Bash function assign value to passed parameter

Question

I've got the following situation:

I'm writing a script that will read its parameters either from a config file (if exists and parameter present) or asks the user to input said parameter if it's not present.

Since I'm doing this for a handful of parameters I thought writing a function would be the way to go.

However, as far as I understand it, the function returns the result value by echoing it or by assigning it to a global variable. I do want to echo to the screen in the function though, so it'll have to be option two. So i tried this:

# parameters: $1=name of parameter, $2=user prompt, $3=value read from config.cfg function getParameter { # If the value couldn't be read from the config file if [ -z "$3" ]; then # Echo the prompt echo "$2" # Read the user input read parameter # If it's empty, fail if [ -z "$parameter" ]; then echo "Parameter $1 not found" exit # Else, try to assign it to $3 <---- This is where it fails else $3="$parameter" fi fi } 

I call it like this:

getParameter "Database username" "Please enter database username" $database_username 

The config.cfg file is sourced before the function is called and $database_username is one of the optional parameters there.

Now this obviously doesn't work. I can't assign to $3 and since I want the method to be generic, I can't do MY_VARIABLE=$parameter either.

Does anyone have any suggestions how I can achieve all of the below:

1. Get variable value from either config.cfg or read it from the user input
2. Do this in a generic fashion, i.e. don't repeat the above code (without a function) for each parameter

Answer 1

Not 100% sure that I follow, but let's say a config file looks like this:

foo database_user tom bar 

The obvius value we want is that of database_user.

In a script you could simply put a line like this:

dbUser=$(sed -nE 's/database_user (.*$)/\1/p' config) 

Then the variable $dbUser will contain this information:

echo $dbUser tom 

Answer 2

Alright, looks like I solved my own problem:

function getParameter { if [ -z "$3" ]; then # User read -p to show a prompt rather than using echo for this read -p "$2`echo $'\n> '`" parameter # Print to sdterr and return 1 to indicate failure if [ -z "$parameter" ]; then >&2 echo "Parameter $1 not found" return 1 else echo $parameter fi else echo $3 fi } 

By using echo -p I was able to show a prompt on the console and still be able to return a string from the function by using regular echo. That way, by calling the function with database_username=$(getParameter ...) I can assign it to a variable.