Minecraft wiki entry for Mac_Scripting_Tutorial, associated text: An exit status of 0 means that no errors were reported, while other numbers indicate either a failure with Tectonicus itself or a failure during rsync. These exit status numbers could be used to determine what to do next based on what failed—for example, you could re-trigger Tectonicus if the script returns an exit code of 3. This script doesn't do any of that, though.