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)每天学一个 Linux 命令（39）：tar
每天学一个 Linux 命令（40）：gzip
每天学一个 Linux 命令（41）：zip