Sourcing a file

Sourcing a file is different from execution, in that all commands are evaluated within the context of the current bash session - this means that any variables, function, or aliases defined will persist throughout your session.

Create the file you wish to source sourceme.sh

#!/bin/bash

export A="hello_world"
alias sayHi="echo Hi"
sayHello() {
    echo Hello
}

From your session, source the file

$ source sourceme.sh

From hencefourth, you have all the resources of the sourced file available

$ echo $A
hello_world

$ sayHi
Hi

$ sayHello
Hello

Note that the command . is synonymous to source, such that you can simply use

$ . sourceme.sh