Proprietary command-line tool with GPL/LGPL GUI

Question

Suppose that I created a command-line tool with a proprietary license.

If I create a GUI, for the command-line tool, using a GPL/LGPL-licensed library/framework such as Qt, does the copyleft apply to the (code of the) command-line tool or the GUI made by the library/framework only?

For example, let's say that I have created a command-line tool that converts an Excel file into a csv file.

Later, I created a GUI for this command-line tool using Qt.

Does the copyleft apply to the code I wrote for the command-line tool or the code that I wrote for the GUI only?

Answer 1

The copyleft requirements could apply if the GUI is considered a derived work of the command-line tool. To determine that, you need to consider how closely related they are.

If the GUI just invokes the command-line tool as an external process and the GUI could have been written by someone who only has access to the command-line tool's executable and public documentation, then the two applications are considered to be independent in terms of copyright and their respective licenses do not affect each other.

If the GUI actually incorporates source code from the command-line tool, or the GUI uses knowledge about the internals of the command-line tool, then the situation becomes more complicated and it also becomes relevant if the GUI used GPL-licensed parts of Qt or only LGPL-licensed parts.