传统的Linux系统启动过程主要由著名的init进程（也被称为SysV init启动系统）处理，而基于init的启动系统被认为有效率不足的问题，systemd是Linux系统机器的另一种启动方式，宣称弥补了以传统Linux SysV init为基础的系统的缺点。
1、列出正在运行的服务
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[root@localhost ~]
# systemctl
UNIT LOAD ACTIVE SUB DESCRIPTION
proc-sys-fs-binfmt_misc.automount loaded active waiting Arbitrary Executable File Formats File System Automount Point
sys-devices-pci0000:00-0000:00:07.1-ata2-host1-target1:0:0-1:0:0:0-block-sr0.device loaded active plugged VMware_Virtual_IDE_CDROM_Drive CentOS_7_x86_64
sys-devices-pci0000:00-0000:00:10.0-host2-target2:0:0-2:0:0:0-block-sda-sda1.device loaded active plugged VMware_Virtual_S 1
sys-devices-pci0000:00-0000:00:10.0-host2-target2:0:0-2:0:0:0-block-sda-sda2.device loaded active plugged LVM PV CSeHGs-hdrA-6Y0N-jXVM-VA3R-3sSe-BraSga on
/dev/sda2
2
sys-devices-pci0000:00-0000:00:10.0-host2-target2:0:0-2:0:0:0-block-sda.device loaded active plugged VMware_Virtual_S
sys-devices-pci0000:00-0000:00:10.0-host2-target2:0:1-2:0:1:0-block-sdb.device loaded active plugged VMware_Virtual_S
sys-devices-pci0000:00-0000:00:11.0-0000:02:01.0-net-ens33.device loaded active plugged 82545EM Gigabit Ethernet Controller (Copper) (PRO
/1000
MT Single Port Adapter)
sys-devices-pci0000:00-0000:00:11.0-0000:02:02.0-sound-card0.device loaded active plugged ES1371
/ES1373
/ Creative Labs CT2518 (Audio PCI 64V
/128/5200
/ Creative CT4810/
sys-devices-platform-serial8250-
tty
-ttyS1.device loaded active plugged
/sys/devices/platform/serial8250/tty/ttyS1
sys-devices-platform-serial8250-
tty
-ttyS2.device loaded active plugged
/sys/devices/platform/serial8250/tty/ttyS2
sys-devices-platform-serial8250-
tty
-ttyS3.device loaded active plugged
/sys/devices/platform/serial8250/tty/ttyS3
sys-devices-pnp0-00:05-
tty
-ttyS0.device loaded active plugged
/sys/devices/pnp0/00
:05
/tty/ttyS0
sys-devices-virtual-block-dm\x2d0.device loaded active plugged
/sys/devices/virtual/block/dm-0
sys-devices-virtual-block-dm\x2d1.device loaded active plugged
/sys/devices/virtual/block/dm-1
sys-module-configfs.device loaded active plugged
/sys/module/configfs
sys-subsystem-net-devices-ens33.device loaded active plugged 82545EM Gigabit Ethernet Controller (Copper) (PRO
/1000
MT Single Port Adapter)
-.
mount
loaded active mounted /
boot.
mount
loaded active mounted
/boot
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2、树状结构显示正在运行的服务
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[root@localhost ~]
# systemd-cgls
├─1
/usr/lib/systemd/systemd
--switched-root --system --deserialize 21
├─user.slice
│ └─user-0.slice
│ └─session-1.scope
│ ├─1649 sshd: root@pts
/0
│ ├─1967 -
bash
│ ├─2860 systemd-cgls
│ └─2861 systemd-cgls
└─system.slice
├─rsyslog.service
│ └─1554
/usr/sbin/rsyslogd
-n
├─umail_postgresql.service
│ ├─1589
/usr/local/u-mail/service/pgsql-9
.4
/bin/postgres
-D
/usr/local/u-mail/service/pgsql-9
.4
/data/data
│ ├─1672 postgres: logger process
│ ├─1719 postgres: checkpointer process
│ ├─1720 postgres: writer process
│ ├─1721 postgres: wal writer process
│ ├─1724 postgres: autovacuum launcher process
│ └─1726 postgres: stats collector process
├─tuned.service
│ └─1546
/usr/bin/python
-Es
/usr/sbin/tuned
-l -P
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3、systemctl常见操作命令
启动服务：systemctl start postfix.service
关闭服务：systemctl stop postfix.service
重启服务：systemctl restart postfix.service
查看服务状态：systemctl status postfix.service
开机启动服务：systemctl enable postfix.service
开机禁用服务：systemctl disable postfix.service
确认开机是否启动服务：systemctl is-enabled postfix.service
查看已启动的服务：systemctl list-unit-files|grep enabled
查看启动失败的服务：systemctl --failed
使用命令 systemctl is-enabled postfix.service返回的结果有enable、disable或static。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。
本文转自 sailikung 51CTO博客，原文链接：http://blog.51cto.com/net881004/2054850，如需转载请自行联系原作者
