在Centos 中 systemctl 是设置系统服务的命令，即 service , 它融合之前service和chkconfig的功能于一体。
可以使用它永久性或只在当前会话中启用/禁用服务。
CentOS 7 的 /etc/rc.d/rc.local 是没有执行权限的， 系统建议创建 systemd service 启动服务。
1.查看服务列表状态:
systemctl list-units --type=service ubuntu@ip-172-31-22-75:~$ systemctl list-units --type=service
UNIT LOAD ACTIVE SUB DESCRIPTION
accounts-daemon.service loaded active running Accounts Service
apparmor.service loaded active exited AppArmor initialization
apport.service loaded active exited LSB: automatic crash repor
atd.service loaded active running Deferred execution schedul
avahi-daemon.service loaded active running Avahi mDNS/DNS-SD Stack
blk-availability.service loaded active exited Availability of block devi
cloud-config.service loaded active exited Apply the settings specifi
cloud-final.service loaded active exited Execute cloud user/final s
cloud-init-local.service loaded active exited Initial cloud-init job (pr
cloud-init.service loaded active exited Initial cloud-init job (me
console-setup.service loaded active exited Set console font and keyma
cron.service loaded active running Regular background program
dbus.service loaded active running D-Bus System Message Bus
...
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.
57 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'.
systemctl list-unit-files
#列出所有已经安装的 服务及状态 （可为人所读, 内容简略、清晰）：ubuntu@ip-172-31-22-75:~$
systemctl list-unit-files
UNIT FILE STATE
proc-sys-fs-binfmt_misc.automount static
-.mount generated
dev-hugepages.mount static
dev-mqueue.mount static
proc-sys-fs-binfmt_misc.mount static
snap-amazon\x2dssm\x2dagent-1455.mount enabled
snap-amazon\x2dssm\x2dagent-1480.mount enabled
snap-core-7713.mount disabled
snap-core-7917.mount disabled
sys-fs-fuse-connections.mount static
sys-kernel-config.mount static
sys-kernel-debug.mount static
acpid.path enabled
apport-autoreport.path enabled
systemd-ask-password-console.path static
systemd-ask-password-plymouth.path static
systemd-ask-password-wall.path static
...
systemctl 可以列出正在运行的服务状态，如图：(基本不为人所读， 内容复杂、全面)
ubuntu@ip-172-31-22-75:~$ systemctl
UNIT LOAD ACTIVE SUB DESCRIPTION
proc-sys-fs-binfmt_misc.automount loaded active waiting Arbitrary Executable
sys-devices-platform-serial8250-tty-ttyS1.device loaded active plugged /sys/
sys-devices-platform-serial8250-tty-ttyS2.device loaded active plugged /sys/
sys-devices-platform-serial8250-tty-ttyS3.device loaded active plugged /sys/
sys-devices-pnp0-00:06-tty-ttyS0.device loaded active plugged /sys/devices/p
sys-devices-vbd\x2d768-block-xvda-xvda1.device loaded active plugged /sys/de
sys-devices-vbd\x2d768-block-xvda.device loaded active plugged /sys/devices/
sys-devices-vif\x2d0-net-eth0.device loaded active plugged /sys/devices/vif-
sys-devices-virtual-block-loop0.device loaded active plugged /sys/devices/vi
sys-devices-virtual-block-loop1.device loaded active plugged /sys/devices/vi
sys-devices-virtual-block-loop2.device loaded active plugged /sys/devices/vi
sys-devices-virtual-block-loop3.device loaded active plugged /sys/devices/vi
sys-devices-virtual-misc-rfkill.device loaded active plugged /sys/devices/vi
sys-devices-virtual-tty-ttyprintk.device loaded active plugged /sys/devices/
sys-module-configfs.device loaded active plugged /sys/module/configfs
sys-module-fuse.device loaded active plugged /sys/module/fuse
sys-subsystem-net-devices-eth0.device loaded active plugged /sys/subsystem/n
...
systemd-cgls 以树形列出正在运行的进程，它可以递归显示控制组内容。如图：
ubuntu@ip-172-31-22-75:~$ systemd-cgls
Control group /:
-.slice
├─user.slice
│ └─user-1000.slice
│ ├─session-175.scope
│ │ ├─15717 sshd: ubuntu [priv]
│ │ ├─15798 sshd: ubuntu@notty
│ │ └─15799 -bash
│ ├─user@1000.service
│ │ └─init.scope
│ │ ├─1455 /lib/systemd/systemd --user
│ │ └─1456 (sd-pam)
│ ├─session-199.scope
│ │ ├─21325 sshd: ubuntu [priv]
│ │ ├─21408 sshd: ubuntu@pts/1
│ │ ├─21409 -bash
│ │ ├─21614 systemd-cgls
│ │ └─21615 systemd-cgls
│ ├─session-196.scope
│ │ ├─20647 sshd: ubuntu [priv]
│ │ ├─20728 sshd: ubuntu@notty
│ │ └─20729 -bash
│ ├─session-183.scope
lines 1-23
2.如何启动/关闭、启用/禁用服务？
启动一个服务：
systemctl start postfix.service
关闭一个服务：
systemctl stop postfix.service重启一个服务：
systemctl restart postfix.service显示一个服务的状态：
systemctl status postfix.service在开机时启用一个服务：systemctl enable postfix.service
在开机时禁用一个服务：systemctl disable postfix.service
查看服务是否开机启动： systemctl is-enabled postfix.service
查看已启动的服务列表： systemctl list-unit-files | grep enabled
查看启动失败的服务列表： systemctl --failed
PS：使用命令 systemctl is-enabled postfix.service 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。
说明：启用服务就是在当前
runlevel的配置文件目录/etc/systemd/system/multi-user.target.wants里，建立/usr/lib/systemd/system里面对应服务配置文件的软链接；
禁用服务就是删除此软链接，添加服务就是添加软连接。
ubuntu@ip-172-31-22-75:~$ cd /etc/systemd/system/multi-user.target.wants/
ubuntu@ip-172-31-22-75:/etc/systemd/system/multi-user.target.wants$ ls
ModemManager.service ondemand.service
atd.service open-vm-tools.service
avahi-daemon.service php7.2-fpm.service
console-setup.service pollinate.service
containerd.service remote-fs.target
cron.service rsync.service
docker.service rsyslog.service
ebtables.service 'snap-amazon\x2dssm\x2dagent-1455.mount'
irqbalance.service 'snap-amazon\x2dssm\x2dagent-1480.mount'
lxcfs.service snap.amazon-ssm-agent.amazon-ssm-agent.service
lxd-containers.service ssh.service
mindocd.service systemd-networkd.service
mysql.service systemd-resolved.service
networkd-dispatcher.service ufw.service
nginx.service unattended-upgrades.service
nodebb.service wpa_supplicant.service
ubuntu@ip-172-31-22-75:/etc/systemd/system/multi-user.target.wants$ ll
total 12
drwxr-xr-x 2 root root 4096 Nov 7 16:35 ./
drwxr-xr-x 13 root root 4096 Nov 7 16:34 ../
lrwxrwxrwx 1 root root 40 Feb 19 2019 ModemManager.service -> /lib/systemd/system/ModemManager.service
lrwxrwxrwx 1 root root 31 Sep 12 2018 atd.service -> /lib/systemd/system/atd.service
lrwxrwxrwx 1 root root 40 Feb 19 2019 avahi-daemon.service -> /lib/systemd/system/avahi-daemon.service
lrwxrwxrwx 1 root root 41 Sep 12 2018 console-setup.service -> /lib/systemd/system/console-setup.service
lrwxrwxrwx 1 root root 38 Feb 19 2019 containerd.service -> /lib/systemd/system/containerd.service
lrwxrwxrwx 1 root root 32 Sep 12 2018 cron.service -> /lib/systemd/system/cron.service
lrwxrwxrwx 1 root root 34 Feb 19 2019 docker.service -> /lib/systemd/system/docker.service
lrwxrwxrwx 1 root root 36 Sep 12 2018 ebtables.service -> /lib/systemd/system/ebtables.service
lrwxrwxrwx 1 root root 38 Sep 12 2018 irqbalance.service -> /lib/systemd/system/irqbalance.service
lrwxrwxrwx 1 root root 33 Sep 12 2018 lxcfs.service -> /lib/systemd/system/lxcfs.service
lrwxrwxrwx 1 root root 42 Sep 12 2018 lxd-containers.service -> /lib/systemd/system/lxd-containers.service文档更新时间: 2019-11-07 17:07 作者：cklx
