But this is what GPL is all about.

In other words, software based on Linux must be open source. Is there a charge? Someone charges, is it infringement?

Free software has nothing to do with charging or not. Freedom means that you must be free to get the source code, modify it and release a new version. Because of the characteristics of free software, many times they are free. But that doesn't mean you can't charge. Just like some PDF e-books, you can choose to download or pay for the same version.

So, the core of Red Hat series server edition is Linux, right? Why do you charge?

It's Linux, so Red Hat must disclose the source code. The official Red Hat server (ftp.redhat.com/pub/red HAT/Linux/Enterprise/4/EN/OS/I386/SRPMS) can download the source code of RHEL for free. Among them, CentOS is a distribution compiled with RHEL source code, which is maintained by the community and has no cost problem.

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& gt Excuse me, "sylecn" uses the GPL license agreement. Do I have strict copyright on my free software while following the open source?

You have complete copyright on the software you wrote, otherwise you are not qualified to protect it with GPL.

Because it is your software, you can ask the author who uses/clones your software to follow the GPL. Ensure that free software continues its "free" feature.

Reference: http://www.gnu.org/licenses/gpl-howto.html