背景: centos7 和 6的重大改变
对于系统管理员而言，系统的服务管理是一件很日常和很重要的工作，而7在6的基础上有了很大的改变，就连命令都完全不一样了。所以要拥抱变化，学习7是如何进行服务的管理和控制的。
system和systemctl的初探
Systemctl是一个systemd工具，主要负责控制systemd系统和服务管理器。
Systemd是一个系统管理守护进程、工具和库的集合，用于取代System V初始进程。Systemd的功能是用于集中管理和配置类UNIX系统。
在Linux生态系统中，Systemd被部署到了大多数的标准Linux发行版中，只有为数不多的几个发行版尚未部署。Systemd通常是所有其它守护进程的父进程，但并非总是如此。
【第一步】
查看systemd的版本
\# systemctl --version
systemd 219
+PAM +AUDIT +SELINUX +IMA -APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ -LZ4 -SECCOMP +BLKID +ELFUTILS +KMOD +IDN
查看进程是否存在
ps -ef | grep [s]ystemd
注意:systemd是作为父进程(pid=1)运行的
分析systemd的启动进程
\# systemd-analyze
Startup finished in 639ms (kernel) + 931ms (initrd) + 12.432s (userspace) = 14.003s
分析各个进程启动发费的时间
\#systemd-analyze blame
分析启动时候的关键链
systemd-analyze critical-chain
重要：Systemctl接受服务（.service），挂载点（.mount），套接口（.socket）和设备（.device）作为单元。
列出所有服务可用单元
systemctl list-unit-files
(200多个)
列出所有运行中的单元
systemctl list-units
列出失败的单元？
systemctl --failed
列出某个单元是否启动
\# systemctl is-enabled crond.service
enabled
或者
\# systemctl is-enabled crond
enabled
检查某个单元或服务是否运行
\# systemctl is-active crond
active
或者
\# systemctl status crond
这个信息更详细
############################# 控制服务 ############################
列出所有服务（包括启用的和禁用的）
systemctl list-unit-files --type=service
(120+)
以httpd为例
yum install httpd
会生成以下文件
/usr/lib/systemd/system/httpd.service
Linux中如何启动、重启、停止、重载服务以及检查服务（如 httpd.service）状态
[root@Centos7-node2 ~]# systemctl status httpd
● httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
Active: inactive (dead)
Docs: man:httpd(8)
man:apachectl(8)
[root@Centos7-node2 ~]# systemctl start httpd
[root@Centos7-node2 ~]# systemctl status httpd
● httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
Active: active (running) since Sun 2016-10-23 00:48:43 CST; 3s ago
Docs: man:httpd(8)
man:apachectl(8)
Main PID: 26811 (httpd)
Status: "Processing requests..."
CGroup: /system.slice/httpd.service
├─26811 /usr/sbin/httpd -DFOREGROUND
├─26812 /usr/sbin/httpd -DFOREGROUND
├─26813 /usr/sbin/httpd -DFOREGROUND
├─26814 /usr/sbin/httpd -DFOREGROUND
├─26815 /usr/sbin/httpd -DFOREGROUND
└─26816 /usr/sbin/httpd -DFOREGROUND
Oct 23 00:48:17 Centos7-node2 systemd[1]: Starting The Apache HTTP Server...
Oct 23 00:48:33 Centos7-node2 httpd[26811]: AH00558: httpd: Could not reliably determine the server's fully qua...ssage
Oct 23 00:48:43 Centos7-node2 systemd[1]: Started The Apache HTTP Server.
Hint: Some lines were ellipsized, use -l to show in full.
[root@Centos7-node2 ~]# systemctl reload httpd
[root@Centos7-node2 ~]# systemctl stop httpd
注意:
Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
在loader中第一个disabled表示系统启动是否自启动，为非自动。 第二个表示工作状态
Active: active (running) since Sun 2016-10-23 00:48:43 CST; 3s ago
Active 绿色表示服务运行正常
如何激活服务并在启动时启用或禁用服务（即系统启动时自动启动服务）
[root@Centos7-node2 ~]# systemctl enable httpd
Created symlink from /etc/systemd/system/multi-user.target.wants/httpd.service to /usr/lib/systemd/system/httpd.service.
禁用
systemctl disabled httpd
[root@Centos7-node2 ~]# systemctl disable httpd
Removed symlink /etc/systemd/system/multi-user.target.wants/httpd.service.
使用systemctl命令杀死服务
[root@Centos7-node2 ~]# systemctl kill httpd
[root@Centos7-node2 ~]# systemctl status httpd
● httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
Active: failed (Result: exit-code) since Sun 2016-10-23 00:56:59 CST; 1s ago
Docs: man:httpd(8)
man:apachectl(8)
Process: 26901 ExecStop=/bin/kill -WINCH ${MAINPID} (code=exited, status=1/FAILURE)
Main PID: 26869 (code=exited, status=0/SUCCESS)
Status: "Total requests: 0; Current requests/sec: 0; Current traffic: 0 B/sec"
Oct 23 00:51:38 Centos7-node2 systemd[1]: Starting The Apache HTTP Server...
Oct 23 00:51:53 Centos7-node2 httpd[26869]: AH00557: httpd: apr_sockaddr_info_get() failed for Centos7-node2
Oct 23 00:51:53 Centos7-node2 httpd[26869]: AH00558: httpd: Could not reliably determine the server's fully qua...ssage
Oct 23 00:52:03 Centos7-node2 systemd[1]: Started The Apache HTTP Server.
Oct 23 00:56:59 Centos7-node2 kill[26901]: kill: cannot find process ""
Oct 23 00:56:59 Centos7-node2 systemd[1]: httpd.service: control process exited, code=exited status=1
Oct 23 00:56:59 Centos7-node2 systemd[1]: Unit httpd.service entered failed state.
Oct 23 00:56:59 Centos7-node2 systemd[1]: httpd.service failed.
Hint: Some lines were ellipsized, use -l to show in full.
注意:Active: failed (Result: exit-code) since Sun 2016-10-23 00:56:59 CST; 1s ago
这个的failed表示的是kill掉的？
\########################## 使用Systemctl控制并管理挂载点 #################3
[root@Centos7-node2 ~]# systemctl list-unit-files --type=mount
UNIT FILE STATE
dev-hugepages.mount static
dev-mqueue.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
挂载、卸载、重新挂载、重载系统挂载点并检查系统中挂载点状态
systemctl start tmp.mount
systemctl status tmp.mount
systemctl reload tmp.mount
在启动时激活、启用或禁用挂载点（系统启动时自动挂载）
\# systemctl is-active tmp.mount
\# systemctl enable tmp.mount
\# systemctl disable tmp.mount
在Linux中屏蔽（让它不能启用）或可见挂载点
\# systemctl mask tmp.mount
ln -s '/dev/null''/etc/systemd/system/tmp.mount'
\# systemctl unmask tmp.mount
rm '/etc/systemd/system/tmp.mount'
#################### 控制系统运行等级 ##################
启动系统救援模式
\# systemctl rescue
Broadcast message from root@tecmint on pts/0(Wed2015-04-2911:31:18 IST):
The system is going down to rescue mode NOW!
进入紧急模式
\# systemctl emergency
Welcome to emergency mode!After logging in, type "journalctl -xb" to view
system logs,"systemctl reboot" to reboot,"systemctl default" to try again
to boot intodefault mode.
列出当前使用的运行等级
注意:init 1也是可以使用的
\# systemctl get-default
multi-user.target
注意:who -r 也是可以查看的
启动运行等级5，即图形模式
\# systemctl isolate runlevel5.target
或
\# systemctl isolate graphical.target
启动运行等级3，即多用户模式（命令行）
\# systemctl isolate runlevel3.target
或
\# systemctl isolate multiuser.target
设置多用户模式或图形模式为默认运行等级
\# systemctl set-default runlevel3.target
\# systemctl set-default runlevel5.target
重启、停止、挂起、休眠系统或使系统进入混合睡眠
\# systemctl reboot
\# systemctl halt
\# systemctl suspend
\# systemctl hibernate
\# systemctl hybrid-sleep
对于不知运行等级为何物的人，说明如下。
Runlevel 0 : 关闭系统
Runlevel 1 : 救援？维护模式
Runlevel 3 : 多用户，无图形系统
Runlevel 4 : 多用户，无图形系统
Runlevel 5 : 多用户，图形化系统
Runlevel 6 : 关闭并重启机器
*注意:在centos7 中仍然可以使用init 0 关机 init 6 启动。