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Date: Tue, 12 Feb 2013 11:10:47 -0600 (CST)
From: Robert Bonomi <bonomi@mail.r-bonomi.com>
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To: tundra@tundraware.com
Subject: Re: Was I Sourced?
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> Date: Tue, 12 Feb 2013 08:53:37 -0600
> From: Tim Daneliuk <tundra@tundraware.com>
> To: FreeBSD Mailing List <freebsd-questions@freebsd.org>
> Subject: Was I Sourced?
>
> Is there a way for script to determine whether is was sourced
> or forked off as a subprocess when it was invoked?
>
> I have a script that needs to be sourced to work properly and
> I want to warn the luser if they exec or subshell it instead.

a 'sourced' script does -not- honor a shebag line.
you can exploit that.

The executable script /usr/local/bin/source_only;
   #!/bin/sh
   echo  Error: this script must be sourced

Your script:
   #!/usr/local/bin/source_only

   {cmd}
   {cmd}
   {cmd}
   {cmd}
   {cmd}
   {cmd}
   ...
   ...

Trying to do it totally self-contained is not easy.