Begin-------------------
Check if variable exists.  Does it?
--Yes
----Echo "Variable exists...  moving on..."
----Call "set-backlight"
--No
----Set variable to some value (arbitrary)
----Echo "xrandr hasn't run yet;  running xrandr..."
----Run xrandr
----Call "set-backlight"

set-backlight()
{
  Echo "Setting backlight brightness as requested..."
  Run xbacklight with appropriate arguments
  return
}

End---------------------

Begin-------------------

Check if variable exists
--Yes
----Echo "Variable exists"
----Exit
--No
----Echo "Variable Does not exist.  Setting now."
----Set variable to whatever

Check if variable exists
--Yes
----Echo "Variable was successfully set"
----Exit
--No (never runs)
----Echo "Variable was NOT set..."

End---------------------

#!/bin/bash
if [  -n "$1"  ];then
       if  [ "$1" = "-F" ]; then
		if [ "$(tail -n1 $0)" = "#END" ]; then
			echo "Clearing END indicator"
			sed -i '$d' $0
			exit 0
		else
			echo "Error - no end indicator to clear"
			exit 1
		fi
	else
		echo "Useage $0 [-F] .. -F forces the prior run indicator to be cleared"
		exit 1
	fi
fi
if [ "$(tail -n1 $0)" = "#END" ];then
	echo "We've been ran before"
else
	echo "First time run"
	echo "#END" >> $0
fi
exit 0

$ xrandr
Screen 0: minimum 320 x 240, current 1440 x 900, maximum 1440 x 900
default connected 1440x900+0+0 0mm x 0mm
   1440x900       50.0*
   1024x768       51.0
   800x600        52.0     53.0
   640x512        54.0
   640x480        55.0     56.0
   512x384        57.0
   400x300        58.0
   320x240        59.0

#!/bin/bash
# by CJ Chitwood
# cj.chitwood <from> gmail.com
# with the help of linuxquestions.org 
#
# http://www.linuxquestions.org/questions/programming-9/bash-setting-a-persistent-variable-that-remains-after-the-script-exits-699591/ 
# (and Google hits too numerous during my searches to remember) 
#
# This file is not maintained.  It is provided as-is and 
# may be considered free-use/public domain.
#
# requires programs "xrandr" and "xbacklight".  Edit PATH to 
# point to the directories that contain these.  In Debian, this 
# is /usr/bin.
##############################################################################

PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin; export PATH

if [ $# -ne 1 ]; then
    echo -e 1>&2 Usage: setbl [arg], where 0 \<\= [arg] \<\= 100.
    exit 127
  else
    ARG=$1;
fi

setBacklight()
{ echo Attempting to set backlight to $ARG%...
  xbacklight -set $ARG
  return
}


TEMPVAR=`xrandr --verbose | grep BACKLIGHT_CONTROL | cut -d : -f 2 | cut -d " " -f 2`

if [ $TEMPVAR == "native" ]; then
    setBacklight
    exit 0
  elif [ $TEMPVAR == "kernel" ]; then
    echo xrandr has not yet been run.  Running xrandr...
    xrandr --output LVDS --set BACKLIGHT_CONTROL native
    setBacklight
  else 
    # This block of code should, in theory, never run; 
    # but, since I only own a VGN-NR160E, I can't test other 
    # models to be sure of this assumption.
    echo xrandr shows an unknown backlight control state...
    echo The current output of "xrandr --verbose | grep BACKLIGHT_CONTROL" is:
    echo     $TEMPVAR
    echo and since I expect either "native" or "kernel", I don\'t know what 
    echo to do with it...
    echo I wonder if you\'re running me on a Sony Vaio VGN NR-series...
    echo Not continuing, since state is unknown.  Try manual controls...  
fi

exit 0