I am now having Screen output to log.txt, and the following applescript works just great!
set trigger to 1
set stopString to ""
repeat until (stopString = "stop")
set testTxt to paragraphs of (read POSIX file "/Users/fed/log.txt")
set countTxt to (count of testTxt) - 1
set lastLine to item countTxt of testTxt
if lastLine contains "Start MP3" and trigger is 1 then
tell application "Spotify" to playpause
set trigger to 0
end if
if lastLine does not contain "Start MP3" then
set trigger to 1
end if
if lastLine contains "Stop Loop" then
set stopString to "stop"
end if
end repeat