This is a question without a simple answer. If by "Linux", you mean the Linux kernel, then yes. The source code is available free of cost, and you are free to modify it with only a few restrictions. If you meant "a Linux distro", then maybe. Most Linux distros, such as Ubuntu, Fedora, Debian, and Slackware are available free of charge as well, and with similar rights to modify and redistribute. However, a few Linux distros, such as Red Hat Enterprise Linux or Xandros use a lot of proprietary software, and are not made available free of charge.
I didn't find the right solution from the Internet.
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