Linux is not public domain software. It is licensed according to the GNU General Public License, which has a strict set of rules for use.

Linux is licensed under the terms of the GNU General Public License (GPL). Source code for any software under the GPL is free. Any modifications to software under a GPL license automatically come under the GPL as well, if the modifications to the GPL code are redistributed. You can't sell source code for GPL code for more than the cost of the distribution media and cost of distribution. If run-times are distributed, the source code must be made public and readily available upon request.

So, what does GPL mean to the embedded Linux developer? If you take the Linux kernel or any Linux utility and modify it, port it, or add features to it, must make the source available to anyone who asks for it. If you are not careful you may give up the rights to your proprietary software unintentionally. It is best to consult your attorney on all open-source copyright issues.

It is OK to ship Linux in a hardware device being sold. If any modifications to the Linux kernel, libraries, or utilities were required, the sources for the modified version must be made available. Application programs that run under Linux may remain proprietary. Just be sure no part of the application is GPL (or other open source license) code. Device drivers can also be kept proprietary. They must be shown to be separate and distinct from the Linux kernel. This is most easily done if the device driver is written as a loadable kernel module. If it is linked as part of the base kernel it is in the gray area of whether it becomes GPL or not. If in doubt consult your intellectual property attorney.