ubuntu recovery

Today I want to make an application to start when system start, then I open the '/etc/profile', and add a command 'exec docky' at the end of this file. Then I restart my system and here the problem comes. When I type in my username and password, the screen flash once, and then jump back to login screen, I do the login again and again, just can't login to the desk. Then I search the Internet and ask some people for help , then they tell me to make a livecd or liveusb, run ubuntu on the liveusb, change back the '/etc/profile' file, then it works! And restart my system from hard disk, it's back to nomal.