‘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]Initializingcgroup subsys cpuset
[0.000000]Initializingcgroup subsys cpu
[0.000000]Initializingcgroup subsys cpuacct
[0.000000]Linuxversion3.11.0-13-generic(buildd@aatxe)(gcc version4.8.1(Ubuntu/Linaro4.8.1-10ubuntu8))#20-Ubuntu SMP Wed Oct 23 17:26:33 UTC 2013
(Ubuntu3.11.0-13.20-generic3.11.6)
[0.000000]KERNEL supported cpus:
[0.000000]IntelGenuineIntel
[0.000000]AMDAuthenticAMD
[0.000000]NSCGeodebyNSC
[0.000000]CyrixCyrixInstead
[0.000000]CentaurCentaurHauls
[0.000000]TransmetaGenuineTMx86
[0.000000]TransmetaTransmetaCPU
[0.000000]UMC UMC UMC UMC
[0.000000]e820:BIOS-provided physical RAM map:
[0.000000]BIOS-e820:[mem0x0000000000000000-0x000000000009fbff]usable
[0.000000]BIOS-e820:[mem0x00000000000f0000-0x00000000000fffff]reserved
[0.000000]BIOS-e820:[mem0x0000000000100000-0x000000007dc08bff]usable
[0.000000]BIOS-e820:[mem0x000000007dc08c00-0x000000007dc5cbff]ACPI NVS
[0.000000]BIOS-e820:[mem0x000000007dc5cc00-0x000000007dc5ebff]ACPI data
[0.000000]BIOS-e820:[mem0x000000007dc5ec00-0x000000007fffffff]reserved
[0.000000]BIOS-e820:[mem0x00000000e0000000-0x00000000efffffff]reserved
[0.000000]BIOS-e820:[mem0x00000000fec00000-0x00000000fed003ff]reserved
[0.000000]BIOS-e820:[mem0x00000000fed20000-0x00000000fed9ffff]reserved
[0.000000]BIOS-e820:[mem0x00000000fee00000-0x00000000feefffff]reserved
[0.000000]BIOS-e820:[mem0x00000000ffb00000-0x00000000ffffffff]reserved
[0.000000]NX(ExecuteDisable)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]sd2:0:0:0:[sda]488281250512-bytelogical blocks:(250GB/232GiB)
[1.281014]sd2:0:0:0:[sda]WriteProtectisoff
[1.281016]sd2:0:0:0:[sda]ModeSense:003a0000
[1.281039]sd2:0:0:0:[sda]Writecache: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***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]Initializingcgroup subsys cpuset
[0.000000]Initializingcgroup subsys cpu
[0.000000]Initializingcgroup subsys cpuacct
[0.000000]Linuxversion3.11.0-13-generic(buildd@aatxe)(gcc version4.8.1(Ubuntu/Linaro4.8.1-10ubuntu8))#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]IntelGenuineIntel
[0.000000]AMDAuthenticAMD
[0.000000]NSCGeodebyNSC
[0.000000]CyrixCyrixInstead
[0.000000]CentaurCentaurHauls
[0.000000]TransmetaGenuineTMx86
[0.000000]TransmetaTransmetaCPU
[0.000000]UMC UMC UMC UMC
[0.000000]e820:BIOS-provided physical RAM map:
[0.000000]BIOS-e820:[mem0x0000000000000000-0x000000000009fbff]usable
[0.000000]BIOS-e820:[mem0x00000000000f0000-0x00000000000fffff]reserved
[0.000000]BIOS-e820:[mem0x0000000000100000-0x000000007dc08bff]usable
[0.000000]BIOS-e820:[mem0x000000007dc08c00-0x000000007dc5cbff]ACPI NVS
[0.000000]BIOS-e820:[mem0x000000007dc5cc00-0x000000007dc5ebff]ACPI data
[0.000000]BIOS-e820:[mem0x000000007dc5ec00-0x000000007fffffff]reserved
4. 只输出dmesg命令***20行日志
在‘dmesg’命令后跟随‘tail’命令(‘ dmesg | tail -20’)来输出‘dmesg’命令的***20行日志，dkfd插入可移动设备时它是非常有用的。
[root@tecmint.com~]#dmesg|tail-20
parport0:PC-style at0x378,irq7[PCSPP,TRISTATE]
ppdev:user-space parallel port driver
EXT4-fs(sda1):mounted filesystemwithordered data mode
Adding2097144kswap on/dev/sda2.Priority:-1extents:1across:2097144k
readahead-disable-service:delaying service auditd
ip_tables:(C)2000-2006NetfilterCoreTeam
nf_conntrack version0.5.0(16384buckets,65536max)
NET:Registeredprotocol family10
lo:DisabledPrivacyExtensions
e1000:eth0 NICLinkisUp1000MbpsFullDuplex,FlowControl:None
Slowwork thread pool:Startingup
Slowwork thread pool:Ready
FS-Cache:Loaded
CacheFiles:Loaded
CacheFiles:Securitydenies permission to nominate security context:error-95
eth0:noIPv6routers present
type=1305audit(1398268784.593:18630):audit_enabled=0old=1auid=4294967295ses=4294967295res=1
readahead-collector:starting delayed service auditd
readahead-collector:sorting
readahead-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
输出
[0.000000]Scanning1areasforlow memory corruption
[0.000000]initial memory mapped:[mem0x00000000-0x01ffffff]
[0.000000]Basememory trampoline at[c009b000]9b000size16384
[0.000000]init_memory_mapping:[mem0x00000000-0x000fffff]
[0.000000]init_memory_mapping:[mem0x37800000-0x379fffff]
[0.000000]init_memory_mapping:[mem0x34000000-0x377fffff]
[0.000000]init_memory_mapping:[mem0x00100000-0x33ffffff]
[0.000000]init_memory_mapping:[mem0x37a00000-0x37bfdfff]
[0.000000]Earlymemory node ranges
[0.000000]PM:Registerednosave memory:[mem0x0009f000-0x000effff]
[0.000000]PM:Registerednosave memory:[mem0x000f0000-0x000fffff]
[0.000000]pleasetry'cgroup_disable=memory'optionifyou 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。
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