linux 命令和教程补充

Linux/command/dmesg.md

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+dmesg
+===
+
+显示Linux系统启动信息
+
+## 补充说明
+
+**dmesg命令** 被用于检查和控制内核的环形缓冲区。kernel会将开机信息存储在ring buffer中。您若是开机时来不及查看信息，可利用dmesg来查看。开机信息保存在`/var/log/dmesg`文件里。
+
+###  语法 
+
+```shell
+dmesg(选项)
+```
+
+###  选项 
+
+```shell
+-c：显示信息后，清除ring buffer中的内容；
+-s<缓冲区大小>：预设置为8196，刚好等于ring buffer的大小；
+-n：设置记录信息的层级。
+```
+
+###  实例 
+
+```shell
+[root@localhost ~]# dmesg | head
+Linux version 2.6.18-348.6.1.el5 (mockbuild@builder17.centos.org) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-54)) #1 SMP Tue May 21 15:34:22 EDT 2013
+BIOS-provided physical RAM map:
+ BIOS-e820: 0000000000010000 - 000000000009f400 (usable)
+ BIOS-e820: 000000000009f400 - 00000000000a0000 (reserved)
+ BIOS-e820: 00000000000f0000 - 0000000000100000 (reserved)
+ BIOS-e820: 0000000000100000 - 000000007f590000 (usable)
+ BIOS-e820: 000000007f590000 - 000000007f5e3000 (ACPI NVS)
+ BIOS-e820: 000000007f5e3000 - 000000007f5f0000 (ACPI data)
+ BIOS-e820: 000000007f5f0000 - 000000007f600000 (reserved)
+ BIOS-e820: 00000000e0000000 - 00000000e8000000 (reserved)
+```
+
+查看硬盘基础信息
+
+```shell
+dmesg | grep sda
+
+[    2.442555] sd 0:0:0:0: [sda] 488281250 512-byte logical blocks: (250 GB/232 GiB)
+[    2.442590] sd 0:0:0:0: [sda] Write Protect is off
+[    2.442592] sd 0:0:0:0: [sda] Mode Sense: 00 3a 00 00
+[    2.442607] sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
+[    2.447533]  sda: sda1
+[    2.448503] sd 0:0:0:0: [sda] Attached SCSI disk
+```
+
+