Here's the modified/improved version of a screencast (record a video+audio track of your current desktop session) script which i originally posted on FreeBSD Forums.

the script relies on ffmpeg for recording and xinput to get $DISPLAY size;
user aragats@ on FBSD Forums contributed refining it;
plain OSS is assumed to be the default audio backend;
the script also provides auto-naming for the new record (depending on date), with exponential indexing

#!/usr/bin/env bash

mouse=6         # get this ID from "xinput --list"
audio=/dev/dsp5
today=$(date +"%Y%m%d")
fname_prefix="/tmp/screen_"
number=0

echo Select top left corner by clicking button 1
while true ; do
    #sleep 1
    eval `xinput --query-state $mouse | grep 'button\|valuator'`
    if [ "down" = "${button[1]}" ] ; then
        X0=${valuator[0]}
        Y0=${valuator[1]}
        break
    fi
done
echo Got: X0=$X0 Y0=$Y0

echo Select bottom right corner by clicking button 3
while true ; do
    #sleep 1
    eval `xinput --query-state $mouse | grep 'button\|valuator'`
    if [ "down" = "${button[3]}" ] ; then
        X1=${valuator[0]}
        Y1=${valuator[1]}
        break
    fi
done
echo Got: X1=$X1 Y1=$Y1

X=$((X1-X0))
Y=$((Y1-Y0))
if [ $X -le 0 ] || [ $Y -le 0 ] ; then
    echo You got negative dimensions: ${X}x${Y}
    exit 1
fi
echo Video size: ${X}x${Y}

while [ -f $fname_prefix$today$count.mkv ]
do
    $((number+=1))
    count=$(printf -- '-%02d' $number)
done
fname="$fname_prefix$today$count.mkv"

ffmpeg -y \
    -thread_queue_size 1024 \
    -f oss -i $audio -ac 2 \
    -f x11grab -r 60 \
    -s ${X}x${Y} \
    -i :0.0+$X0,$Y0 -c:v libx264rgb -crf 0 -preset ultrafast -c:a flac \
    $fname

echo $fname