Free software and copyleft

Most software programs under Linux come under the GPL license. The so-called copyleft agreement. It may be confusing to new Linux programmers. Here are the main features of the license, and why you may or may not want to use it for your programs. The full license is available from the GNU site at http://www.gnu.org.

The GPL license provides software free. Not as in free of charge, but in freedom. You can charge money for free software or software under the GPL. What you can't do is restrict its use.

You must provide source code, or a way for the user to have access to that source code. This means that if you release a program as a binary only, it can't be under the GPL license and may not contain GPL code from other people.

If you use code available from other people under the GPL license, you can modify it and redistribute it under the same license, with source code. Also, you must keep the previous copyright notices, and write down in the program what changes you have made to it. This also means that anyone can modify your programs, as long as they keep your copyright notices there, and release their modifications under the same license.