#1. systemctl 介紹

Systemctl是一個systemd工具，主要負責控制systemd系統和服務管理器。

Systemd是一個系統管理守護程式、工具和庫的集合，用於取代System V初始程式。Systemd的功能是用於集中管理和配置類UNIX系統。

在Linux生態系統中，Systemd被部署到了大多數的標準Linux發行版中，只有為數不多的幾個發行版尚未部署。Systemd通常是所有其它守護程式的父程式，但並非總是如此。

# 2. `systemctl` 基本功能：

1. 分析systemd啟動過程

$ systemd-analyze
		Startup finished in 767ms (kernel) + 6.540s (initrd) + 20.174s (userspace) = 27.483s
	
2. 分析每個程式在引導時花費的時間

$ systemd-analyze
		Startup finished in 767ms (kernel) + 6.540s (initrd) + 20.174s (userspace) = 27.483s
		systemd-analyze blame
		7.301s tuned.service
		5.075s systemd-vconsole-setup.service
		4.984s network.service
		3.829s etcd.service
		3.806s postfix.service
		1.512s docker.service
		....
	
3. 分析啟動時的關鍵鏈

$ 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 @20.163s
		└─tuned.service @7.413s +7.301s
		  └─network.target @7.395s
		    └─network.service @2.410s +4.984s
		      └─basic.target @2.359s
		        └─sockets.target @2.359s
		          └─dbus.socket @2.357s
		            └─sysinit.target @2.357s
		              └─systemd-update-utmp.service @2.343s +13ms
		                └─auditd.service @2.145s +196ms
		                  └─systemd-tmpfiles-setup.service @2.138s +5ms
		                    └─rhel-import-state.service @2.103s +33ms
		                      └─local-fs.target @2.102s
		                        └─boot.mount @1.836s +266ms
		                          └─local-fs-pre.target @1.835s
		                            └─lvm2-monitor.service @849ms +985ms
		                              └─lvm2-lvmetad.service @906ms
		                                └─lvm2-lvmetad.socket @831ms
		                                  └─-.slice

> 重要：Systemctl接受服務（.service），掛載點（.mount），套接字（.socket）和裝置（.device）作為單位。

4. 列出所有可用的單位

$ 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  
		...
	
5. 列出所有執行單元

$ systemctl list-units
		UNIT                                        LOAD   ACTIVE SUB       DESCRIPTION
		proc-sys-fs-binfmt_misc.automount           loaded active waiting   Arbitrary Executable File Formats File Syste
		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 Desktop Adapter
		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-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
		...

6. 列出所有失敗的單元

$ 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'.

7. 檢查單元(cron.service)是否啟用?

$ systemctl is-enabled crond.service
		enabled

8. 檢查單元或服務是否正在執行？

$ systemctl status firewalld.service
		firewalld.service - firewalld - dynamic firewall daemon
		Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled)
		Active: active (running) since Tue 2018-04-28 16:27:55 IST; 34min ago
		Main PID: 549 (firewalld)
		CGroup: /system.slice/firewalld.service
		└─549 /usr/bin/python -Es /usr/sbin/firewalld --nofork --nopid
		Apr 28 16:27:51 tecmint systemd[1]: Starting firewalld - dynamic firewall daemon...
		Apr 28 16:27:55 tecmint systemd[1]: Started firewalld - dynamic firewall daemon.

9. 列出所有服務（包括啟用和禁用）

$ 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 
		....

10. 如何在Linux中啟動，重新啟動，停止，重新載入和檢查服務（httpd.service）的狀態

# systemctl start httpd.service
		# systemctl restart httpd.service
		# systemctl stop httpd.service
		# systemctl reload httpd.service
		# systemctl status httpd.service
		httpd.service - The Apache HTTP Server
		Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled)
		Active: active (running) since Tue 2018-04-28 17:21:30 IST; 6s ago
		Process: 2876 ExecStop=/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
		Apr 28 17:21:30 tecmint systemd[1]: Starting The Apache HTTP Server...
		Apr 28 17:21:30 tecmint httpd[2881]: AH00558: httpd: Could not 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：當我們使用systemctl等啟動，重啟，停止和過載等命令時，我們將不會在終端上獲得任何輸出，只有status命令會列印輸出。

11. 如何在引導時啟用服務並啟用或禁用服務（系統引導時自動啟動服務）

# systemctl is-active httpd.service
		# systemctl enable httpd.service
		# systemctl disable httpd.service

12. 如何遮蔽（使其無法啟動）或取消遮蔽服務（httpd.service)
		
		ln -s '/dev/null' '/etc/systemd/system/httpd.service'
		# systemctl unmask httpd.service
		rm '/etc/systemd/system/httpd.service'

13. 如何使用systemctl命令終止服務

# systemctl kill httpd
		# systemctl status httpd
		httpd.service - The Apache HTTP Server
		Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled)
		Active: failed (Result: exit-code) since Tue 2018-04-28 18:01:42 IST; 28min ago
		Main PID: 2881 (code=exited, status=0/SUCCESS)
		Status: "Total requests: 0; Current requests/sec: 0; Current traffic:   0 B/sec"
		Apr 28 17:37:29 tecmint systemd[1]: httpd.service: Got notification message from PID 2881, but recepti...bled.
		Apr 28 17:37:29 tecmint systemd[1]: httpd.service: Got notification message from PID 2881, but recepti...bled.
		Apr 28 17:37:39 tecmint systemd[1]: httpd.service: Got notification message from PID 2881, but recepti...bled.
		Apr 28 17:37:39 tecmint systemd[1]: httpd.service: Got notification message from PID 2881, but recepti...bled.
		Apr 28 17:37:49 tecmint systemd[1]: httpd.service: Got notification message from PID 2881, but recepti...bled.
		Apr 28 17:37:49 tecmint systemd[1]: httpd.service: Got notification message from PID 2881, but recepti...bled.
		Apr 28 17:37:59 tecmint systemd[1]: httpd.service: Got notification message from PID 2881, but recepti...bled.
		Apr 28 17:37:59 tecmint systemd[1]: httpd.service: Got notification message from PID 2881, but recepti...bled.
		Apr 28 18:01:42 tecmint systemd[1]: httpd.service: control process exited, code=exited status=226
		Apr 28 18:01:42 tecmint systemd[1]: Unit httpd.service entered failed state.
		Hint: Some lines were ellipsized, use -l to show in full.

14. 列出所有系統安裝點

# 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

15. 如何裝載，解除安裝，重新裝載，重新裝載系統裝載點，以及檢查系統上裝載點的狀態

# systemctl start tmp.mount
		# systemctl stop tmp.mount
		# systemctl restart tmp.mount
		# systemctl reload tmp.mount
		# systemctl status tmp.mount
		tmp.mount - Temporary Directory
		Loaded: loaded (/usr/lib/systemd/system/tmp.mount; disabled)
		Active: active (mounted) since Tue 2018-04-28 17:46:06 IST; 2min 48s ago
		Where: /tmp
		What: tmpfs
		Docs: man:hier(7)
		http://www.freedesktop.org/wiki/Software/systemd/APIFileSystems
		Process: 3908 ExecMount=/bin/mount tmpfs /tmp -t tmpfs -o mode=1777,strictatime (code=exited, status=0/SUCCESS)
		Apr 28 17:46:06 tecmint systemd[1]: Mounting Temporary Directory...
		Apr 28 17:46:06 tecmint systemd[1]: tmp.mount: Directory /tmp to mount over is not empty, mounting anyway.
		Apr 28 17:46:06 tecmint systemd[1]: Mounted Temporary Directory.

16. 如何在引導時啟用，啟用或禁用裝入點（在系統引導時自動裝入）

# systemctl is-active tmp.mount
		# systemctl enable tmp.mount
		# systemctl disable  tmp.mount

17. 如何在Linux中遮蔽（使其無法啟動）或取消遮蔽掛載點

# systemctl mask tmp.mount
		ln -s '/dev/null' '/etc/systemd/system/tmp.mount'
		# systemctl unmask tmp.mount
		rm '/etc/systemd/system/tmp.mount'

18. 列出所有可用的系統套接字。

# 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.

19. 如何在Linux中啟動，重新啟動，停止，重新載入和檢查套接字的狀態（例如：cups.socket）

# systemctl start cups.socket
		# systemctl restart cups.socket
		# systemctl stop cups.socket
		# systemctl reload cups.socket
		# systemctl status cups.socket
		cups.socket - CUPS Printing Service Sockets
		Loaded: loaded (/usr/lib/systemd/system/cups.socket; enabled)
		Active: active (listening) since Tue 2015-04-28 18:10:59 IST; 8s ago
		Listen: /var/run/cups/cups.sock (Stream)
		Apr 28 18:10:59 tecmint systemd[1]: Starting CUPS Printing Service Sockets.
		Apr 28 18:10:59 tecmint systemd[1]: Listening on CUPS Printing Service Sockets.

20. 如何啟用套接字並在引導時啟用或禁用（在系統引導時自動啟動套接字）

# systemctl is-active cups.socket
		# systemctl enable cups.socket
		# systemctl disable cups.socket
	
21. 如何遮蔽（使其無法啟動）或取消遮蔽插座（cups.socket）

# systemctl mask cups.socket
		ln -s '/dev/null' '/etc/systemd/system/cups.socket'
		# systemctl unmask cups.socket
		rm '/etc/systemd/system/cups.socket'

22. 獲取服務的當前CPU份額（比如httpd）

# systemctl show -p CPUShares httpd.service
		CPUShares=1024
	
	> 注意：每個服務的預設值都為CPUShare = 1024.您可以增加/減少程式的CPU份額。

23. 將服務的CPU份額（httpd.service）限制為2000 CPUShares /

# systemctl set-property httpd.service CPUShares=2000
		# systemctl show -p CPUShares httpd.service
		CPUShares=2000

> 注意：為服務設定CPUShare時，會建立一個名為service的目錄（httpd.service.d），其中包含一個包含CPUShare Limit資訊的檔案90-CPUShares.conf。 您可以將檔案視為：

# vi /etc/systemd/system/httpd.service.d/90-CPUShares.conf 
		[Service]
		CPUShares=2000

24. 檢查服務的所有配置詳細資訊

# 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=The Apache HTTP Server
		LoadState=loaded
		ActiveState=active
		SubState=running
		FragmentPath=/usr/lib/systemd/system/httpd.service
		....

25. 分析服務的關鍵鏈（httpd）

# 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-by\x2duuid-79f594ad\x2da332\x2d4730\x2dbb5f\x2d85d196080964.service @4.092s +149ms
		└─dev-disk-by\x2duuid-79f594ad\x2da332\x2d4730\x2dbb5f\x2d85d196080964.device @4.092s

26. 獲取服務的依賴項列表（httpd）

# 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
		....

27. 按層次列出控制組

# 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
		....
	
28. 根據CPU，記憶體，輸入和輸出列出控制組

# systemd-cgtop
		Path                                                              Tasks   %CPU   Memory  Input/s Output/s
		/                                                                    83    1.0   437.8M        -        -
		/system.slice                                                         -    0.1        -        -        -
		/system.slice/mariadb.service                                         2    0.1        -        -        -
		/system.slice/tuned.service                                           1    0.0        -        -        -
		/system.slice/httpd.service                                           6    0.0        -        -        -
		/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.scope                              3      -        -        -        -

29. 如何啟動系統救援模式

# systemctl rescue
		Broadcast message from root@tecmint on pts/0 (Wed 2015-04-29 11:31:18 IST):
		The system is going down to rescue mode NOW!

30. 如何進入緊急模式。

# 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 into default mode.

31. 列出當前使用的執行級別

# systemctl get-default
		multi-user.target

32. 如何啟動Runlevel 5 aka圖形模式。

# systemctl isolate runlevel5.target
		OR
		# systemctl isolate graphical.target

33. 如何啟動Runlevel 3又稱多使用者模式（命令列）

# systemctl isolate runlevel3.target
		OR
		# systemctl isolate multiuser.target

34. 如何將多使用者模式或圖形模式設定為預設執行級別

# systemctl set-default runlevel3.target
		# systemctl set-default runlevel5.target

35. 如何重啟，暫停，暫停，休眠或將系統置於混合睡眠狀態

# systemctl reboot
		# systemctl halt
		# systemctl suspend
		# systemctl hibernate
		# systemctl hybrid-sleep

CentOS 7 systemctl 命令總結

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