Merging Bash sources

[Simon Wistow]

I have a small problem in that I'm trying to modify a bash script so 
that currently does this

    . config
    # then inspect command line args
    getArgs

so that you can specify the config file on the command line. Which 
necessarily requires do

    config_file="config"
    # inspect command line args first
    getArgs
    . ${config_file}

however that means that anything in the config file will override 
anything passed in on the command line. Which sort of defeats the point. 

Is there an easy way to say "source this config file but don't override 
any variable already set?" or some sort of standard recipe? Or amy I 
going to have to write something that reads the config file line by 
line, splits out any variable name left of a '=' checks to see if it's 
set and then evals the line? Cos that's potentially prone to failure.