CentOS7 systemctl命令初探
CentOS7在系统结构上和CentOS5、6有了很大的区别，它用systemd代替了init作为用户的初始进程。systemctl命令是CentOS7下的管理命令。
1、用户启动进程类别
Linux系统主要有以下几种主流选择：
（1）以Ubuntu为代表的Linux发行版采用upstart。
（2）以7.0版本之前的CentOS为代表的System V init。
（3）CentOS7.0版本的systemd。
2、systemctl命令
[root@server201 ~]# systemctl UNIT LOAD ACTIVE SUB DESCRIPTION sys-devices-virtual-net-venet0.device loaded active plugged /sys/devices/virtual/net/venet0 sys-subsystem-net-devices-venet0.device loaded active plugged /sys/subsystem/net/devices/venet0 -.mount loaded active mounted / dev-mqueue.mount loaded active mounted POSIX Message Queue File System run-user-0.mount loaded active mounted /run/user/0 brandbot.path loaded active waiting Flexible branding systemd-ask-password-console.path loaded active waiting Dispatch Password Requests to Console Directory Watch systemd-ask-password-wall.path loaded active waiting Forward Password Requests to Wall Directory Watch session-29566954.scope loaded active running Session 29566954 of user root console-getty.service loaded active running Console Getty crond.service loaded active running Command Scheduler dbus.service loaded active running D-Bus System Message Bus firewalld.service loaded active running firewalld - dynamic firewall daemon getty@tty2.service loaded active running Getty on tty2 httpd.service loaded active running The Apache HTTP Server network.service loaded active exited LSB: Bring up/down networking quotaon.service loaded active exited Enable File System Quotas rhel-dmesg.service loaded active exited Dump dmesg to /var/log/dmesg rhel-readonly.service loaded active exited Configure read-only root support rsyslog.service loaded active running System Logging Service saslauthd.service loaded active running SASL authentication daemon. sendmail.service loaded active running Sendmail Mail Transport Agent sm-client.service loaded active running Sendmail Mail Transport Client sshd.service loaded active running OpenSSH server daemon systemd-journal-flush.service loaded active exited Flush Journal to Persistent Storage systemd-journald.service loaded active running Journal Service systemd-logind.service loaded active running Login Service systemd-random-seed.service loaded active exited Load/Save Random Seed systemd-remount-fs.service loaded active exited Remount Root and Kernel File Systems * systemd-sysctl.service loaded failed failed Apply Kernel Variables systemd-tmpfiles-setup.service loaded active exited Create Volatile Files and Directories systemd-udev-trigger.service loaded active exited udev Coldplug all Devices systemd-udevd.service loaded active running udev Kernel Device Manager systemd-update-utmp.service loaded active exited Update UTMP about System Boot/Shutdown systemd-user-sessions.service loaded active exited Permit User Sessions * systemd-vconsole-setup.service loaded failed failed Setup Virtual Console xinetd.service loaded active running Xinetd A Powerful Replacement For Inetd xinetd.service loaded active running Xinetd A Powerful Replacement For Inetd -.slice loaded active active Root Slice system-getty.slice loaded active active system-getty.slice system.slice loaded active active System Slice user-0.slice loaded active active User Slice of root user.slice loaded active active User and Session Slice dbus.socket loaded active running D-Bus System Message Bus Socket rpcbind.socket loaded active listening RPCbind Server Activation Socket systemd-initctl.socket loaded active listening /dev/initctl Compatibility Named Pipe systemd-journald.socket loaded active running Journal Socket systemd-shutdownd.socket loaded active listening Delayed Shutdown Socket systemd-udevd-control.socket loaded active running udev Control Socket systemd-udevd-kernel.socket loaded active running udev Kernel Socket 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-pre.target loaded active active Network (Pre) network.target loaded active active Network paths.target loaded active active Paths 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 systemd-tmpfiles-clean.timer loaded active waiting Daily Cleanup of Temporary Directories 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. 67 loaded units listed. Pass --all to see loaded but inactive units, too. To show all installed unit files use 'systemctl list-unit-files'.
可以看到systemctl命令显示分为五列：
UNIT：单元名称
LOAD：单元的加载状态
ACTIVE：单元的启动状态
SUB：单元的运行状态
DESCRIPTION：单元说明
3、Unit分类
unit有许多种后缀，分为不同的类型：
1）.automount：用于控制自动挂载文件系统。自动挂载即当某一目录被访问时系统自动挂载该目录，这类unit取代了传统Linux系统的autofs相应功能
2）.device：对应 /dev 目录下设备，主要用于定义设备之间的依赖关系
3）.mount：定义系统结构层次中的一个挂载点，可以替代过去的/etc/fstab配置文件
4）.path：用于监控指定目录变化，并触发其他unit运行
5）.scope：这类unit文件不是用户创建的，而是Systemd运行时自己产生的，描述一些系统服务的分组信息
6）.service：封装守护进程的启动、停止、重启和重载操作，是最常见的一种unit类型
7）.slice：用于描述cgroup的一些信息，极少使用到，一般用户就忽略它吧
8）.snapshot：这种unit其实是systemctl snapshot命令创建的一个描述Systemd unit运行状态的快照
9）.socket：监控系统或互联网中的socket消息，用于实现基于网络数据自动触发服务启动
10）.swap：定义一个用于做虚拟内存的交换分区
11）.target：用于对unit进行逻辑分组，引导其他unit的执行。它替代了SysV中运行级别的作用，并提供更灵活的基于特定设备事件的启动方式。例如multi-user.target相当于过去的运行级别5，而bluetooth.target在有蓝牙设备接入时就会被触发
12）.timer：封装由system的里面由时间触发的动作, 替代了crontab的功能
4、systemctl命令代替service、chkconfig
实现启动、停止、管理服务功能，以httpd服务为例：
1）设置服务自启动
systemctl enable httpd
2）关闭服务自启动
systemctl disable httpd
3）启动httpd服务
systemctl start httpd.service
4）查看当前httpd状态
[root@server201 ~]# systemctl status httpd.service
* httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)
Active: active (running) since Sun 2017-11-26 05:17:11 UTC; 46min ago
Docs: man:httpd(8)
man:apachectl(8)
Main PID: 602 (httpd)
Status: "Total requests: 14; Current requests/sec: 0; Current traffic: 0 B/sec"
CGroup: /system.slice/httpd.service
|-602 /usr/sbin/httpd -DFOREGROUND
|-613 /usr/sbin/httpd -DFOREGROUND
|-630 /usr/sbin/httpd -DFOREGROUND
|-648 /usr/sbin/httpd -DFOREGROUND
|-649 /usr/sbin/httpd -DFOREGROUND
`-650 /usr/sbin/httpd -DFOREGROUND
Nov 26 05:11:50 server201 systemd[1]: Stopping The Apache HTTP Server...
Nov 26 05:11:51 server201 systemd[1]: Starting The Apache HTTP Server...
Nov 26 05:11:52 server201 httpd[1494]: AH00558: httpd: Could not reliably determine the server's fully qualified domain name, using 107.181.152.135. Set the 'ServerN...his message
Nov 26 05:11:52 server201 systemd[1]: Started The Apache HTTP Server.
Nov 26 05:16:54 server201 systemd[1]: Stopping The Apache HTTP Server...
Nov 26 05:16:55 server201 systemd[1]: Stopped The Apache HTTP Server.
Nov 26 05:17:10 server201 systemd[1]: Starting The Apache HTTP Server...
Nov 26 05:17:11 server201 httpd[602]: AH00558: httpd: Could not reliably determine the server's fully qualified domain name, using 107.181.152.135. Set the 'ServerN...this message
Nov 26 05:17:11 server201 systemd[1]: Started The Apache HTTP Server.
Hint: Some lines were ellipsized, use -l to show in full.
5）查看所有服务自启动状态
systemctl list-unit-files
得到的值可以是enable、disable或static，这里的 static 它是指对应的 Unit 文件中没有定义[Install]区域，因此无法配置为开机启动服务。
参考资料：
http://blog.51cto.com/carllai/1439562
http://blog.csdn.net/smstong/article/details/39317491