Let's consider a BASH shell script named example.sh that contains the following.

#!/bin/bash
GREETING="Hello World"

Running the script and then attempting to echo $GREETING will produce no output.

~]# bash example.sh
~]# echo $GREETING

The source command can be used make the variables set in the script available in your shell.

~]# source example.sh

Or, just a single period can be used.

~]# . example.sh

Now, echo $GREETING should produce output.

~]# echo $GREETING
Hello World