I am not sure why Caffeine does not keep your computer awake. I suspect it is because idle time on Ubuntu is equivalent to the time there is no user activity detected via keyboard or mouse.

How to prevent the computer from falling asleep

If you are looking for a solution to keep your computer awake like Caffeine does (should do), the solution below should work; I tested it on 14.04. It exists of two small scripts that you should store together in one and the same folder. You can switch it of or on (toggle) with a key combination. Once you press it, it will show the current state in a message:

or:

It basically exists of a small background script that simulates a minor user action (keypress Ctrl) if idle time exceeds a defined amount of time, thus preventing the computer to go asleep or blank the screen. The keypress itself is meaningless and has no effect on playing video in full screen.

How to use

• You will need to have xprintidle and xdotool installed:

  
  
  

  sudo apt-get install xprintidle xdotool
  
  

  
  
  

  (inotify-tools should be installed by default on Ubuntu, might not be the case on Xu- or Lubuntu)




• The script below is to toggle on/off, copy it into an empty file, save it as insomnia.py (keep the name as it is!) and make it executable(!)

#!/usr/bin/env python3

import os

import subprocess



script_dir = os.path.dirname(os.path.abspath(__file__))

try:

    pid = subprocess.check_output(["pgrep", "-f", "caf.py"]).decode("utf-8").strip()

    subprocess.Popen(["kill", pid])

    subprocess.Popen(["notify-send", "Computer doesn't stay awake..."])

except:

    subprocess.Popen(["/bin/bash", "-c", script_dir+"/"+"caf.py"])

    subprocess.Popen(["notify-send", "Computer stays awake..."])

• The (main) script below is to keep the computer awake, copy the script into an empty file, save it as caf.py (keep the name as it is!) and make it executable(!)

#!/usr/bin/env python3



import subprocess

import time



seconds = 120 # number of seconds to start preventing blank screen / suspend



while True:

    curr_idle = subprocess.check_output(["xprintidle"]).decode("utf-8").strip()

    if int(curr_idle) > seconds*1000:

        subprocess.call(["xdotool", "key", "Control_L"])

    time.sleep(10)

It is important that you keep both scripts in the same folder!

• Now add a keyboard shortcut to toggle your Caffeine replacement on and off: "System Settings" > "Keyboard" > "Shortcuts" > "Custom Shortcuts"

  
  
  

  Add the command:

  
  
  

  /path/to/insomnia.py
  
  

  
  
  

  To a key combination of your choice

That's it.