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?