A script to capture a 'screencast' on your Chromebook via 'crouton' (http://goo.gl/fd3zc) by David Schneider. This is a modified version of the original script 'screencast.sh' (http://goo.gl/UmWAta) by Francois Beaufort.

screencast

#!/bin/bash -e
## A script to capture a 'screencast' on your Chromebook via 'crouton' (http://goo.gl/fd3zc) by David Schneider. 
#+ This is a modified version of the original script 'screencast.sh'   (http://goo.gl/UmWAta) by Francois Beaufort.
#+ This 'screencast' modified source (and author) can be found here -  (https://gist.github.com/DennisLfromGA/8441689)

# Needs to run in bash so do it.
if [ -z "$BASH_VERSION" ]; then exec bash -e "$0" "$@"; fi

APPLICATION="${0##*/}"
ARGNUM="$#"
CHROOT=''
DFLTNAME='screencast'
PRILOC='/var/crouton/chroots' # crispy   crouton
ALTLOC='/usr/local/chroots'   # standard crouton
if   [ -d $PRILOC ]; then
  CHROOTS="$PRILOC"
elif [ -d $ALTLOC ]; then
  CHROOTS="$ALTLOC"
fi
if [ -d "$CHROOTS/$DFLTNAME" ]; then
  CHROOTNAME="$DFLTNAME"
else
  CHROOTNAME="$(ls -1 $CHROOTS 2>/dev/null | head -n 1)"
fi
DELAY='10'
if [ -d "$HOME/Extensions/pibhkhofcjfhelinjppfgdcoccedkhmm/" ]; then
  MINIMIRROR="y"
else
  MINIMIRROR=''
fi
FILENAME="${APPLICATION}-$(date +%Y%m%d-%H%M%S)"
VERSION='20150309200657'
VERHIST="\
20150309200657 ## 'MiniMirror' detection and suggestion added.
               #+  also tweaked the countdown display.
20150306124703 ## Added 'screencast' as default CHROOTNAME
20141108235808 ## Light cleanup, touchup comments, etc.
20141108113743 ## Removed non-functional $HOME and logging
               #+ also enforce bash invocation.
20141108101455 ## Added 'crispy crouton' /var/crouton/chroots location
               #+ and '-l' option to specify CHROOTS location
               #+ also updated enter-chroot to add '-p ...' option
20140129132631 ## Added option to display instructions 
               #+ to create a new chroot if none exists.
20140129123043 ## Changed date format to lead with the year
               #+ for both the file name and version number.
01152014162638 ## Original script (based on screencast.sh by Francois Beaufort)
"

# Helpers function to color terminal.
textreset=$(tput sgr0)
colorred=$(tput setaf 1) 
coloryel=$(tput setaf 3) 
colorblue=$(tput setaf 6) 
 
INSTRUCTIONS="
${coloryel}### You can create a new chroot like this:${textreset}
${colorblue}### First, download 'crouton' from: ${textreset}http://goo.gl/fd3zc
${colorblue}### Then open a crosh shell and enter:${textreset}

    sudo sh ~/Downloads/crouton -p `dirname $CHROOTS` -t audio -n $DFLTNAME

${colorblue}### Then re-run this script: ${textreset}'sh $0 [options]'
"

if [ -z "$CHROOTNAME" ]
then
  CHROOT="${colorred}None Found!${textreset}"
else
  CHROOT="$CHROOTNAME"
fi

USAGE="
'$APPLICATION' - A script to capture a 'screencast' on your Chromebook via crouton.

${colorblue}FORMAT: $APPLICATION [-d Delay] [-f File name] [-l Chroots folder] [-n Chroot name]${textreset}

Options:
    -d    Delay in seconds before starting the screencast - Default: '$DELAY'
    -f    Screencast file name - Default: '~/Downloads/${FILENAME}.mkv'
    -n    Chroot name - Default: '$CHROOT' ('$DFLTNAME' or alphabetically 1st chroot)
    -l    Chroots location - Default: '$CHROOTS'
Also:
    -h    Help - prints this usage summary and exits.
    -i    Instructions - prints instructions to create a chroot and exits.
    -v    Version - displays the current version of '$APPLICATION' and exits.
    -V    Version History - displays version history of '$APPLICATION' and exits.
"

# Function to exit with exit code $1, spitting out message $@ to stderr
error() {
  local ecode="$1"
  shift
  echo "$*" 1>&2
  exit "$ecode"
}

if [ "$1" = "--help" ]; then error 0 "$USAGE"; fi

while getopts d:D:f:F:l:L:n:N:hHiIvV OPT; do
  case ${OPT} in
    [dD])  DELAY="${OPTARG}";;
    [fF])  FILENAME="${OPTARG}";;
    [lL])  CHROOTS="`readlink -f ${OPTARG}`";;
    [nN])  CHROOTNAME="${OPTARG}";;
    [hH]*) error 255 "$USAGE";;
    [iI]*) error 255 "$INSTRUCTIONS";;
    [v]*)  error 255 "$APPLICATION version: $VERSION";;
    [V]*)  error 255 "$VERHIST";;
    \?)    error 2 "$USAGE";;
  esac
done
shift "$((OPTIND-1))"

if [ -z "$CHROOTNAME" ]
then
  echo "$USAGE"
  echo "${colorred}*** No chroot found in $CHROOTS ...${textreset}"
  error 1 "$INSTRUCTIONS"
elif [ ! -d $CHROOTS/$CHROOTNAME ]
then
  echo "${colorred}*** Sorry, no chroot named '$CHROOTNAME' found in $CHROOTS ...${textreset}"
  echo
  ls $CHROOTS
  echo
  error 1 "${coloryel}*** Try one of the above ...${textreset}"
else
  CHROOT="$CHROOTNAME"
fi

echo "${colorblue}*** Filename: '${textreset}~/Downloads/${FILENAME}.mkv${colorblue}'\
 - Chrootname: '${textreset}$CHROOT${colorblue}'\
 - Delay: '${textreset}$DELAY${colorblue}'${textreset}"

if [ "$ARGNUM" -le 0 ]; then read -p "Press ENTER to continue - Ctrl-C to abort "; fi

# Checking package requirements.
echo "${colorblue}### Checking requirements ...${textreset}"
INSTALLED_PACKAGES=$(sudo enter-chroot -c $CHROOTS -n $CHROOT -b dpkg -l | awk '{print $2}')
REQUIRED_PACKAGES="libav-tools libavcodec-extra-53 libavdevice-extra-53 \
    libavfilter-extra-2 libavformat-extra-53 libavutil-extra-51 \
    libpostproc-extra-52 libswscale-extra-2 x264 libx264-dev pulseaudio"
for package in $REQUIRED_PACKAGES; do
  if ! echo $INSTALLED_PACKAGES | \grep -q -E "^ii $package"; then
    echo "${colorblue}*** Installing avconv and its friends - if needed ...${textreset}"
    sudo enter-chroot -c $CHROOTS -n $CHROOT -x sudo apt-get install -qq -y $REQUIRED_PACKAGES
    break
  fi
done

echo
echo "${coloryel}*** Now is a good time to switch to the desired window to be captured ...${textreset}"
echo "${coloryel}*** It's also a good time to adjust your volume or maybe even mute it ...${textreset}"
[ -n "$MINIMIRROR" ] && \
echo "${colorblue}*** Mini Mirror is installed, have you got it running and situated ? ...${textreset}"
echo
echo -e "*** Starting screen capture in $DELAY seconds - Press Ctrl+C to abort.${colorred}\c"
echo -e "\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\c"
for ((i=$DELAY; i>0; i--)); do
  echo -e " $i\b\b\c"; sleep 1
done
echo    " 0${textreset}"

# Go $HOME and Start screen capture.
cd
RESOLUTION=`xrandr |sed -n 's/.*current \([0-9]\+\) x \([0-9]\+\).*/\1x\2/p'`
TRAP="echo \"${coloryel}*** See Filename: ${FILENAME}.mkv for output ...${textreset}\""
trap "${TRAP}" INT HUP 2 0
sudo enter-chroot -c $CHROOTS -n $CHROOT host-x11 avconv -f alsa \
    -ac 2 -ab 128k -i pulse -f x11grab -r 30 -s $RESOLUTION -i :0.0  \
    -vcodec libx264 -preset ultrafast -crf 0 Downloads/${FILENAME}.mkv
exit 0

Checkout 'VERSHIST' for updated info.
For more in-depth info., click on 'Revisions'.