Simple way to make bash script configurable

In order to simplify testing, I needed to make a bash script configurable. 
Yes bash is quite convenient to archive some tasks. 
I had trouble finding the syntax to override environment variable.

Here is a simple way to make to do it:

• set default environment variables (recommendation = pointing to stable code)
• make override config file a parameter 
• add this to make config override working (config is argument 4):

# check override configuration

if [ ! -z $4 ]

then

    echo "load config file:" $4

    source $4

else

    echo "default config (stable branch)"

fi

for VARIABLE in PYTHON INPUT OUTPUT

do

    eval var=\\$VARIABLE

    [ -z ${!var} ] && { echo "Need to set $VARIABLE"; exit 1; }

    echo $VARIABLE":"${!var}

done

example override_config.sh:

PYTHON=/work/fpieraut/py27/bin/python

Yes the syntax is a little cryptic but that is the only way I found to make is working;)

Btw, this isn't working in sh (calling source to set variables).