Mac OS X comes with bash as the default shell, but it does not include the very convenient completion functionality commonly available in Linux distributions. So here are the steps to enable it.

First, you need to install the bash-completion package using the great Homebrew package manager:

brew install bash-completion

Then, enable the completions for the brew command itself:

ln -s `brew --prefix`/Library/Contributions/brew_bash_completion.sh `brew --prefix`/etc/bash_completion.d/

Now, edit your ~/.bash_profile to add the following lines:

export USER_BASH_COMPLETION_DIR=~/.bash_completion.d
if [ -f `brew --prefix`/etc/bash_completion ]; then
    . `brew --prefix`/etc/bash_completion
fi

Finally, create a file named ~/.bash_completion with the following lines (this will allow you to add your own completion scripts in the ~/.bash_completion.d/ directory):

if [ -d $USER_BASH_COMPLETION_DIR -a -r $USER_BASH_COMPLETION_DIR -a \
     -x $USER_BASH_COMPLETION_DIR ]; then
        for i in $USER_BASH_COMPLETION_DIR/*; do
                [[ ${i##*/} != @(*~|*.bak|*.swp|\#*\#|*.dpkg*|.rpm*) ]] &&
                        [ \( -f $i -o -h $i \) -a -r $i ] && . $i
        done
fi
unset i

That’s it! All new shells will now be completion enabled.