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