Last night I came up with some stuff to use vim for blogging. It involves some bash scripts and aliases, and you end up with a blog command you can invoke to create a new post. Once there I thought it'd been cool if doing blog<TAB> listed all my current posts and offered tab completation for partially typed names. Bash programmable completation is what I wanted, but it turned out to be less straightforward and well documented than I expected.

Some readings to start

First of all some reading material:
1. man bash , search for "Programmable Completion"
2. Relevant chapters in the bash reference manual
3. Bash completation (useful as example)

Concepts

Conceptually what you do is creating a function that returns a listing and assign that function to some command, so when you run it and TAB that function will be invoked, producing a list of items.
In my case I have blog posts in a well-known path with extension .blog, so all I want to do when I do blog<TAB> is to return all the files in that dir with extension .blog or all the files whose name starts with whatever I typed after the command.

Code

Stick this in a file under /etc/bash_completion.d/ and you're ready to go (or at the bottom of /etc/bash_completion if your distro doesn't provide a .d mechanism)

have blog &&
_blog()
{
    COMPREPLY=()
    if [ -z "${COMP_WORDS[COMP_CWORD]}" ] ; then
        cur="*.blog"
    else
        cur="${COMP_WORDS[COMP_CWORD]}*"
    fi
    COMPREPLY=( $(find /path/to/your/blog/ -name "$cur" -printf "%f ") )
}

complete -F _blog blog

The have() function is defined in the /etc/bash_completion and simply checks if that function has already been defined. COMPREPLY is where you're supposed to store your file listing to be used by complete command. complete -F _blog blog assigns the function we just wrote to the blog command making the magic happening.
As I mentioned in the warning I suspect that version to be slightly broken as compgen should be used instead of find, but I couldn't get my head around it. Also, complete has got a -X switch you can use to filter out extensions, so I suspect somewhere that should be used too.