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每天学一个 Linux 命令(43):dmesg

每天学一个 Linux 命令(43):dmesg_mooc

命令简介

dmesg 命令用于显示系统开机信息,可用于诊断系统故障。

内核会将系统开机信息存储在ring buffer中,可以使用dmesg命令来查看,开机信息保存在/var/log/dmesg文件中。

命令语法

dmesg [options]

选项说明

-c  #显示信息后,清除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 #打印帮助文本并退出
-k #打印内核消息
-n #设置将消息记录到控制台的级别
-r #打印原始消息缓冲区
-s #使用多少大小的缓冲区来查询内核环缓冲区。 默认情况下为16392
-T #打印人类可读时间戳
-t #不打印内核的时间戳
-u #打印用户空间消息
-V #输出版本信息并退出
-x #将设施和级别(优先级)编号解码为可读的前缀

应用举例

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.

查看与内存相关的开机信息

[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

查看与磁盘相关的开机信息

[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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