.target启动之后再启动basic.target，如果没有这行After，这些目标将会和basic.target并行启动
- 查看wants目录
- [root@newhostname system]# cd basic.target.wants/
- [root@newhostname basic.target.wants]# ll
- 总用量 0
- lrwxrwxrwx. 1 root root 23 11月 7 16:12 alsa-restore.service -> ../alsa-restore.service
- lrwxrwxrwx. 1 root root 21 11月 7 16:12 alsa-state.service -> ../alsa-state.service
- lrwxrwxrwx. 1 root root 32 11月 7 16:12 rhel-autorelabel-mark.service -> ../rhel-autorelabel-mark.service
- lrwxrwxrwx. 1 root root 27 11月 7 16:12 rhel-autorelabel.service -> ../rhel-autorelabel.service
- lrwxrwxrwx. 1 root root 25 11月 7 16:12 rhel-configure.service -> ../rhel-configure.service
- lrwxrwxrwx. 1 root root 21 11月 7 16:12 rhel-dmesg.service -> ../rhel-dmesg.service
- lrwxrwxrwx. 1 root root 27 11月 7 16:12 rhel-loadmodules.service -> ../rhel-loadmodules.service
- lrwxrwxrwx. 1 root root 48 1月 1 23:07 selinux-policy-migrate-local-changes@targeted.service -> ../selinux-policy-migrate-local-changes@.service
再向上看sysinit.target
[root@newhostname system]# cat sysinit.target# This file is part of systemd.## systemd is free software; you can redistribute it and/or modify it# under the terms of the GNU Lesser General Public License as published by# the Free Software Foundation; either version 2.1 of the License, or# (at your option) any later version.[Unit]Description=System InitializationDocumentation=man:systemd.special(7)Conflicts=emergency.service emergency.targetWants=local-fs.target swap.targetAfter=local-fs.target swap.target emergency.service emergency.target这是systemd的第一个运行目标[root@newhostname system]# ll sysinit.target.wants/总用量 0lrwxrwxrwx. 1 root root 20 11月 7 16:12 cryptsetup.target -> ../cryptsetup.targetlrwxrwxrwx. 1 root root 22 11月 7 16:12 dev-hugepages.mount -> ../dev-hugepages.mountlrwxrwxrwx. 1 root root 19 11月 7 16:12 dev-mqueue.mount -> ../dev-mqueue.mountlrwxrwxrwx. 1 root root 28 11月 7 16:12 kmod-static-nodes.service -> ../kmod-static-nodes.servicelrwxrwxrwx. 1 root root 30 11月 7 16:12 plymouth-read-write.service -> ../plymouth-read-write.servicelrwxrwxrwx. 1 root root 25 11月 7 16:12 plymouth-start.service -> ../plymouth-start.servicelrwxrwxrwx. 1 root root 36 11月 7 16:12 proc-sys-fs-binfmt_misc.automount -> ../proc-sys-fs-binfmt_misc.automountlrwxrwxrwx. 1 root root 32 11月 7 16:12 sys-fs-fuse-connections.mount -> ../sys-fs-fuse-connections.mountlrwxrwxrwx. 1 root root 26 11月 7 16:12 sys-kernel-config.mount -> ../sys-kernel-config.mountlrwxrwxrwx. 1 root root 25 11月 7 16:12 sys-kernel-debug.mount -> ../sys-kernel-debug.mountlrwxrwxrwx. 1 root root 36 11月 7 16:12 systemd-ask-password-console.path -> ../systemd-ask-password-console.pathlrwxrwxrwx. 1 root root 25 11月 7 16:12 systemd-binfmt.service -> ../systemd-binfmt.servicelrwxrwxrwx. 1 root root 28 11月 7 16:12 systemd-firstboot.service -> ../systemd-firstboot.servicelrwxrwxrwx. 1 root root 30 11月 7 16:12 systemd-hwdb-update.service -> ../systemd-hwdb-update.servicelrwxrwxrwx. 1 root root 41 11月 7 16:12 systemd-journal-catalog-update.service -> ../systemd-journal-catalog-update.servicelrwxrwxrwx. 1 root root 27 11月 7 16:12 systemd-journald.service -> ../systemd-journald.servicelrwxrwxrwx. 1 root root 32 11月 7 16:12 systemd-journal-flush.service -> ../systemd-journal-flush.servicelrwxrwxrwx. 1 root root 36 11月 7 16:12 systemd-machine-id-commit.service -> ../systemd-machine-id-commit.servicelrwxrwxrwx. 1 root root 31 11月 7 16:12 systemd-modules-load.service -> ../systemd-modules-load.servicelrwxrwxrwx. 1 root root 30 11月 7 16:12 systemd-random-seed.service -> ../systemd-random-seed.servicelrwxrwxrwx. 1 root root 25 11月 7 16:12 systemd-sysctl.service -> ../systemd-sysctl.servicelrwxrwxrwx. 1 root root 37 11月 7 16:12 systemd-tmpfiles-setup-dev.service -> ../systemd-tmpfiles-setup-dev.servicelrwxrwxrwx. 1 root root 33 11月 7 16:12 systemd-tmpfiles-setup.service -> ../systemd-tmpfiles-setup.servicelrwxrwxrwx. 1 root root 24 11月 7 16:12 systemd-udevd.service -> ../systemd-udevd.servicelrwxrwxrwx. 1 root root 31 11月 7 16:12 systemd-udev-trigger.service -> ../systemd-udev-trigger.servicelrwxrwxrwx. 1 root root 30 11月 7 16:12 systemd-update-done.service -> ../systemd-update-done.servicelrwxrwxrwx. 1 root root 30 11月 7 16:12 systemd-update-utmp.service -> ../systemd-update-utmp.servicelrwxrwxrwx. 1 root root 33 11月 7 16:12 systemd-vconsole-setup.service -> ../systemd-vconsole-setup.service启动sysinit.target之后会启动的服务
在systemd中，target其实就是一组unit的集合
五、其他的一些操作
切换至紧急救援模式：
systemctl rescue 相当于 systemctl isolate rescue.target
切换至emergency模式：
systemctl emergency 相当于 systemctl isolate emergency.target
参考文献：https://wiki.archlinux.org/index.php/systemd_(%E7%AE%80%E4%BD%93%E4%B8%AD%E6%96%87)
http://blog.csdn.net/dubendi/article/details/78792169
文档更新时间: 2018-12-13 18:37 作者：张尚
