gistfile1.rb

# File you want to get before it's deleted
fileToSave = 'blarblarblar'
# If you want to specify a little bit we can trim off the end
# do it here in bytes. This may make the script more likely to
# save your file.
buffer = 0;

# Wait for the file to be created
print "Okay, please start the process that outputs "+fileToSave+"\n";
while true
  break if !File.size?(fileToSave).nil?
end

# First pass to get maximum file size before nuke
print "Thanks, running loop to determine the maximum file size before the file is deleted\n";
maxSize = 0;
while true
  size = File.size?(fileToSave)
  if size
    if size > maxSize
      maxSize = size
    end
  else
    break
  end
end

print "Max size appears to be "+maxSize.to_s+" bytes. Please start the same process again.\n"

# Wait for file to be written again
while true
  break if !File.size?(fileToSave).nil?
end

print "Sweet. Will move the file to saved-"+fileToSave+" when it reaches that size again\n";

# Watch it until it gets to our target size
sizeToCheck = maxSize-buffer
while true
  size = File.size?(fileToSave)
  if size
    if size >= sizeToCheck
      File.rename(fileToSave, 'saved-'+fileToSave)
      break
    end
  end
end

print "Done!\n\n";