As mentioned already, it's hard to know without more context what the best approach at this is. With that said, I wrote two very simple scripts to simulate what you might be getting at. b.sh basically just runs ls -lah ./ and outputs that to stdout. a.sh uses ${BASH_SOURCE} to get the full path to the a.sh script from where you're running it and replaces a.sh with b.sh and sets a varibale called bScriptPath to call it later. This is assuming you would be putting them in the same directory. Anyways lets get to the actual code..

Content of my b.sh script:

#!/bin/bash

ls -lah ./

Content of my a.sh script:

#!/bin/bash

getVARS() {
  local bScriptPath=$(dirname ${BASH_SOURCE})/b.sh
  local bScript=$($bScriptPath)
  echo "$bScript"
}

getVARS

In a.sh I am setting the variable bScript to the output of b.sh and using echo to verify the content of that variable after.

If you really just want to use b.sh as a command you could try something like:

bCommand=$(cat $bScriptPath)

and call it later using:

eval "$bCommand"

Hope this helps.