For computer software, including nontrivial scripts, the "Artistic License" covers the Perl programming language interpreter, and the GNU General Public License covers the Linux operating system kernel.
Both of these licenses allow third parties to use the work of the original artist very freely; however, if a third party makes changes to the work, the GNU General Public License requires those changes to be released under the very same license -- and shared with the world, just like the original work. It is also common to see licenses that grant free use for noncommercial purposes only.
All of these licenses, of course, depend on copyright law for their legal enforcement. For more information, see how do I copyright my website?
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