freebsd-dev/sbin/dmesg/dmesg.8
Warner Losh 2f44ad86f5 dmesg: Better wording from review
I pushed the last changes before I'd noticed the better wording
suggestions in the review. Also include a note that not all lines will
have a timestampe. Some multi-line messages are generated with sbuf, and
when those are pushed, only the first line will have the
timestamp. Document this quirky behavior as well since fixing it likely
won't happen soon. CAM periph drivers generate all the lines in their
announce message together so they aren't intermingled with other things,
for example.

Suggested by:		allanjude, emaste, rpokala
Sponsored by:		Netflix
Differential Revision:	https://reviews.freebsd.org/D35139
Differential Revision:	https://reviews.freebsd.org/D35141
2022-05-07 09:55:40 -06:00

105 lines
3.5 KiB
Groff

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.\" @(#)dmesg.8 8.1 (Berkeley) 6/5/93
.\" $FreeBSD$
.\"
.Dd May 7, 2022
.Dt DMESG 8
.Os
.Sh NAME
.Nm dmesg
.Nd "display the system message buffer"
.Sh SYNOPSIS
.Nm
.Op Fl ac
.Op Fl M Ar core Op Fl N Ar system
.Sh DESCRIPTION
The
.Nm
utility displays the contents of the system message buffer.
If the
.Fl M
option is not specified, the buffer is read from the currently running kernel
via the
.Xr sysctl 3
interface.
Otherwise, the buffer is read from the specified core file,
using the name list from the specified kernel image (or from
the default image).
.Pp
The options are as follows:
.Bl -tag -width indent
.It Fl a
Show all data in the message buffer.
This includes any syslog records and
.Pa /dev/console
output.
.It Fl c
Clear the kernel buffer after printing.
.It Fl M
Extract values associated with the name list from the specified core.
.It Fl N
If
.Fl M
is also specified,
extract the name list from the specified system instead of the default,
which is the kernel image the system has booted from.
.El
.Sh SYSCTL VARIABLES
The following
.Xr sysctl 8
variables control how the kernel timestamps entries in the message buffer:
The default value is shown next to each variable.
.Bl -tag -width indent
.It kern.msgbuf_show_timestamp : No 0
If set to 0, no timetamps are added.
If set to 1, then a 1-second granularity timestamp will be added to most lines
in the message buffer.
If set to 2, then a microsecond granularity timestamp will be added.
This may also be set as a boot
.Xr loader 8
tunable.
The timestamps are placed at the start of most lines that the kernel generates.
Some multi-line messages will have only the first line tagged with a timestamp.
.El
.Sh FILES
.Bl -tag -width ".Pa /var/run/dmesg.boot" -compact
.It Pa /var/run/dmesg.boot
usually a snapshot of the buffer contents
taken soon after file systems are mounted
at startup time
.El
.Sh SEE ALSO
.Xr sysctl 3 ,
.Xr syslogd 8
.Sh HISTORY
The
.Nm
utility appeared in
.Bx 3 .