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昨日举荐：每天学一个 Linux 命令（42）：unzip
命令简介
dmesg 命令用于显示零碎开机信息，可用于诊断系统故障。
内核会将零碎开机信息存储在 ring buffer 中，能够应用 dmesg 命令来查看，开机信息保留在 /var/log/dmesg 文件中。
命令语法
dmesg [options]
选项阐明
-c #显示信息后，革除 ring buffer 中的内容
-s< 缓冲区大小 > #默认值为 8196，刚好等于 ring buffer 的大小
-n #设置记录信息的层级
-D #禁用打印消息到控制台
-E #启用打印消息到控制台
-h #打印帮忙文本并退出
-k #打印内核音讯
-n #设置将音讯记录到控制台的级别
-r #打印原始音讯缓冲区
-s #应用多少大小的缓冲区来查问内核环缓冲区。默认状况下为 16392
-T #打印人类可读工夫戳
-t #不打印内核的工夫戳
-u #打印用户空间音讯
-V #输入版本信息并退出
-x #将设施和级别（优先级）编号解码为可读的前缀
利用举例
查看前 20 行开机信息
[root@centos7 ~]# dmesg | head -n 20
[0.000000] Initializing cgroup subsys cpuset
[0.000000] Initializing cgroup subsys cpu
[0.000000] Initializing cgroup subsys cpuacct
[0.000000] Linux version 3.10.0-1127.18.2.el7.x86_64 (mockbuild@kbuilder.bsys.centos.org) (gcc version 4.8.5 20150623 (Red Hat 4.8.5-39) (GCC) ) #1 SMP Sun Jul 26 15:27:06 UTC 2020
[0.000000] Command line: BOOT_IMAGE=/vmlinuz-3.10.0-1127.18.2.el7.x86_64 root=/dev/mapper/centos-root ro crashkernel=auto spectre_v2=retpoline rd.lvm.lv=centos/root rd.lvm.lv=centos/swap rhgb quiet LANG=en_US.UTF-8
[0.000000] Disabled fast string operations
[0.000000] e820: BIOS-provided physical RAM map:
[0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009ebff] usable
[0.000000] BIOS-e820: [mem 0x000000000009ec00-0x000000000009ffff] reserved
[0.000000] BIOS-e820: [mem 0x00000000000dc000-0x00000000000fffff] reserved
[0.000000] BIOS-e820: [mem 0x0000000000100000-0x000000003fedffff] usable
[0.000000] BIOS-e820: [mem 0x000000003fee0000-0x000000003fefefff] ACPI data
[0.000000] BIOS-e820: [mem 0x000000003feff000-0x000000003fefffff] ACPI NVS
[0.000000] BIOS-e820: [mem 0x000000003ff00000-0x000000003fffffff] usable
[0.000000] BIOS-e820: [mem 0x00000000f0000000-0x00000000f7ffffff] reserved
[0.000000] BIOS-e820: [mem 0x00000000fec00000-0x00000000fec0ffff] reserved
[0.000000] BIOS-e820: [mem 0x00000000fee00000-0x00000000fee00fff] reserved
[0.000000] BIOS-e820: [mem 0x00000000fffe0000-0x00000000ffffffff] reserved
[0.000000] NX (Execute Disable) protection: active
[0.000000] SMBIOS 2.4 present.
查看与内存相干的开机信息
[root@centos7 ~]# dmesg | grep -i memory
[0.000000] Base memory trampoline at [ffff9102c0098000] 98000 size 24576
[0.000000] crashkernel=auto resulted in zero bytes of reserved memory.
[0.000000] Early memory node ranges
[0.000000] PM: Registered nosave memory: [mem 0x0009e000-0x0009efff]
[0.000000] PM: Registered nosave memory: [mem 0x0009f000-0x0009ffff]
[0.000000] PM: Registered nosave memory: [mem 0x000a0000-0x000dbfff]
[0.000000] PM: Registered nosave memory: [mem 0x000dc000-0x000fffff]
[0.000000] PM: Registered nosave memory: [mem 0x3fee0000-0x3fefefff]
[0.000000] PM: Registered nosave memory: [mem 0x3feff000-0x3fefffff]
[0.000000] Memory: 972140k/1048576k available (7784k kernel code, 524k absent, 75912k reserved, 5958k data, 1980k init)
[0.000000] please try 'cgroup_disable=memory' option if you don't want memory cgroups
[0.755288] Initializing cgroup subsys memory
[1.619645] x86/mm: Memory block size: 128MB
[3.669071] Freeing initrd memory: 20628k freed
[3.933907] Non-volatile memory driver v1.3
[3.935079] crash memory driver: version 1.1
[4.000261] Freeing unused kernel memory: 1980k freed
[4.001692] Freeing unused kernel memory: 396k freed
[4.003171] Freeing unused kernel memory: 540k freed
[5.956205] [drm] Max dedicated hypervisor surface memory is 0 kiB
[5.956206] [drm] Maximum display memory size is 32768 kiB
[5.968934] [TTM] Zone kernel: Available graphics memory: 497842 kiB
查看与磁盘相干的开机信息
[root@centos7 ~]# dmesg | grep -i disk
[0.000000] RAMDISK: [mem 0x357a7000-0x36bcbfff]
[3.738914] VFS: Disk quotas dquot_6.5.2
[4.013333] systemd[1]: Running in initial RAM disk.
[6.295432] sd 0:0:0:0: [sda] Attached SCSI disk
[root@centos7 ~]# dmesg | grep -i sda
[6.291698] sd 0:0:0:0: [sda] 41943040 512-byte logical blocks: (21.4 GB/20.0 GiB)
[6.291830] sd 0:0:0:0: [sda] Write Protect is off
[6.291831] sd 0:0:0:0: [sda] Mode Sense: 61 00 00 00
[6.292032] sd 0:0:0:0: [sda] Cache data unavailable
[6.292033] sd 0:0:0:0: [sda] Assuming drive cache: write through
[6.294046] sda: sda1 sda2
[6.295432] sd 0:0:0:0: [sda] Attached SCSI disk
[15.843965] XFS (sda1): Mounting V5 Filesystem
[16.778908] XFS (sda1): Starting recovery (logdev: internal)
[16.801659] XFS (sda1): Ending recovery (logdev: internal)
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