bootup检查，系统状态查看
在linux bootup时，会显示大量信息，可以使用 systemctl 命令查看所有服务，磁盘，挂载，定时器等的状态信息。
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$ systemctl
UNIT LOAD ACTIVE SUB DESCRIPTION >
proc-sys-fs-binfmt_misc.automount loaded active running Arbitrary Executable Fil>
dev-fuse.device loaded activating tentative /dev/fuse >
sys-devices-pci0000:00-0000:00:11.5-ata5-host4-target4:0:0-4:0:0:0-block-sda-sda1.device loaded active plugged WDC_WUH721816ALE6L4 1 >
sys-devices-pci0000:00-0000:00:11.5-ata5-host4-target4:0:0-4:0:0:0-block-sda.device loaded active plugged WDC_WUH721816ALE6L4 >
sys-devices-pci0000:00-0000:00:14.0-usb1-1\x2d4-1\x2d4:1.1-ttyUSB1-tty-ttyUSB1.device loaded active plugged FT4232H Quad HS USB-UART>
sys-devices-pci0000:00-0000:00:14.0-usb1-1\x2d4-1\x2d4:1.2-ttyUSB2-tty-ttyUSB2.device loaded active plugged FT4232H Quad HS USB-UART>
sys-devices-pci0000:00-0000:00:14.0-usb1-1\x2d4-1\x2d4:1.3-ttyUSB3-tty-ttyUSB3.device loaded active plugged FT4232H Quad HS USB-UART>
sys-devices-pci0000:64-0000:64:02.0-0000:65:00.0-nvme-nvme0-nvme0n1-nvme0n1p1.device loaded active plugged WD_BLACK SN770 500GB EFI>
sys-devices-pci0000:64-0000:64:02.0-0000:65:00.0-nvme-nvme0-nvme0n1-nvme0n1p2.device loaded active plugged WD_BLACK SN770 500GB 2 >
sys-devices-pci0000:64-0000:64:02.0-0000:65:00.0-nvme-nvme0-nvme0n1-nvme0n1p3.device loaded active plugged WD_BLACK SN770 500GB 3 >
sys-devices-pci0000:64-0000:64:02.0-0000:65:00.0-nvme-nvme0-nvme0n1-nvme0n1p4.device loaded active plugged WD_BLACK SN770 500GB 4 >
sys-devices-pci0000:64-0000:64:02.0-0000:65:00.0-nvme-nvme0-nvme0n1.device loaded active plugged WD_BLACK SN770 500GB
...
-.mount loaded active mounted Root Mount >
boot-efi.mount loaded active mounted /boot/efi >
data.mount loaded active mounted /data >
dev-hugepages.mount loaded active mounted Huge Pages File System >
dev-mqueue.mount loaded active mounted POSIX Message Queue File>
home-hjm-thinclient_drives.mount loaded active mounted /home/hjm/thinclient_dri>
proc-sys-fs-binfmt_misc.mount loaded active mounted Arbitrary Executable Fil>
run-snapd-ns-snap\x2dstore.mnt.mount loaded active mounted /run/snapd/ns/snap-store>
run-snapd-ns.mount loaded active mounted /run/snapd/ns >
run-user-1001-doc.mount loaded active mounted /run/user/1001/doc >
run-user-1001-gvfs.mount loaded active mounted /run/user/1001/gvfs >
run-user-1001.mount loaded active mounted /run/user/1001
...
init.scope loaded active running System and Service Manag>
session-11.scope loaded active running Session 11 of user test >
session-16.scope loaded active running Session 16 of user test >
session-17.scope loaded active running Session 17 of user test >
session-2.scope loaded active running Session 2 of user test >
session-221.scope loaded active running Session 221 of user test >
session-222.scope loaded active running Session 222 of user test >
session-230.scope loaded active running Session 230 of user test >
...
getty@tty1.service loaded active running Getty on tty1 >
irqbalance.service loaded active running irqbalance daemon >
kerneloops.service loaded active running Tool to automatically co>
keyboard-setup.service loaded active exited Set the console keyboard>
kmod-static-nodes.service loaded active exited Create list of static de>
lightdm.service loaded active running Light Display Manager >
ModemManager.service loaded active running Modem Manager >
networkd-dispatcher.service loaded active running Dispatcher daemon for sy>
NetworkManager-wait-online.service loaded active exited Network Manager Wait Onl>
...
basic.target loaded active active Basic System >
cryptsetup.target loaded active active Local Encrypted Volumes >
getty-pre.target loaded active active Login Prompts (Pre) >
getty.target loaded active active Login Prompts >
graphical.target loaded active active Graphical Interface >
local-fs-pre.target loaded active active Local File Systems (Pre)>
local-fs.target loaded active active Local File Systems >
multi-user.target loaded active active Multi-User System >
...
anacron.timer loaded active waiting Trigger anacron every ho>
apt-daily-upgrade.timer loaded active waiting Daily apt upgrade and cl>
apt-daily.timer loaded active waiting Daily apt download activ>
e2scrub_all.timer loaded active waiting Periodic ext4 Online Met>
fstrim.timer loaded active waiting Discard unused blocks on>
...
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.
285 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'.
如上所示，可以看到所有服务的简单状态信息，可以进一步查看详细信息，由于是当服务出错时。 使用 systemctl status ，如：
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$ systemctl status smbd.service
● smbd.service - Samba SMB Daemon
Loaded: loaded (/lib/systemd/system/smbd.service; enabled; vendor preset: enabled)
Active: active (running) since Thu 2020-08-30 13:44:38 CST; 6 days ago
Docs: man:smbd(8)
man:samba(7)
man:smb.conf(5)
Process: 1893 ExecStartPre=/usr/share/samba/update-apparmor-samba-profile (code=exited, status=0/SUCCESS)
Main PID: 1917 (smbd)
Status: "smbd: ready to serve connections..."
Tasks: 4 (limit: 308968)
Memory: 32.8M
CGroup: /system.slice/smbd.service
├─1917 /usr/sbin/smbd --foreground --no-process-group
├─1929 /usr/sbin/smbd --foreground --no-process-group
├─1931 /usr/sbin/smbd --foreground --no-process-group
└─1932 /usr/sbin/smbd --foreground --no-process-group
Warning: some journal files were not opened due to insufficient permissions.