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。
使用Systemctl管理Linux服务
本文旨在阐明在运行systemd的系统上“如何控制系统和服务”。
Systemd初体验和Systemctl基础
1. 首先检查你的系统中是否安装有systemd并确定当前安装的版本
# systemd --version
systemd 215
+PAM +AUDIT +SELINUX +IMA +SYSVINIT +LIBCRYPTSETUP +GCRYPT +ACL +XZ -SECCOMP -APPARMOR
上例中很清楚地表明，我们安装了215版本的systemd。
2. 检查systemd和systemctl的二进制文件和库文件的安装位置
# whereis systemd
systemd:/usr/lib/systemd /etc/systemd /usr/share/systemd /usr/share/man/man1/systemd.1.gz
# whereis systemctl
systemctl:/usr/bin/systemctl /usr/share/man/man1/systemctl.1.gz
3. 检查systemd是否运行
# ps -eaf | grep systemd
root 10016:27?00:00:00/usr/lib/systemd/systemd --switched-root --system --deserialize 23
root 4441016:27?00:00:00/usr/lib/systemd/systemd-journald
root 4691016:27?00:00:00/usr/lib/systemd/systemd-udevd
root 5551016:27?00:00:00/usr/lib/systemd/systemd-logind
dbus 5561016:27?00:00:00/bin/dbus-daemon --system --address=systemd:--nofork --nopidfile --systemd-activation
注意：systemd是作为父进程（PID=1）运行的。在上面带（-e）参数的ps命令输出中，选择所有进程，（-a）选择除会话前导外的所有进程，并使用（-f）参数输出完整格式列表（即 -eaf）。
也请注意上例中后随的方括号和例子中剩余部分。方括号表达式是grep的字符类表达式的一部分。
4. 分析systemd启动进程
# systemd-analyze
Startup finished in487ms(kernel)+2.776s(initrd)+20.229s(userspace)=23.493s
5. 分析启动时各个进程花费的时间
# systemd-analyze blame
8.565s mariadb.service
7.991s webmin.service
6.095s postfix.service
4.311s httpd.service
3.926s firewalld.service
3.780s kdump.service
3.238s tuned.service
1.712s network.service
1.394s lvm2-monitor.service
1.126s systemd-logind.service
....
6. 分析启动时的关键链
# systemd-analyze critical-chain
The time after the unit is active or started is printed after the "@" character.
The time the unit takes to start is printed after the "+" character.
multi-user.target @20.222s
└─mariadb.service @11.657s+8.565s
└─network.target @11.168s
└─network.service @9.456s+1.712s
└─NetworkManager.service @8.858s+596ms
└─firewalld.service @4.931s+3.926s
└─basic.target @4.916s
└─sockets.target @4.916s
└─dbus.socket @4.916s
└─sysinit.target @4.905s
└─systemd-update-utmp.service @4.864s+39ms
└─auditd.service @4.563s+301ms
└─systemd-tmpfiles-setup.service @4.485s+69ms
└─rhel-import-state.service @4.342s+142ms
└─local-fs.target @4.324s
└─boot.mount @4.286s+31ms
└─systemd-fsck@dev-disk-byx2duuid-79f594adx2da332x2d4730x2dbb5fx2d85d19608096
└─dev-disk-byx2duuid-79f594adx2da332x2d4730x2dbb5fx2d85d196080964.device@4
重要：Systemctl接受服务（.service），挂载点（.mount），套接口（.socket）和设备（.device）作为单元。
7. 列出所有可用单元
# systemctl list-unit-files
UNIT FILE STATE
proc-sys-fs-binfmt_misc.automount static
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
brandbot.path disabled
.....
8. 列出所有运行中单元
# systemctl list-units
UNIT LOAD ACTIVE SUB DESCRIPTION
proc-sys-fs-binfmt_misc.automount loaded active waiting ArbitraryExecutableFileFormatsFileSyste
sys-devices-pc...0-1:0:0:0-block-sr0.device loaded active plugged VBOX_CD-ROM
sys-devices-pc...:00:03.0-net-enp0s3.device loaded active plugged PRO/1000 MT DesktopAdapter
sys-devices-pc...00:05.0-sound-card0.device loaded active plugged 82801AA AC'97 Audio Controller
sys-devices-pc...:0:0-block-sda-sda1.device loaded active plugged VBOX_HARDDISK
sys-devices-pc...:0:0-block-sda-sda2.device loaded active plugged LVM PV Qzyo3l-qYaL-uRUa-Cjuk-pljo-qKtX-VgBQ8
sys-devices-pc...0-2:0:0:0-block-sda.device loaded active plugged VBOX_HARDDISK
sys-devices-pl...erial8250-tty-ttyS0.device loaded active plugged /sys/devices/platform/serial8250/tty/ttyS0
sys-devices-pl...erial8250-tty-ttyS1.device loaded active plugged /sys/devices/platform/serial8250/tty/ttyS1
sys-devices-pl...erial8250-tty-ttyS2.device loaded active plugged /sys/devices/platform/serial8250/tty/ttyS2
sys-devices-pl...erial8250-tty-ttyS3.device loaded active plugged /sys/devices/platform/serial8250/tty/ttyS3
sys-devices-virtual-block-dmx2d0.device loaded active plugged /sys/devices/virtual/block/dm-0
sys-devices-virtual-block-dmx2d1.device loaded active plugged /sys/devices/virtual/block/dm-1
sys-module-configfs.device loaded active plugged /sys/module/configfs
...
9. 列出所有失败单元
# systemctl --failed
UNIT LOAD ACTIVE SUB DESCRIPTION
kdump.service loaded failed failed Crash recovery kernel arming
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.
1 loaded units listed.Pass--all to see loaded but inactive units, too.
To show all installed unit files use'systemctl list-unit-files'.
10. 检查某个单元（如 cron.service）是否启用
# systemctl is-enabled crond.service
enabled
11. 检查某个单元或服务是否运行
# systemctl status firewalld.service
firewalld.service - firewalld -dynamic firewall daemon
Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled)
Active: active (running) since Tue2015-04-2816:27:55 IST;34min ago
Main PID:549(firewalld)
CGroup:/system.slice/firewalld.service
└─549/usr/bin/python -Es/usr/sbin/firewalld --nofork --nopid
Apr2816:27:51 tecmint systemd[1]:Starting firewalld -dynamic firewall daemon...
Apr2816:27:55 tecmint systemd[1]:Started firewalld -dynamic firewall daemon.
使用Systemctl控制并管理服务
12. 列出所有服务（包括启用的和禁用的）
# systemctl list-unit-files --type=service
UNIT FILE STATE
arp-ethers.service disabled
auditd.service enabled
autovt@.service disabled
blk-availability.service disabled
brandbot.service static
collectd.service disabled
console-getty.service disabled
console-shell.service disabled
cpupower.service disabled
crond.service enabled
dbus-org.Fedoraproject.FirewallD1.service enabled
....
13. 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
# systemctl start httpd.service
# systemctl restart httpd.service
# systemctl stop httpd.service
# systemctl reload httpd.service
# systemctl status httpd.service
httpd.service -TheApache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled)
Active: active (running) since Tue2015-04-2817:21:30 IST;6s ago
Process:2876ExecStop=/bin/kill -WINCH ${MAINPID}(code=exited, status=0/SUCCESS)
Main PID:2881(httpd)
Status:"Processing requests..."
CGroup:/system.slice/httpd.service
├─2881/usr/sbin/httpd -DFOREGROUND
├─2884/usr/sbin/httpd -DFOREGROUND
├─2885/usr/sbin/httpd -DFOREGROUND
├─2886/usr/sbin/httpd -DFOREGROUND
├─2887/usr/sbin/httpd -DFOREGROUND
└─2888/usr/sbin/httpd -DFOREGROUND
Apr2817:21:30 tecmint systemd[1]:StartingTheApache HTTP Server...
Apr2817:21:30 tecmint httpd[2881]: AH00558: httpd:Couldnot reliably determine the server's fully q...ssage
Apr 28 17:21:30 tecmint systemd[1]: Started The Apache HTTP Server.
Hint: Some lines were ellipsized, use -l to show in full.
注意：当我们使用systemctl的start，restart，stop和reload命令时，我们不会从终端获取到任何输出内容，只有status命令可以打印输出。
14. 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）
# systemctl is-active httpd.service
# systemctl enable httpd.service
# systemctl disable httpd.service
15. 如何屏蔽（让它不能启动）或显示服务（如 httpd.service）
# systemctl mask httpd.service
ln -s '/dev/null''/etc/systemd/system/httpd.service'
# systemctl unmask httpd.service
rm '/etc/systemd/system/httpd.service'
16. 使用systemctl命令杀死服务
# systemctl kill httpd
# systemctl status httpd
httpd.service -TheApache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled)
Active: failed (Result:exit-code) since Tue2015-04-2818:01:42 IST;28min ago
Main PID:2881(code=exited, status=0/SUCCESS)
Status:"Total requests: 0; Current requests/sec: 0; Current traffic: 0 B/sec"
Apr2817:37:29 tecmint systemd[1]: httpd.service:Got notification message from PID 2881, but recepti...bled.
Apr2817:37:29 tecmint systemd[1]: httpd.service:Got notification message from PID 2881, but recepti...bled.
Apr2817:37:39 tecmint systemd[1]: httpd.service:Got notification message from PID 2881, but recepti...bled.
Apr2817:37:39 tecmint systemd[1]: httpd.service:Got notification message from PID 2881, but recepti...bled.
Apr2817:37:49 tecmint systemd[1]: httpd.service:Got notification message from PID 2881, but recepti...bled.
Apr2817:37:49 tecmint systemd[1]: httpd.service:Got notification message from PID 2881, but recepti...bled.
Apr2817:37:59 tecmint systemd[1]: httpd.service:Got notification message from PID 2881, but recepti...bled.
Apr2817:37:59 tecmint systemd[1]: httpd.service:Got notification message from PID 2881, but recepti...bled.
Apr2818:01:42 tecmint systemd[1]: httpd.service: control process exited, code=exited status=226
Apr2818:01:42 tecmint systemd[1]:Unit httpd.service entered failed state.
Hint:Some lines were ellipsized,use-l to show in full.
使用Systemctl控制并管理挂载点
17. 列出所有系统挂载点
# 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
18. 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 stop tmp.mount
# systemctl restart tmp.mount
# systemctl reload tmp.mount
# systemctl status tmp.mount
tmp.mount -TemporaryDirectory
Loaded: loaded (/usr/lib/systemd/system/tmp.mount; disabled)
Active: active (mounted) since Tue2015-04-2817:46:06 IST;2min48s ago
Where:/tmp
What: tmpfs
Docs: man:hier(7)
http://www.freedesktop.org/wiki/Software/systemd/APIFileSystems
Process:3908ExecMount=/bin/mount tmpfs /tmp -t tmpfs -o mode=1777,strictatime (code=exited, status=0/SUCCESS)
Apr2817:46:06 tecmint systemd[1]:MountingTemporaryDirectory...
Apr2817:46:06 tecmint systemd[1]: tmp.mount:Directory/tmp to mount over isnot empty, mounting anyway.
Apr2817:46:06 tecmint systemd[1]:MountedTemporaryDirectory.
19. 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
20. 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'
使用Systemctl控制并管理套接口
21. 列出所有可用系统套接口
# systemctl list-unit-files --type=socket
UNIT FILE STATE
dbus.socket static
dm-event.socket enabled
lvm2-lvmetad.socket enabled
rsyncd.socket disabled
sshd.socket disabled
syslog.socket static
systemd-initctl.socket static
systemd-journald.socket static
systemd-shutdownd.socket static
systemd-udevd-control.socket static
systemd-udevd-kernel.socket static
11 unit files listed.
22. 在Linux中启动、重启、停止、重载套接口并检查其状态
# systemctl start cups.socket
# systemctl restart cups.socket
# systemctl stop cups.socket
# systemctl reload cups.socket
# systemctl status cups.socket
cups.socket - CUPS PrintingServiceSockets
Loaded: loaded (/usr/lib/systemd/system/cups.socket; enabled)
Active: active (listening) since Tue2015-04-2818:10:59 IST;8s ago
Listen:/var/run/cups/cups.sock (Stream)
Apr2818:10:59 tecmint systemd[1]:Starting CUPS PrintingServiceSockets.
Apr2818:10:59 tecmint systemd[1]:Listening on CUPS PrintingServiceSockets.
23. 在启动时激活套接口，并启用或禁用它（系统启动时自启动）
# systemctl is-active cups.socket
# systemctl enable cups.socket
# systemctl disable cups.socket
24. 屏蔽（使它不能启动）或显示套接口
# systemctl mask cups.socket
ln -s '/dev/null''/etc/systemd/system/cups.socket'
# systemctl unmask cups.socket
rm '/etc/systemd/system/cups.socket'
服务的CPU利用率（分配额）
25. 获取当前某个服务的CPU分配额（如httpd）
# systemctl show -p CPUShares httpd.service
CPUShares=1024
注意：各个服务的默认CPU分配份额=1024，你可以增加/减少某个进程的CPU分配份额。
26. 将某个服务（httpd.service）的CPU分配份额限制为2000 CPUShares/
# systemctl set-property httpd.service CPUShares=2000
# systemctl show -p CPUShares httpd.service
CPUShares=2000
注意：当你为某个服务设置CPUShares，会自动创建一个以服务名命名的目录（如 httpd.service），里面包含了一个名为90-CPUShares.conf的文件，该文件含有CPUShare限制信息，你可以通过以下方式查看该文件：
# vi /etc/systemd/system/httpd.service.d/90-CPUShares.conf
[Service]
CPUShares=2000
27. 检查某个服务的所有配置细节
# systemctl show httpd
Id=httpd.service
Names=httpd.service
Requires=basic.target
Wants=system.slice
WantedBy=multi-user.target
Conflicts=shutdown.target
Before=shutdown.target multi-user.target
After=network.target remote-fs.target nss-lookup.target systemd-journald.socket basic.target system.slice
Description=TheApache HTTP Server
LoadState=loaded
ActiveState=active
SubState=running
FragmentPath=/usr/lib/systemd/system/httpd.service
....
28. 分析某个服务（httpd）的关键链
# systemd-analyze critical-chain httpd.service
The time after the unit is active or started is printed after the "@" character.
The time the unit takes to start is printed after the "+" character.
httpd.service +142ms
└─network.target @11.168s
└─network.service @9.456s+1.712s
└─NetworkManager.service @8.858s+596ms
└─firewalld.service @4.931s+3.926s
└─basic.target @4.916s
└─sockets.target @4.916s
└─dbus.socket @4.916s
└─sysinit.target @4.905s
└─systemd-update-utmp.service @4.864s+39ms
└─auditd.service @4.563s+301ms
└─systemd-tmpfiles-setup.service @4.485s+69ms
└─rhel-import-state.service @4.342s+142ms
└─local-fs.target @4.324s
└─boot.mount @4.286s+31ms
└─systemd-fsck@dev-disk-byx2duuid-79f594adx2da332x2d4730x2dbb5fx2d85d196080964.service@4.092s+149ms
└─dev-disk-byx2duuid-79f594adx2da332x2d4730x2dbb5fx2d85d196080964.device@4.092s
29. 获取某个服务（httpd）的依赖性列表
# systemctl list-dependencies httpd.service
httpd.service
├─system.slice
└─basic.target
├─firewalld.service
├─microcode.service
├─rhel-autorelabel-mark.service
├─rhel-autorelabel.service
├─rhel-configure.service
├─rhel-dmesg.service
├─rhel-loadmodules.service
├─paths.target
├─slices.target
│├─-.slice
│└─system.slice
├─sockets.target
│├─dbus.socket
....
30. 按等级列出控制组
# systemd-cgls
├─1/usr/lib/systemd/systemd --switched-root --system --deserialize 23
├─user.slice
│└─user-0.slice
│└─session-1.scope
│├─2498 sshd: root@pts/0
│├─2500-bash
│├─4521 systemd-cgls
│└─4522 systemd-cgls
└─system.slice
├─httpd.service
│├─4440/usr/sbin/httpd -DFOREGROUND
│├─4442/usr/sbin/httpd -DFOREGROUND
│├─4443/usr/sbin/httpd -DFOREGROUND
│├─4444/usr/sbin/httpd -DFOREGROUND
│├─4445/usr/sbin/httpd -DFOREGROUND
│└─4446/usr/sbin/httpd -DFOREGROUND
├─polkit.service
│└─721/usr/lib/polkit-1/polkitd --no-debug
....
31. 按CPU、内存、输入和输出列出控制组
# systemd-cgtop
PathTasks%CPU MemoryInput/s Output/s
/831.0437.8M--
/system.slice -0.1---
/system.slice/mariadb.service 20.1---
/system.slice/tuned.service 10.0---
/system.slice/httpd.service 60.0---
/system.slice/NetworkManager.service 1----
/system.slice/atop.service 1----
/system.slice/atopacct.service 1----
/system.slice/auditd.service 1----
/system.slice/crond.service 1----
/system.slice/dbus.service 1----
/system.slice/firewalld.service 1----
/system.slice/lvm2-lvmetad.service 1----
/system.slice/polkit.service 1----
/system.slice/postfix.service 3----
/system.slice/rsyslog.service 1----
/system.slice/system-getty.slice/getty@tty1.service 1----
/system.slice/systemd-journald.service 1----
/system.slice/systemd-logind.service 1----
/system.slice/systemd-udevd.service 1----
/system.slice/webmin.service 1----
/user.slice/user-0.slice/session-1.scope3----
控制系统运行等级
32. 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!
33. 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.
34. 列出当前使用的运行等级
# systemctl get-default
multi-user.target
35. 启动运行等级5，即图形模式
# systemctl isolate runlevel5.target
或
# systemctl isolate graphical.target
36. 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
36. 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
37. 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
到此为止吧。保持连线，进行评论。别忘了在下面的评论中为我们提供一些有价值的反馈哦。喜欢我们、与我们分享，求扩散。
教大家几个基本systemctl的用法：http://www.linuxdiyf.com/linux/11642.html
CentOS 7上systemctl的用法：http://www.linuxdiyf.com/linux/3800.html
在CentOS 7上利用systemctl添加自定义系统服务：http://www.linuxdiyf.com/linux/1075.html