‘dmesg’命令显示linux内核的环形缓冲区信息，我们可以从中获得诸如系统架构、cpu、挂载的硬件，RAM等多个运行级别的大量的系统信息。当计算机启动时，系统内核（操作系统的核心部分）将会被加载到内存中。在加载的过程中会显示很多的信息，在这些信息中我们可以看到内核检测硬件设备。

dmesg 命令的使用范例

‘dmesg’命令设备故障的诊断是非常重要的。在‘dmesg’命令的帮助下进行硬件的连接或断开连接操作时，我们可以看到硬件的检测或者断开连接的信息。‘dmesg’命令在多数基于Linux和Unix的操作系统中都可以使用。

下面我们展示一些最负盛名的‘dmesg’命令工具以及其实际使用举例。‘dmesg’命令的使用语法如下。

# dmesg [options...]

1. 列出加载到内核中的所有驱动

我们可以使用如‘more’。 ‘tail’, ‘less ’或者‘grep’文字处理工具来处理‘dmesg’命令的输出。由于dmesg日志的输出不适合在一页中完全显示，因此我们使用管道（pipe）将其输出送到more或者less命令单页显示。

[root@tecmint.com ~]# dmesg | more[root@tecmint.com ~]# dmesg | less

输出

[    0.000000] Initializing cgroup subsys cpuset[    0.000000] Initializing cgroup subsys cpu[    0.000000] Initializing cgroup subsys cpuacct[    0.000000] Linux version 3.11.0-13-generic (buildd@aatxe) (gcc version 4.8.1 (Ubuntu/Linaro 4.8.1-10ubuntu8) ) #20-Ubuntu SMP Wed Oct 23 17:26:33 UTC 2013 (Ubuntu 3.11.0-13.20-generic 3.11.6)[    0.000000] KERNEL supported cpus:[    0.000000]   Intel GenuineIntel[    0.000000]   AMD AuthenticAMD[    0.000000]   NSC Geode by NSC[    0.000000]   Cyrix CyrixInstead[    0.000000]   Centaur CentaurHauls[    0.000000]   Transmeta GenuineTMx86[    0.000000]   Transmeta TransmetaCPU[    0.000000]   UMC UMC UMC UMC[    0.000000] e820: BIOS-provided physical RAM map:[    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable[    0.000000] BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff] reserved[    0.000000] BIOS-e820: [mem 0x0000000000100000-0x000000007dc08bff] usable[    0.000000] BIOS-e820: [mem 0x000000007dc08c00-0x000000007dc5cbff] ACPI NVS[    0.000000] BIOS-e820: [mem 0x000000007dc5cc00-0x000000007dc5ebff] ACPI data[    0.000000] BIOS-e820: [mem 0x000000007dc5ec00-0x000000007fffffff] reserved[    0.000000] BIOS-e820: [mem 0x00000000e0000000-0x00000000efffffff] reserved[    0.000000] BIOS-e820: [mem 0x00000000fec00000-0x00000000fed003ff] reserved[    0.000000] BIOS-e820: [mem 0x00000000fed20000-0x00000000fed9ffff] reserved[    0.000000] BIOS-e820: [mem 0x00000000fee00000-0x00000000feefffff] reserved[    0.000000] BIOS-e820: [mem 0x00000000ffb00000-0x00000000ffffffff] reserved[    0.000000] NX (Execute Disable) protection: active.....

2. 列出所有被检测到的硬件

要显示所有被内核检测到的硬盘设备，你可以使用‘grep’命令搜索‘sda’关键词，如下：

[root@tecmint.com ~]# dmesg | grep sda
[    1.280971] sd 2:0:0:0: [sda] 488281250 512-byte logical blocks: (250 GB/232 GiB)[    1.281014] sd 2:0:0:0: [sda] Write Protect is off[    1.281016] sd 2:0:0:0: [sda] Mode Sense: 00 3a 00 00[    1.281039] sd 2:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA[    1.359585]  sda: sda1 sda2 < sda5 sda6 sda7 sda8 >[    1.360052] sd 2:0:0:0: [sda] Attached SCSI disk[    2.347887] EXT4-fs (sda1): mounted filesystem with ordered data mode. Opts: (null)[   22.928440] Adding 3905532k swap on /dev/sda6.  Priority:-1 extents:1 across:3905532k FS[   23.950543] EXT4-fs (sda1): re-mounted. Opts: errors=remount-ro[   24.134016] EXT4-fs (sda5): mounted filesystem with ordered data mode. Opts: (null)[   24.330762] EXT4-fs (sda7): mounted filesystem with ordered data mode. Opts: (null)[   24.561015] EXT4-fs (sda8): mounted filesystem with ordered data mode. Opts: (null)

注解 ‘sda’表示第一块 SATA硬盘，‘sdb’表示第二块SATA硬盘。若想查看IDE硬盘搜索‘hda’或‘hdb’关键词。

3. 只输出dmesg命令的前20行日志

在‘dmesg’命令后跟随‘head’命令来显示开始几行，‘dmesg | head -20′命令将显示开始的前20行。

[root@tecmint.com ~]# dmesg | head  -20
[    0.000000] Initializing cgroup subsys cpuset[    0.000000] Initializing cgroup subsys cpu[    0.000000] Initializing cgroup subsys cpuacct[    0.000000] Linux version 3.11.0-13-generic (buildd@aatxe) (gcc version 4.8.1 (Ubuntu/Linaro 4.8.1-10ubuntu8) ) #20-Ubuntu SMP Wed Oct 23 17:26:33 UTC 2013 (Ubuntu 3.11.0-13.20-generic 3.11.6)[    0.000000] KERNEL supported cpus:[    0.000000]   Intel GenuineIntel[    0.000000]   AMD AuthenticAMD[    0.000000]   NSC Geode by NSC[    0.000000]   Cyrix CyrixInstead[    0.000000]   Centaur CentaurHauls[    0.000000]   Transmeta GenuineTMx86[    0.000000]   Transmeta TransmetaCPU[    0.000000]   UMC UMC UMC UMC[    0.000000] e820: BIOS-provided physical RAM map:[    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable[    0.000000] BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff] reserved[    0.000000] BIOS-e820: [mem 0x0000000000100000-0x000000007dc08bff] usable[    0.000000] BIOS-e820: [mem 0x000000007dc08c00-0x000000007dc5cbff] ACPI NVS[    0.000000] BIOS-e820: [mem 0x000000007dc5cc00-0x000000007dc5ebff] ACPI data[    0.000000] BIOS-e820: [mem 0x000000007dc5ec00-0x000000007fffffff] reserved

4. 只输出dmesg命令最后20行日志

在‘dmesg’命令后跟随‘tail’命令（‘ dmesg | tail -20’）来输出‘dmesg’命令的最后20行日志，当你插入可移动设备时它是非常有用的。

[root@tecmint.com ~]# dmesg | tail -20 
parport0: PC-style at 0x378, irq 7 [PCSPP,TRISTATE]ppdev: user-space parallel port driverEXT4-fs (sda1): mounted filesystem with ordered data modeAdding 2097144k swap on /dev/sda2.  Priority:-1 extents:1 across:2097144kreadahead-disable-service: delaying service auditdip_tables: (C) 2000-2006 Netfilter Core Teamnf_conntrack version 0.5.0 (16384 buckets, 65536 max)NET: Registered protocol family 10lo: Disabled Privacy Extensionse1000: eth0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: NoneSlow work thread pool: Starting upSlow work thread pool: ReadyFS-Cache: LoadedCacheFiles: LoadedCacheFiles: Security denies permission to nominate security context: error -95eth0: no IPv6 routers presenttype=1305 audit(1398268784.593:18630): audit_enabled=0 old=1 auid=4294967295 ses=4294967295 res=1readahead-collector: starting delayed service auditdreadahead-collector: sortingreadahead-collector: finished

5. 搜索包含特定字符串的被检测到的硬件

由于‘dmesg’命令的输出实在太长了，在其中搜索某个特定的字符串是非常困难的。因此，有必要过滤出一些包含‘usb’ ‘dma’ ‘tty’ ‘memory’等字符串的日志行。grep 命令 的‘-i’选项表示忽略大小写。

[root@tecmint.com log]# dmesg | grep -i usb
[root@tecmint.com log]# dmesg | grep -i dma
[root@tecmint.com log]# dmesg | grep -i tty
[root@tecmint.com log]# dmesg | grep -i memory

[    0.000000] Scanning 1 areas for low memory corruption[    0.000000] initial memory mapped: [mem 0x00000000-0x01ffffff][    0.000000] Base memory trampoline at [c009b000] 9b000 size 16384[    0.000000] init_memory_mapping: [mem 0x00000000-0x000fffff][    0.000000] init_memory_mapping: [mem 0x37800000-0x379fffff][    0.000000] init_memory_mapping: [mem 0x34000000-0x377fffff][    0.000000] init_memory_mapping: [mem 0x00100000-0x33ffffff][    0.000000] init_memory_mapping: [mem 0x37a00000-0x37bfdfff][    0.000000] Early memory node ranges[    0.000000] PM: Registered nosave memory: [mem 0x0009f000-0x000effff][    0.000000] PM: Registered nosave memory: [mem 0x000f0000-0x000fffff][    0.000000] please try 'cgroup_disable=memory' option if you don't want memory cgroups[    0.000000] Memory: 2003288K/2059928K available (6352K kernel code, 607K rwdata, 2640K rodata, 880K init, 908K bss, 56640K reserved, 1146920K highmem)[    0.000000] virtual kernel memory layout:[    0.004291] Initializing cgroup subsys memory[    0.004609] Freeing SMP alternatives memory: 28K (c1a3e000 - c1a45000)[    0.899622] Freeing initrd memory: 23616K (f51d0000 - f68e0000)[    0.899813] Scanning for low memory corruption every 60 seconds[    0.946323] agpgart-intel 0000:00:00.0: detected 32768K stolen memory[    1.360318] Freeing unused kernel memory: 880K (c1962000 - c1a3e000)[    1.429066] [drm] Memory usable by graphics device = 2048M

6. 清空dmesg缓冲区日志

我们可以使用如下命令来清空dmesg的日志。该命令会清空dmesg环形缓冲区中的日志。但是你依然可以查看存储在‘/var/log/dmesg’文件中的日志。你连接任何的设备都会产生dmesg日志输出。

[root@tecmint.com log]# dmesg -c

7. 实时监控dmesg日志输出

在某些发行版中可以使用命令‘tail -f /var/log/dmesg’来实时监控dmesg的日志输出。

[root@tecmint.com log]# watch "dmesg | tail -20"

结论：dmesg命令在系统dmesg记录实时更改或产生的情况下是非常有用的。你可以使用man dmesg来获取更多关于dmesg的信息。