Stop Apps from Auto-Quitting

You might have noticed an odd thing while using OS X: if you haven’t used it for a while and it has no open files, an app might seem to quit in the background without asking you. If actual fact, the app enters a quasi-sleep mode: as far as the user is concerned, it appears to have stopped running but is still held in memory.

Sometimes apps quit themselves almost instantly: if I close the Messages window, for example, the program usually quits shortly afterward.

Apple calls this feature automatic termination4 if you find it annoying by issuing a secret command. Open a Terminal window (open Finder, select the Applications list, and then in the list of applications double-click Terminal within the Utilities folder) and type the following: 

defaults write -g NSDisableAutomaticTermination -bool TRUE

Then log out and back in again for the changes to take effect.

To reactivate automatic pruning of inactive applications later, again open a Terminal window, and type the following, before logging out and 
for the changes to take effect:back in again

defaults delete -g NSDisableAutomaticTermination