The GNU GPL is the most widely used free software license and has a strong copyleft requirement. When distributing derived works, the source code of the work must be made available under the same license. There are multiple variants of the GNU GPL, each with different requirements.
The GNU General Public License (GPL)Version 2, June 1991Copyright (C) 1989, 1991 Free Software Foundation, Inc.51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USAEveryone is permitted to copy and distribute verbatim copies of this licensedocument, but changing it is not allowed.PreambleThe licenses for most software are designed to take away your freedom to shareand change it. By contrast, the GNU General Public License is intended toguarantee your freedom to share and change free software--to make sure thesoftware is free for all its users. This General Public License applies tomost of the Free Software Foundation's software and to any other program whoseauthors commit to using it. (Some other Free Software Foundation software iscovered by the GNU Library General Public License instead.) You can apply it toyour programs, too.When we speak of free software, we are referring to freedom, not price. OurGeneral Public Licenses are designed to make sure that you have the freedom todistribute copies of free software (and charge for this service if you wish),that you receive source code or can get it if you want it, that you can changethe software or use pieces of it in new free programs; and that you know youcan do these things.To protect your rights, we need to make restrictions that forbid anyone to denyyou these rights or to ask you to surrender the rights. These restrictionstranslate to certain responsibilities for you if you distribute copies of thesoftware, or if you modify it.For example, if you distribute copies of such a program, whether gratis or fora fee, you must give the recipients all the rights that you have. You mustmake sure that they, too, receive or can get the source code. And you mustshow them these terms so they know their rights.We protect your rights with two steps: (1) copyright the software, and (2)offer you this license which gives you legal permission to copy, distributeand/or modify the software.Also, for each author's protection and ours, we want to make certain thateveryone understands that there is no warranty for this free software. If thesoftware is modified by someone else and passed on, we want its recipients toknow that what they have is not the original, so that any problems introducedby others will not reflect on the original authors' reputations.Finally, any free program is threatened constantly by software patents. Wewish to avoid the danger that redistributors of a free program willindividually obtain patent licenses, in effect making the program proprietary.To prevent this, we have made it clear that any patent must be licensed foreveryone's free use or not licensed at all.The precise terms and conditions for copying, distribution and modificationfollow.TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION0. This License applies to any program or other work which contains a noticeplaced by the copyright holder saying it may be distributed under the terms ofthis General Public License. The "Program", below, refers to any such programor work, and a "work based on the Program" means either the Program or anyderivative work under copyright law: that is to say, a work containing theProgram or a portion of it, either verbatim or with modifications and/ortranslated into another language. (Hereinafter, translation is includedwithout limitation in the term "modification".) Each licensee is addressed as"you".Activities other than copying, distribution and modification are not covered bythis License; they are outside its scope. The act of running the Program isnot restricted, and the output from the Program is covered only if its contentsconstitute a work based on the Program (independent of having been made byrunning the Program). Whether that is true depends on what the Program does.1. You may copy and distribute verbatim copies of the Program's source code asyou receive it, in any medium, provided that you conspicuously andappropriately publish on each copy an appropriate copyright notice anddisclaimer of warranty; keep intact all the notices that refer to this Licenseand to the absence of any warranty; and give any other recipients of theProgram a copy of this License along with the Program.You may charge a fee for the physical act of transferring a copy, and you mayat your option offer warranty protection in exchange for a fee.2. You may modify your copy or copies of the Program or any portion of it, thusforming a work based on the Program, and copy and distribute such modificationsor work under the terms of Section 1 above, provided that you also meet all ofthese conditions:a) You must cause the modified files to carry prominent notices statingthat you changed the files and the date of any change.b) You must cause any work that you distribute or publish, that in whole orin part contains or is derived from the Program or any part thereof, to belicensed as a whole at no charge to all third parties under the terms ofthis License.c) If the modified program normally reads commands interactively when run,you must cause it, when started running for such interactive use in themost ordinary way, to print or display an announcement including anappropriate copyright notice and a notice that there is no warranty (orelse, saying that you provide a warranty) and that users may redistributethe program under these conditions, and telling the user how to view a copyof this License. (Exception: if the Program itself is interactive but doesnot normally print such an announcement, your work based on the Program isnot required to print an announcement.)These requirements apply to the modified work as a whole. If identifiablesections of that work are not derived from the Program, and can be reasonablyconsidered independent and separate works in themselves, then this License, andits terms, do not apply to those sections when you distribute them as separateworks. But when you distribute the same sections as part of a whole which is awork based on the Program, the distribution of the whole must be on the termsof this License, whose permissions for other licensees extend to the entirewhole, and thus to each and every part regardless of who wrote it.Thus, it is not the intent of this section to claim rights or contest yourrights to work written entirely by you; rather, the intent is to exercise theright to control the distribution of derivative or collective works based onthe Program.In addition, mere aggregation of another work not based on the Program with theProgram (or with a work based on the Program) on a volume of a storage ordistribution medium does not bring the other work under the scope of thisLicense.3. You may copy and distribute the Program (or a work based on it, underSection 2) in object code or executable form under the terms of Sections 1 and2 above provided that you also do one of the following:a) Accompany it with the complete corresponding machine-readable sourcecode, which must be distributed under the terms of Sections 1 and 2 aboveon a medium customarily used for software interchange; or,b) Accompany it with a written offer, valid for at least three years, togive any third party, for a charge no more than your cost of physicallyperforming source distribution, a complete machine-readable copy of thecorresponding source code, to be distributed under the terms of Sections 1and 2 above on a medium customarily used for software interchange; or,c) Accompany it with the information you received as to the offer todistribute corresponding source code. (This alternative is allowed onlyfor noncommercial distribution and only if you received the program inobject code or executable form with such an offer, in accord withSubsection b above.)The source code for a work means the preferred form of the work for makingmodifications to it. For an executable work, complete source code means allthe source code for all modules it contains, plus any associated interfacedefinition files, plus the scripts used to control compilation and installationof the executable. However, as a special exception, the source codedistributed need not include anything that is normally distributed (in eithersource or binary form) with the major components (compiler, kernel, and so on)of the operating system on which the executable runs, unless that componentitself accompanies the executable.If distribution of executable or object code is made by offering access to copyfrom a designated place, then offering equivalent access to copy the sourcecode from the same place counts as distribution of the source code, even thoughthird parties are not compelled to copy the source along with the object code.4. You may not copy, modify, sublicense, or distribute the Program except asexpressly provided under this License. Any attempt otherwise to copy, modify,sublicense or distribute the Program is void, and will automatically terminateyour rights under this License. However, parties who have received copies, orrights, from you under this License will not have their licenses terminated solong as such parties remain in full compliance.5. You are not required to accept this License, since you have not signed it.However, nothing else grants you permission to modify or distribute the Programor its derivative works. These actions are prohibited by law if you do notaccept this License. Therefore, by modifying or distributing the Program (orany work based on the Program), you indicate your acceptance of this License todo so, and all its terms and conditions for copying, distributing or modifyingthe Program or works based on it.6. Each time you redistribute the Program (or any work based on the Program),the recipient automatically receives a license from the original licensor tocopy, distribute or modify the Program subject to these terms and conditions.You may not impose any further restrictions on the recipients' exercise of therights granted herein. You are not responsible for enforcing compliance bythird parties to this License.7. If, as a consequence of a court judgment or allegation of patentinfringement or for any other reason (not limited to patent issues), conditionsare imposed on you (whether by court order, agreement or otherwise) thatcontradict the conditions of this License, they do not excuse you from theconditions of this License. If you cannot distribute so as to satisfysimultaneously your obligations under this License and any other pertinentobligations, then as a consequence you may not distribute the Program at all.For example, if a patent license would not permit royalty-free redistributionof the Program by all those who receive copies directly or indirectly throughyou, then the only way you could satisfy both it and this License would be torefrain entirely from distribution of the Program.If any portion of this section is held invalid or unenforceable under anyparticular circumstance, the balance of the section is intended to apply andthe section as a whole is intended to apply in other circumstances.It is not the purpose of this section to induce you to infringe any patents orother property right claims or to contest validity of any such claims; thissection has the sole purpose of protecting the integrity of the free softwaredistribution system, which is implemented by public license practices. Manypeople have made generous contributions to the wide range of softwaredistributed through that system in reliance on consistent application of thatsystem; it is up to the author/donor to decide if he or she is willing todistribute software through any other system and a licensee cannot impose thatchoice.This section is intended to make thoroughly clear what is believed to be aconsequence of the rest of this License.8. If the distribution and/or use of the Program is restricted in certaincountries either by patents or by copyrighted interfaces, the originalcopyright holder who places the Program under this License may add an explicitgeographical distribution limitation excluding those countries, so thatdistribution is permitted only in or among countries not thus excluded. Insuch case, this License incorporates the limitation as if written in the bodyof this License.9. The Free Software Foundation may publish revised and/or new versions of theGeneral Public License from time to time. Such new versions will be similar inspirit to the present version, but may differ in detail to address new problemsor concerns.Each version is given a distinguishing version number. If the Programspecifies a version number of this License which applies to it and "any laterversion", you have the option of following the terms and conditions either ofthat version or of any later version published by the Free Software Foundation.If the Program does not specify a version number of this License, you maychoose any version ever published by the Free Software Foundation.10. If you wish to incorporate parts of the Program into other free programswhose distribution conditions are different, write to the author to ask forpermission. For software which is copyrighted by the Free Software Foundation,write to the Free Software Foundation; we sometimes make exceptions for this.Our decision will be guided by the two goals of preserving the free status ofall derivatives of our free software and of promoting the sharing and reuse ofsoftware generally.NO WARRANTY11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FORTHE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISESTATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THEPROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED,INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY ANDFITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY ANDPERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE,YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILLANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THEPROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANYGENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE ORINABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATABEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR AFAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDEROR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.END OF TERMS AND CONDITIONSHow to Apply These Terms to Your New ProgramsIf you develop a new program, and you want it to be of the greatest possibleuse to the public, the best way to achieve this is to make it free softwarewhich everyone can redistribute and change under these terms.To do so, attach the following notices to the program. It is safest to attachthem to the start of each source file to most effectively convey the exclusionof warranty; and each file should have at least the "copyright" line and apointer to where the full notice is found.One line to give the program's name and a brief idea of what it does.Copyright (C) <year> <name of author>This program is free software; you can redistribute it and/or modify itunder the terms of the GNU General Public License as published by the FreeSoftware Foundation; either version 2 of the License, or (at your option)any later version.This program is distributed in the hope that it will be useful, but WITHOUTANY WARRANTY; without even the implied warranty of MERCHANTABILITY orFITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License formore details.You should have received a copy of the GNU General Public License alongwith this program; if not, write to the Free Software Foundation, Inc.,51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.Also add information on how to contact you by electronic and paper mail.If the program is interactive, make it output a short notice like this when itstarts in an interactive mode:Gnomovision version 69, Copyright (C) year name of author Gnomovision comeswith ABSOLUTELY NO WARRANTY; for details type 'show w'. This is freesoftware, and you are welcome to redistribute it under certain conditions;type 'show c' for details.The hypothetical commands 'show w' and 'show c' should show the appropriateparts of the General Public License. Of course, the commands you use may becalled something other than 'show w' and 'show c'; they could even bemouse-clicks or menu items--whatever suits your program.You should also get your employer (if you work as a programmer) or your school,if any, to sign a "copyright disclaimer" for the program, if necessary. Hereis a sample; alter the names:Yoyodyne, Inc., hereby disclaims all copyright interest in the program'Gnomovision' (which makes passes at compilers) written by James Hacker.signature of Ty Coon, 1 April 1989Ty Coon, President of ViceThis General Public License does not permit incorporating your program intoproprietary programs. If your program is a subroutine library, you mayconsider it more useful to permit linking proprietary applications with thelibrary. If this is what you want to do, use the GNU Library General PublicLicense instead of this License."CLASSPATH" EXCEPTION TO THE GPLCertain source files distributed by Oracle America and/or its affiliates aresubject to the following clarification and special exception to the GPL, butonly where Oracle has expressly included in the particular source file's headerthe words "Oracle designates this particular file as subject to the "Classpath"exception as provided by Oracle in the LICENSE file that accompanied this code."Linking this library statically or dynamically with other modules is makinga combined work based on this library. Thus, the terms and conditions ofthe GNU General Public License cover the whole combination.As a special exception, the copyright holders of this library give youpermission to link this library with independent modules to produce anexecutable, regardless of the license terms of these independent modules,and to copy and distribute the resulting executable under terms of yourchoice, provided that you also meet, for each linked independent module,the terms and conditions of the license of that module. An independentmodule is a module which is not derived from or based on this library. Ifyou modify this library, you may extend this exception to your version ofthe library, but you are not obligated to do so. If you do not wish to doso, delete this exception statement from your version.
：Code submit frequency
：React/respond to issue & PR etc.
：Well-balanced team members and collaboration
：Recent popularity of project
：Star counts, download counts etc.