I added a .sh file (shell script) to my Login items, for OS X to run when I startup and then login. It starts xcode to edit the file, instead of running it.
Solution: Rename the file to a .command file.
Why: .sh files are owned by xcode by default; .command files are owned by Terminal. Ergo, a .command file will be run by the Terminal app instead of by xcode.
Alternative: Change system preferences (via Get Information on the file) to open .sh files with Terminal. That could be risky. I don’t have a known risk in mind, except that OS X might expect .sh files to open with xcode, somewhere, somehow.