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d3905a3b07
Event: Advanced UNIX Programming Course (Fall'23) at NTHU. Pull Request: https://github.com/freebsd/freebsd-src/pull/1036
102 lines
3.4 KiB
Groff
102 lines
3.4 KiB
Groff
.\" Copyright (c) 1980, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.Dd May 7, 2022
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.Dt DMESG 8
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.Os
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.Sh NAME
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.Nm dmesg
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.Nd "display the system message buffer"
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.Sh SYNOPSIS
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.Nm
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.Op Fl ac
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.Op Fl M Ar core Op Fl N Ar system
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.Sh DESCRIPTION
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The
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.Nm
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utility displays the contents of the system message buffer.
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If the
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.Fl M
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option is not specified, the buffer is read from the currently running kernel
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via the
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.Xr sysctl 3
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interface.
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Otherwise, the buffer is read from the specified core file,
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using the name list from the specified kernel image (or from
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the default image).
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.Pp
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The options are as follows:
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.Bl -tag -width indent
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.It Fl a
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Show all data in the message buffer.
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This includes any syslog records and
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.Pa /dev/console
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output.
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.It Fl c
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Clear the kernel buffer after printing.
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.It Fl M
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Extract values associated with the name list from the specified core.
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.It Fl N
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If
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.Fl M
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is also specified,
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extract the name list from the specified system instead of the default,
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which is the kernel image the system has booted from.
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.El
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.Sh SYSCTL VARIABLES
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The following
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.Xr sysctl 8
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variables control how the kernel timestamps entries in the message buffer:
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The default value is shown next to each variable.
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.Bl -tag -width indent
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.It kern.msgbuf_show_timestamp : No 0
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If set to 0, no timestamps are added.
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If set to 1, then a 1-second granularity timestamp will be added to most lines
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in the message buffer.
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If set to 2, then a microsecond granularity timestamp will be added.
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This may also be set as a boot
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.Xr loader 8
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tunable.
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The timestamps are placed at the start of most lines that the kernel generates.
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Some multi-line messages will have only the first line tagged with a timestamp.
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.El
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.Sh FILES
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.Bl -tag -width ".Pa /var/run/dmesg.boot" -compact
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.It Pa /var/run/dmesg.boot
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usually a snapshot of the buffer contents
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taken soon after file systems are mounted
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at startup time
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.El
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.Sh SEE ALSO
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.Xr sysctl 3 ,
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.Xr syslogd 8
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.Sh HISTORY
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The
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.Nm
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utility appeared in
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.Bx 3 .
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