如何控制服务的运行状态？如何切换不同的运行级别？

服务控制

ntsysv

• 仿图形交互界面，集中配置各种服务启动状态

--level 35：同时对指定运行级别中的服务进行管理，不加仅管理当前运行级别中的服务。

[root@localhost ~]# ntsysv --level 35

systemctl

• systemctl {选项} name.servive

start：启动
stop：停止

[root@localhost ~]# systemctl start httpd.service
[root@localhost ~]# systemctl stop httpd.service

restart：重启，先停止，再启动
reload：重新加载配置文件，不会完全停止和启动服务

[root@localhost ~]# systemctl restart httpd.service
[root@localhost ~]# systemctl reload httpd.service

status：状态
is-active：是否启动成功
is-failed：是否启动失败

[root@localhost ~]# systemctl status httpd.service
● httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
   Active: active (running) since Tue 2019-08-27 17:31:39 CST; 40s ago
     Docs: man:httpd(8)
           man:apachectl(8)
  Process: 8833 ExecReload=/usr/sbin/httpd $OPTIONS -k graceful (code=exited, status=0/SUCCESS)
 Main PID: 8821 (httpd)
   Status: "Total requests: 0; Current requests/sec: 0; Current traffic:   0 B/sec"
   CGroup: /system.slice/httpd.service
           ├─8821 /usr/sbin/httpd -DFOREGROUND
           ├─8835 /usr/sbin/httpd -DFOREGROUND
           ├─8836 /usr/sbin/httpd -DFOREGROUND
           ├─8837 /usr/sbin/httpd -DFOREGROUND
           ├─8838 /usr/sbin/httpd -DFOREGROUND
           └─8839 /usr/sbin/httpd -DFOREGROUND

Aug 27 17:31:39 localhost.localdomain systemd[1]: Starting The Apache HTTP Server...
Aug 27 17:31:39 localhost.localdomain httpd[8821]: AH00558: httpd: Could not reliably determine the server's fully qualified domain name, using localhost.l...s message
Aug 27 17:31:39 localhost.localdomain systemd[1]: Started The Apache HTTP Server.
Aug 27 17:31:45 localhost.localdomain httpd[8833]: AH00558: httpd: Could not reliably determine the server's fully qualified domain name, using localhost.l...s message
Aug 27 17:31:45 localhost.localdomain systemd[1]: Reloaded The Apache HTTP Server.
Hint: Some lines were ellipsized, use -l to show in full.

[root@localhost ~]# systemctl is-active httpd.service
active
[root@localhost ~]# systemctl is-failed httpd.service
active

enable：开机自启
disable：关闭开机自启
is-enabled：是否开机自启

[root@localhost ~]# systemctl enable httpd.service
Created symlink from /etc/systemd/system/multi-user.target.wants/httpd.service to /usr/lib/systemd/system/httpd.service.
[root@localhost ~]# systemctl disable httpd.service
Removed symlink /etc/systemd/system/multi-user.target.wants/httpd.service.
[root@localhost ~]# systemctl is-enabled httpd.service
disabled

list-dependencies：查看依赖关系

[root@localhost ~]# systemctl list-dependencies httpd.service
httpd.service
● ├─-.mount
● ├─system.slice
● └─basic.target
●   ├─microcode.service
●   ├─rhel-autorelabel-mark.service
●   ├─rhel-autorelabel.service
●   ├─rhel-configure.service
●   ├─rhel-dmesg.service
●   ├─rhel-loadmodules.service
●   ├─selinux-policy-migrate-local-changes@targeted.service
●   ├─paths.target
●   ├─slices.target
●   │ ├─-.slice
●   │ └─system.slice
●   ├─sockets.target
●   │ ├─dbus.socket
●   │ ├─dm-event.socket
●   │ ├─systemd-initctl.socket
●   │ ├─systemd-journald.socket
●   │ ├─systemd-shutdownd.socket
●   │ ├─systemd-udevd-control.socket
●   │ └─systemd-udevd-kernel.socket
●   ├─sysinit.target
●   │ ├─dev-hugepages.mount
●   │ ├─dev-mqueue.mount
●   │ ├─kmod-static-nodes.service
●   │ ├─lvm2-lvmetad.socket
●   │ ├─lvm2-lvmpolld.socket
●   │ ├─lvm2-monitor.service
●   │ ├─plymouth-read-write.service
●   │ ├─plymouth-start.service
●   │ ├─proc-sys-fs-binfmt_misc.automount
●   │ ├─sys-fs-fuse-connections.mount
●   │ ├─sys-kernel-config.mount
●   │ ├─sys-kernel-debug.mount
●   │ ├─systemd-ask-password-console.path
●   │ ├─systemd-binfmt.service
●   │ ├─systemd-firstboot.service
●   │ ├─systemd-hwdb-update.service
●   │ ├─systemd-journal-catalog-update.service
●   │ ├─systemd-journal-flush.service
●   │ ├─systemd-journald.service
●   │ ├─systemd-machine-id-commit.service
●   │ ├─systemd-modules-load.service
●   │ ├─systemd-random-seed.service
●   │ ├─systemd-sysctl.service
●   │ ├─systemd-tmpfiles-setup-dev.service
●   │ ├─systemd-tmpfiles-setup.service
●   │ ├─systemd-udev-trigger.service
●   │ ├─systemd-udevd.service
●   │ ├─systemd-update-done.service
●   │ ├─systemd-update-utmp.service
●   │ ├─systemd-vconsole-setup.service
●   │ ├─cryptsetup.target
●   │ ├─local-fs.target
●   │ │ ├─-.mount
●   │ │ ├─boot.mount
●   │ │ ├─rhel-import-state.service
●   │ │ ├─rhel-readonly.service
●   │ │ └─systemd-remount-fs.service
●   │ └─swap.target
●   │   └─dev-mapper-centosx2dswap.swap
●   └─timers.target
●     └─systemd-tmpfiles-clean.timer

mask：屏蔽服务
unmak：取消屏蔽

• 主要使用于相互冲突的服务，屏蔽后，不会被其他的systemd单元启动，也无法被手动启动。例如iptables和firewalld就是互相冲突的。

[root@localhost ~]# systemctl mask firewalld.service
Created symlink from /etc/systemd/system/firewalld.service to /dev/null.
[root@localhost ~]# systemctl unmask firewalld.service
Removed symlink /etc/systemd/system/firewalld.service.

运行级别

runlevel

• 显示系统的运行级别

[root@localhost ~]# runlevel
N 5

切换多用户模式

两种方法：

[root@localhost ~]# init 3

[root@localhost ~]# systemctl isolate multi-user.target

切换图形化界面

两种方法：

[root@localhost ~]# init 5

[root@localhost ~]# systemctl isolate graphical.target

开机默认 target

1.查看默认启动target

[root@localhost ~]# systemctl get-default
graphical.target

2.切换默认启动target，两种方法：

[root@localhost ~]# systemctl set-default multi-user.target
Removed symlink /etc/systemd/system/default.target.
Created symlink from /etc/systemd/system/default.target to /usr/lib/systemd/system/multi-user.target.

[root@localhost ~]# ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target

3.查看默认启动target

[root@localhost ~]# systemctl get-default
multi-user.target