Difference between 3 commands which are used to execute shell script?

Question

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Different ways to execute a shell script

What are the specific differences between the commands listed below?

./<scriptname>
. <scriptname>
bash <scriptname>

Can anyone explain the differences in simple words, please?

Answer 1

./<scriptname> executes <scriptname> in a child process and waits for it to exit.

. <scriptname> executes <scriptname> in the same process; the reasons this is used are usually to allow <scriptname> to modify the environment, and to perform additional actions without the resource cost of spawning a new process.

bash <scriptname> tells bash to run <scriptname>. If <scriptname> has a shebang with the path to bash (#!/bin/bash) at the top, it will have the same effect as just executing <scriptname>. If <scriptname> points to another interpreter (#!/bin/sh), then <scriptname> would normally be interpreted by that program, but you are overriding that by asking bash to interpret it.