【Linux命令每日一知】dmesg--内核日志输出

骑猪看日落 2023-02-26 12:30 2阅读 0赞

> 2020-07-15 上海

## 命令解释 ##

dmesg命令用来输出linux内核的相关日志信息

man page: `dmesg - print or control the kernel ring buffer`

> 日志内容保存在文件: **/var/log/dmesg**

内核的日志也有对应的日志级别,可以在日志内容输出的时候使用`--level=xx1,xxx2`格式来控制

<table> 
 <thead> 
  <tr> 
   <th>日志级别</th> 
   <th>描述</th> 
   <th>对应命令的参数值</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>LOG_EMERG</td> 
   <td>system is unusable</td> 
   <td>emerg</td> 
  </tr> 
  <tr> 
   <td>LOG_ALERT</td> 
   <td>action must be taken immediately</td> 
   <td>alert</td> 
  </tr> 
  <tr> 
   <td>LOG_CRIT</td> 
   <td>critical conditions</td> 
   <td>crit</td> 
  </tr> 
  <tr> 
   <td>LOG_ERR</td> 
   <td>error conditions</td> 
   <td>err</td> 
  </tr> 
  <tr> 
   <td>LOG_WARNING</td> 
   <td>warning conditions</td> 
   <td>warn</td> 
  </tr> 
  <tr> 
   <td>LOG_NOTICE</td> 
   <td>normal, but significant, condition</td> 
   <td>notice</td> 
  </tr> 
  <tr> 
   <td>LOG_INFO</td> 
   <td>informational message</td> 
   <td>info</td> 
  </tr> 
  <tr> 
   <td>LOG_DEBUG</td> 
   <td>debug-level message</td> 
   <td>debug</td> 
  </tr> 
 </tbody> 
</table>

比如，输出emerge和error级别的日志,可以使用如下命令:  
`dmesg --level=error,emerg`

通常，主要将dmesg的输出结合grep来进行过滤查询问题,  
比如了解内存问题，可以使用如下命令:  
`dmesg | grep -i memory`

### 参数选项 ###

<table> 
 <thead> 
  <tr> 
   <th>参数</th> 
   <th>说明</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>-T</td> 
   <td>输出已读的时间戳</td> 
  </tr> 
  <tr> 
   <td>-u</td> 
   <td>控制输出用户空间的日志</td> 
  </tr> 
  <tr> 
   <td>-c</td> 
   <td>清除dmesg日志,但是日志保存的位置内容不会被删除</td> 
  </tr> 
 </tbody> 
</table>

### 示例 ###

[root@107 ~]# dmesg -h
    
    Usage:
     dmesg [options]
    
    Options:
     -C, --clear                 clear the kernel ring buffer
     -c, --read-clear            read and clear all messages
     -D, --console-off           disable printing messages to console
     -d, --show-delta            show time delta between printed messages
     -e, --reltime               show local time and time delta in readable format
     -E, --console-on            enable printing messages to console
     -F, --file <file>           use the file instead of the kernel log buffer
     -f, --facility <list>       restrict output to defined facilities
     -H, --human                 human readable output
     -k, --kernel                display kernel messages
     -L, --color                 colorize messages
     -l, --level <list>          restrict output to defined levels
     -n, --console-level <level> set level of messages printed to console
     -P, --nopager               do not pipe output into a pager
     -r, --raw                   print the raw message buffer
     -S, --syslog                force to use syslog(2) rather than /dev/kmsg
     -s, --buffer-size <size>    buffer size to query the kernel ring buffer
     -T, --ctime                 show human readable timestamp (could be
                                   inaccurate if you have used SUSPEND/RESUME)
     -t, --notime                don't print messages timestamp -u, --userspace display userspace messages -w, --follow wait for new messages -x, --decode decode facility and level to readable string -h, --help display this help and exit -V, --version output version information and exit Supported log facilities: kern - kernel messages user - random user-level messages mail - mail system daemon - system daemons auth - security/authorization messages syslog - messages generated internally by syslogd lpr - line printer subsystem news - network news subsystem Supported log levels (priorities): emerg - system is unusable alert - action must be taken immediately crit - critical conditions err - error conditions warn - warning conditions notice - normal but significant condition info - informational debug - debug-level messages For more details see dmesg(q). [root@107 ~]# dmesg -H [Jun13 21:27] Linux version 5.5.2-1.el7.elrepo.x86_64 (mockbuild@Build64R7) (gcc version 4.8.5 20150623 (Red Hat 4.8.5-39) (GCC)) # [ +0.000000] Command line: BOOT_IMAGE=/vmlinuz-5.5.2-1.el7.elrepo.x86_64 root=UUID=3437f1a0-f850-4f1b-8a7c-819c5f6a29e4 ro crashke [ +0.000000] x86/fpu: Supporting XSAVE feature 0x001: 'x87 floating point registers' [ +0.000000] x86/fpu: Supporting XSAVE feature 0x002: 'SSE registers'
    [root@107 ~]# dmesg -H
    [root@107 ~]#