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I'm developing a wordpress plugin for a client. I modified two different plugins (licensed under gpl2) with similar functionality and combined them into one to match my client's needs. I'm giving my client the modified plugin under gpl2 licence. Am I required to tell him it's modified version of another plugin?

  • Required, probably not. It would be courteous to do so. – vonbrand Jul 6 '18 at 16:04
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Yes, you must indicate that you modified the GPL software. Section 2a of the GPLv2 license answers your question specifically:

  1. You may modify your copy or copies of the Program or any portion of it ... and copy and distribute such modifications or work under the terms of Section 1 above, provided that you also meet all of these conditions:

    a) You must cause the modified files to carry prominent notices stating that you changed the files and the date of any change.

    b) ...

    c) ...

https://www.gnu.org/licenses/old-licenses/gpl-2.0.html

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If it is "take 1/3 of A, 1/5 of B, mush in with a whole lot of my own", the "modified" part is quite murky. As a courtesy, I'd say something along the lines of "Parts of A and of B used" in the general README, perhaps (if identifiable pieces remain), state in the pieces (files) "Adapted from file F of A" or some such.

I'm not any kind of lawyer, just trying to apply common sense/courtesy. In the end, the idea is that if A is redone, bugs are fixed, ... the happy user of your product C can trace back where it has to be updated to match.

  • The GPL license says that you must make note of the changes, period. It does not matter whether your changes are to fix bugs or for some other reason. Basically it means that you must mark modified files with a message to the effect "I modified this on this date." If another developer wants to know exacrly what changed later, she can always perform a diff of your version, or load it into her version control system, or whatever. You don't need to mark each line that you changed or something like that. The license doesn't require that you say what bugs were fixed, if any. – Brandin Jul 11 '18 at 7:46
  • @Brandin, that is the "normal" case of fixes/modifications to some work. OP specifically asks about combining parts and pieces of several separate works. – vonbrand Jul 11 '18 at 13:55

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