> For the complete documentation index, see [llms.txt](https://book.bsdcn.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://book.bsdcn.org/man/man8/dmesg.8.md).

# dmesg.8

`dmesg` — 显示内核消息缓冲区

## 名称

`dmesg`

## 概要

`dmesg [-ac] [-t [-f output_fmt]] [-M core [-N system]]`

## 描述

`dmesg` 工具显示内核消息缓冲区的内容。如果未指定 `-M` 选项，则通过 sysctl(3) 接口从当前运行的内核读取缓冲区。否则，从指定的核心文件读取缓冲区，使用指定内核映像（或默认映像）中的名称列表。

选项如下：

**`-a`** 显示消息缓冲区中的所有数据。这包括任何 syslog 记录和 **/dev/console** 输出。

**`-c`** 打印后清除内核缓冲区。

**`-t`** 将时间戳（参见下文的 SYSCTL 变量章节）转换为绝对日期和时间。

**`-f`** 如果同时指定了 `-t`，则使用指定的 `output_fmt` 进行转换（参见 strftime(3) 手册页）。

**`-M`** 从指定的核心中提取与名称列表关联的值。

**`-N`** 如果同时指定了 `-M`，则从指定的系统而不是默认系统（即系统引导时所用的内核映像）提取名称列表。

## SYSCTL 变量

以下 [sysctl(8)](https://github.com/FreeBSD-Ask/freebsd-man-sc/blob/main/man8/sysctl.8.md) 变量控制内核如何在消息缓冲区中为条目添加时间戳：每个变量旁边显示了默认值。

**`kern.msgbuf_show_timestamp`**: 0 如果设为 0，则不添加时间戳。如果设为 1，则会在消息缓冲区的大多数行中添加 1 秒粒度的时间戳。如果设为 2，则会添加微秒粒度的时间戳。这也可以设置为引导 [loader(8)](https://github.com/FreeBSD-Ask/freebsd-man-sc/blob/main/man8/loader.8.md) 可调参数。时间戳放置在内核生成的大多数行的开头。某些多行消息只有第一行会标记时间戳。

## 文件

**/var/run/dmesg.boot** 通常是在启动时挂载文件系统后不久获取的缓冲区内容快照

## 参见

sysctl(3), syslogd(8)

## 历史

`dmesg` 工具出现于 3BSD。


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