I think you're describing aggregation which the GPL does not consider a derived work. Mere use of a GPL-licensed executable does not extend the GPLs terms to what you are doing (e.g. shell scripts, system() calls, etc). For each binary, the license terms of each statically or dynamically linked component must be simultaneously satisfied. The GPL includes a "system library" exception that excludes libraries shipped with the OS from that calculus.

The situation with combining interpreter code that doesn't access compiled libraries (e.g. pure octave M-files, or python code) isn't really clear to me. The FSF's opinion has been that a script that uses GPL'd interpreter code is subject to the terms of the GPL. I personally accept that interpretation out of deference to the FSF, but I've never understood specifically how that conclusion arises from the text of the GPL.