Unix 中的 systemctl
systemctl用于检查和控制“systemd”系统和服务管理器的状态。
systemd是类 Unix 操作系统（大多数发行版，不是全部）的系统和服务管理器。当系统启动时，创建的第一个进程，即PID=1的init进程，是启动用户空间服务的systemd系统。
句法 ：
systemctl [OPTIONS...] COMMAND [NAME...]
systemctl 命令的部分输出
systemctl
输出：
snap-core-3887.mount loaded active mounted Mount unit for core
snap-simplescreenrecorder-1.mount loaded active mounted Mount unit for simplescreenrecorder
sys-fs-fuse-connections.mount loaded active mounted FUSE Control File System
sys-kernel-config.mount loaded active mounted Configuration File System
sys-kernel-debug.mount loaded active mounted Debug File System
acpid.path loaded active running ACPI Events Check
cups.path loaded active running CUPS Scheduler
systemd-ask-password-plymouth.path loaded active waiting Forward Password Requests to Plymouth Directory Watch
systemd-ask-password-wall.path loaded active waiting Forward Password Requests to Wall Directory Watch
systemd-networkd-resolvconf-update.path loaded active waiting Trigger resolvconf update for networkd DNS
init.scope loaded active running System and Service Manager
session-1.scope loaded active running Session 1 of user mandeep
另一个例子，如果你想挂起你的系统，使用下面的命令
systemctl suspend
or
systemctl -i suspend
systemctl 选项：
-t, --type= : the argument in this case should be comma-separated list of unit types such as service and socket.
--state= : this is used to show only specified states, for example --state=failed, will show only failed units
-a, --all: show all units, regardless of their states
-r, --recursive: while listing units, show units of local containers also
--show-types: it shows types of sockets along with showing sockets.
--job-mode : this option controls how to deal with already queued jobs in case of queuing a new job.
--fail : similar to --job-mode=fail
-i, --ignore-inhibitors: When you request system shutdown or sleep state, it ignores inhibitor locks
-q, --quite : it suppresses printing of results of various commands and the hints about the truncated lines
--no-block : don't wait synchronously for process to finish.
--user : talk to service manager of calling user, instead of system.
--no-wall : don't send wall message before power-off, halt or reboot.
-f, --force : when used with enable override any conflicting symlinks.
--message= : When used with halt, poweroff, reboot, kexec, you can display a short message specifying reason of operation.
-n, --lines : When used with status, controls the no. of journal lines to show.
-H, --host= : execute operation remotely.
例子 ：
1)
systemctl --fail
它将显示处于失败状态的作业，类似于 –job-mode=fail
输出：
这是完整输出的一部分
UNIT LOAD ACTIVE SUB DESCRIPTION
proc-sys-fs-binfmt_misc.automount loaded active waiting Arbitrary Executable File Formats File System Automount Poi
sys-devices-pci0000:00-0000:00:02.0-drm-card0-card0\x2deDP\x2d1-intel_backlight.device loaded active plugged /sys/devices/pci0000:00/0000:00:
sys-devices-pci0000:00-0000:00:03.0-sound-card0.device loaded active plugged Haswell-ULT HD Audio Controller
sys-devices-pci0000:00-0000:00:1b.0-sound-card1.device loaded active plugged 8 Series HD Audio Controller
sys-devices-pci0000:00-0000:00:1c.0-0000:01:00.0-net-enp1s0.device loaded active plugged RTL8101/2/6E PCI Express Fast/Gigabit Ethernet contr
sys-devices-pci0000:00-0000:00:1c.3-0000:02:00.0-net-wlp2s0.device loaded active plugged Wireless 3160 (Dual Band Wireless AC 3160)
sys-devices-pci0000:00-0000:00:1d.0-usb1-1\x2d1-1\x2d1.5-1\x2d1.5:1.0-bluetooth-hci0.device loaded active plugged /sys/devices/pci0000:00/000
sys-devices-pci0000:00-0000:00:1f.2-ata1-host0-target0:0:0-0:0:0:0-block-sda-sda1.device loaded active plugged ST500LT012-1DG142 ESP
sys-devices-pci0000:00-0000:00:1f.2-ata1-host0-target0:0:0-0:0:0:0-block-sda-sda10.device loaded active plugged ST500LT012-1DG142 10
sys-devices-pci0000:00-0000:00:1f.2-ata1-host0-target0:0:0-0:0:0:0-block-sda-sda11.device loaded active plugged ST500LT012-1DG142 PBR_Image
2) -i 选项，有时在您尝试执行命令时发生，例如systemctl suspend ，它不会被执行，当某些操作（例如复制项目或刻录 CD）时，在该系统中使用抑制锁来执行此操作时可能会发生确保这些操作成功，您可以使用 -i 选项覆盖这些锁
例如：
systemctl -i suspend
systemctl 命令：
1) list-units :列出所有的单位
list-units [PATTERN...]
例子：
systemctl list-units
部分输出：
sys-devices-pci0000:00-0000:00:1f.2-ata1-host0-target0:0:0-0:0:0:0-block-sda-sda9.device loaded active plugged ST500LT012-1DG142 9
sys-devices-pci0000:00-0000:00:1f.2-ata1-host0-target0:0:0-0:0:0:0-block-sda.device loaded active plugged ST500LT012-1DG142
sys-devices-platform-coretemp.0-hwmon-hwmon1.device loaded active plugged /sys/devices/platform/coretemp.0/hwmon/hwmon1
sys-devices-platform-dell\x2dlaptop-leds-dell::kbd_backlight.device loaded active plugged /sys/devices/platform/dell-laptop/leds/dell::kbd_ba
sys-devices-platform-serial8250-tty-ttyS0.device loaded active plugged /sys/devices/platform/serial8250/tty/ttyS0
sys-devices-platform-serial8250-tty-ttyS1.device loaded active plugged /sys/devices/platform/serial8250/tty/ttyS1
sys-devices-platform-serial8250-tty-ttyS10.device loaded active plugged /sys/devices/platform/serial8250/tty/ttyS10
sys-devices-platform-serial8250-tty-ttyS11.device loaded active plugged /sys/devices/platform/serial8250/tty/ttyS11
sys-devices-platform-serial8250-tty-ttyS12.device loaded active plugged /sys/devices/platform/serial8250/tty/ttyS12
2) list-sockets ：它列出了按监听地址排序的套接字地址。
systemctl list-sockets
输出：
LISTEN UNIT ACTIVATES
/dev/rfkill systemd-rfkill.socket systemd-rfkill.service
/run/acpid.socket acpid.socket acpid.service
/run/snapd-snap.socket snapd.socket snapd.service
/run/snapd.socket snapd.socket snapd.service
/run/systemd/fsck.progress systemd-fsckd.socket systemd-fsckd.service
/run/systemd/initctl/fifo systemd-initctl.socket systemd-initctl.service
/run/systemd/journal/dev-log systemd-journald-dev-log.socket systemd-journald.service
/run/systemd/journal/socket systemd-journald.socket systemd-journald.service
/run/systemd/journal/stdout systemd-journald.socket systemd-journald.service
/run/systemd/journal/syslog syslog.socket rsyslog.service
/run/udev/control systemd-udevd-control.socket systemd-udevd.service
/run/uuidd/request uuidd.socket uuidd.service
/var/run/avahi-daemon/socket avahi-daemon.socket avahi-daemon.service
/var/run/cups/cups.sock cups.socket cups.service
/var/run/dbus/system_bus_socket dbus.socket dbus.service
audit 1 systemd-journald-audit.socket systemd-journald.service
kobject-uevent 1 systemd-udevd-kernel.socket systemd-udevd.service
同样，您可以使用list-timers列出按它们接下来经过的时间排序的计时器单元。
3) start, stop, reload, restart ：顾名思义，它们分别用于启动、停止、重新加载或重新启动服务
start PATTERN...
reload PATTERN...
stop PATTERN...
restart PATTERN...
4) is-active PATTERN... ：检查是否有任何指定的服务处于活动状态，类似的 is-failed 可用于确定处于失败状态的单元。
5) status [PATTERN…|PID…]] :显示关于一个或多个单元的简洁的运行时状态信息
systemctl status
对于输出，请参见下图
systemctl 状态输出
6) show [PATTERN…|JOB…] ：显示一个或多个单位、工作或经理本身的属性。
systemctl show
输出：
Version=229
Features=+PAM +AUDIT +SELINUX +IMA +APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ -LZ4 +SECCOMP +BLKID +ELFUTILS +KMO
Architecture=x86-64
FirmwareTimestampMonotonic=0
LoaderTimestampMonotonic=0
KernelTimestamp=Tue 2018-02-13 08:07:08 IST
KernelTimestampMonotonic=0
InitRDTimestampMonotonic=0
UserspaceTimestamp=Tue 2018-02-13 08:07:13 IST
UserspaceTimestampMonotonic=5502424
FinishTimestamp=Tue 2018-02-13 08:07:53 IST
FinishTimestampMonotonic=45214960
SecurityStartTimestamp=Tue 2018-02-13 08:07:13 IST
7) enable NAME...：启用一个或多个单元文件或其实例，同理disable可用于禁用。
8)暂停
systemctl supend
它可以用来悬挂系统，同样存在对休眠，混合睡眠，重启，关机和停机选项
9) kexec :它关闭系统并通过 kexec 重新启动它。
我在本文中提到了最常用的命令和选项，更多你可以访问 systemctl 的 linux 手册页。
– 曼迪普·辛格