On my Linux systems, for a couple years my /etc/rc.d/dmesg has:

#!/bin/sh
#

# PROVIDE: dmesg
# REQUIRE: mountcritremote
# BEFORE:  DAEMON

# todo: maybe save to /var/run and copy to /var/log first

. /etc/rc.subr

name="dmesg"
rcvar=$name
dmesg_file="/var/log/dmesg.boot"
start_cmd="do_dmesg"
stop_cmd=":"

do_dmesg()
{
	# keep only a few rotations
	[ -e "${dmesg_file}.1" ] && cp -pf ${dmesg_file}.1 ${dmesg_file}.2
	[ -e "${dmesg_file}.0" ] && cp -pf ${dmesg_file}.0 ${dmesg_file}.1
	cp -pf ${dmesg_file} ${dmesg_file}.0
	( umask 022 ; dmesg $rc_flags > ${dmesg_file} )
}

load_rc_config $name
run_rc_command "$1"


May I update src/etc/rc.d/dmesg to save a few older versions?

Should it be a setting like: dmesg_backup_count=3 and I will save up to 
that many old versions?

This has been quite useful for me. (And will take little space.)

I recall that my BSD/OS systems also saved previous versions of the boot 
messages too.

Should it save them under /var/run/ or /var/log/?

I know that /var/log/messages keeps same, but on long running systems, by 
default, you will lose your old dmesg output.

Any thoughts on this?

Jeremy C. Reed