Using VI to edit shell commands in UNIX

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http://unix.stackexchange.com – I used to have a co-worker who was really good at UNIX. He showed me how to use Vi to edit my shell commands. He placed the command in a file that ran every time I logged in. Since then, I've moved to a different project. Unfortunately I don't remember how to set this up. Is there anyone here who knows how to use Vi to edit commands in the terminal? How can I make that setting permanent? (HowTos)