Mac Terminal Auto Complete

1. Type in terminal nano ~/.inputrc

2. Paste the following on separate lines

   set completion-ignore-case on
   set show-all-if-ambiguous on
   TAB: menu-complete
   

3. Hit Control+O to save changes to .inputrc followed by control+X to quit
4. Open a new Terminal window or tab, or type “login” to open a new session with the rules in effect
5. Type and hit the tab key

Src: http://osxdaily.com/2012/08/02/improve-tab-completion-in-mac-os-x-terminal/

Put this in your ~/.inputrc:

set show-all-if-ambiguous on

You'll need to restart your shell (for example by opening a new terminal window or typing exec /bin/bash).

I am adding a new answer because in my case adding the line set show-all-if-ambiguous on was not enough.

I had also to remove the line: TAB: menu-complete.

My complete ~/.inputrc file is the following:

set completion-ignore-case on
set show-all-if-ambiguous on