While browsing this site Text Files/Programming which contains historical data pertaining to programming and a lot of other stuff I came across this file Info sheet LinuxThe file in question has a peculier paragraph with the following contents:Although Linux is supplied with  the complete source code, it is           copyrighted   software,  not  public   domain.  However,  it  is           available for free under the GNU Public License. See the GPL for           more information.  The programs that  run under  Linux have each           their own copyright, although  much of it uses  the GPL as well.           All of the software on the  FTP site is freely distributable (or           else it shouldn't be there)It says although Linux is available for free, but still its source code is copyrighted not public domainWhat does it mean by saying that its copyrighted by still available for free, the file specifically says that the source is not under public domain, so is it against the law to modify linux kernel?
Is Linux kernel free to modify?
linux;licenses;gpl
Copyrighted means there is a copyright and license protecting that. The license in the case of the Linux kernel is GPL (http://www.gnu.org/copyleft/gpl.html).In a nutshell, you are allowed to modify the code in any way you wish. However, if you distribute your modified code, you have to license it GPL and keep the credit to the original authors. Also, if you distribute compiled versions of the modified source, you have to distribute that modified source code.The kernel's license is a so called copyleft, you do what you want but you have to let others do the same to your modifications.PS: this is a very simple explanation, for more information and details see the above link.