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Linux Master 2· AssociateWeek 1 · Day 3~25 min read

Day 3 - Free Software and Open Source: What a License Decides

📌 Key Takeaways

The "free" in free software means "freedom," not "no cost." It is defined by four freedoms: run, study, distribute, and improve.
The FSF (Free Software Foundation) emphasizes freedom, while the OSI (Open Source Initiative) emphasizes the practicality of the development method.
Licenses fall broadly into copyleft (GPL family) and permissive (BSD, MIT, Apache).
The GPL is viral. If you use GPL code, you must release the result under the GPL too.
The LGPL softens that virality by making an exception when you only link against a library.

The Four Freedoms of Free Software

This is the standard defined by Richard Stallman. A distinguishing feature is that the numbering starts from 0.

Number	Freedom
Freedom 0	The freedom to run it for any purpose
Freedom 1	The freedom to study and modify how it works (requires access to the source code)
Freedom 2	The freedom to redistribute copies
Freedom 3	The freedom to distribute your improved versions

A common misconception: "free software = gratis" is not correct.
The confusion arises because the English word "free" means both "no cost" and "freedom," so the FSF explains it as "free as in free speech, not as in free beer."
Selling it for money does not violate free software either. However, you must guarantee the four freedoms above to the buyer.

FSF and OSI: Same Code, Different Emphasis

Category	Free software (FSF)	Open source (OSI)
Founded	1985, Richard Stallman	1998
Core value	User freedom (ethics, philosophy)	Superiority of the development method (pragmatism)
Term	Free Software	Open Source
Representative license	GPL	Varied (including BSD, MIT, Apache, etc.)

The lists of licenses recognized by the two camps overlap almost entirely. Only the emphasis differs.
Together they are called FOSS or FLOSS (Free/Libre and Open Source Software).

The Open Source Definition (OSD)

The OSI was created in 1998 with Eric Raymond and Bruce Perens at the center. To ease the aversion the word "free" provoked in businesses, they put forward the name open source.
The OSI has the OSD (Open Source Definition), the criteria for judging which licenses are open source. It consists of 10 clauses based on the Debian Free Software Guidelines.

Number	Clause	In plain words
1	Free redistribution	Doesn't prevent selling or giving it away
2	Source code	Source must be included or easily obtainable
3	Derived works	Modified versions must be distributable under the same terms
4	Integrity of the author's source code	May require modified versions to carry a different name or version
5	No discrimination against persons or groups	Can't set conditions saying certain people can't use it
6	No discrimination against fields of endeavor	Can't impose limits like "no commercial use" or "research only"
7	Distribution of license	The same rights follow to everyone who receives it
8	License must not be specific to a product	It must not be valid only when included in a particular distribution
9	License must not restrict other software	Doesn't force conditions on other programs distributed alongside
10	License must be technology-neutral	Doesn't presuppose a specific technology or interface

⚠️ Trap: Because of clauses 5 and 6, software that is "usable for non-profit purposes only" is not open source. The same holds even if the source is published. It is wrong to jump to "source published = open source."

Similarly Named Software Categories

Not everything labeled "free" is the same. Whether the source is published and whether you pay are independent axes.

Category	Source code	Price	Key point
Commercial (proprietary) software	Closed	Paid	You buy only a right to use. No modification or redistribution
Shareware	Closed	Free for a limited period or limited features	Try it, and pay if you want to keep using it
Freeware	Closed	Free	Costs nothing, but you have no right to modify or redistribute
Public domain	No restrictions	Free	Copyright has been waived or has expired. No conditions at all
Free / open source software	Published	Can be free or paid	Copyright remains alive, and you must follow the license terms

Freeware ≠ free software. Freeware merely costs zero; it lacks the four freedoms.
Public domain ≠ open source. Open source is permission granted by the copyright holder who keeps the rights and attaches conditions, while public domain has no conditions to attach in the first place.

The Two Branches of Licenses

                    Open source licenses
                            │
          ┌─────────────────┴─────────────────┐
          ▼                                   ▼
      Copyleft                            Permissive
   (copyleft, viral)                  (permissive, lenient)
          │                                   │
   ┌──────┴──────┐                    ┌───────┼───────┐
   ▼             ▼                    ▼       ▼       ▼
  GPL          LGPL                 BSD     MIT    Apache
 (strong       (weak
  copyleft)     copyleft)

Copyleft — The GPL Family

Core rule: When you distribute a work that uses GPL code, that work must also be released with source under the GPL.
This property is called viral. It is copyleft in the sense that it flips copyright.
Note: the obligation arises only when you distribute. If you use it only inside a company and don't distribute it, there is no obligation to release.

License	Feature
GPL v2	The license the Linux kernel uses
GPL v3	Adds a patent clause and a clause preventing hardware locking (Tivoization)
LGPL	If you only link a library, you don't have to release your own code
AGPL	The source-release obligation arises even if you only provide a service over the network

Permissive — BSD, MIT, Apache

There are almost no conditions. As long as you keep the copyright notice, you can put it into a commercial product and keep the source closed.

License	Feature
BSD	Just keep the copyright notice. Very simple
MIT	Practically identical to BSD. The shortest and widely used
Apache 2.0	Copyright notice + a clause explicitly granting patent rights
MPL	File-level copyleft. You only need to release the files you modified

The exam has questions asking about "the obligations when selling a modified program distributed under the GPL." The answer is that selling itself is allowed, but you must release the source code along with it.

License Comparison at a Glance

License	Release source when distributing modifications	Combining with closed programs	Patent clause	Representative software
GPL v2	Required	The whole combined work becomes GPL	No explicit clause	Linux kernel
GPL v3	Required	The whole combined work becomes GPL	Yes	GCC, bash
LGPL	Only the parts of the library you modified	Linking is allowed	Depends on version	glibc (GNU C Library)
AGPL	Required + also for network services	The whole combined work becomes AGPL	Yes	—
MPL	Only the modified files	Allowed	Yes	Firefox
BSD	Not required	Allowed	No	FreeBSD
MIT	Not required	Allowed	No	X Window System
Apache 2.0	Not required	Allowed	Yes	Apache web server, Android (mostly)

Reading the table from top to bottom goes from strongest conditions to weakest. The GPL and AGPL are the strongest, and BSD and MIT the weakest.
Where Apache departs from BSD and MIT is patents. It includes a sentence in which contributors license their patents to users, and modified files must also state the fact that they were modified.
BSD originally had an advertising clause requiring "mention the University of California, Berkeley in ads for products that use this software" (4-clause BSD). The version without this clause is the 3-clause and 2-clause BSD commonly used today.

What the GPL Actually Requires

When you hand over a GPL program (or a modified version of it) to others, what you must do boils down to four things.

Provide the source code along with it, or attach in writing a promise to provide it on request.
Provide the full GPL text (a copy of the license) along with it.
Don't remove the original copyright notices.
Don't impose additional restrictions on recipients — you can't add conditions like "no redistribution."

Version	Released	What to remember
GPL v1	1989	The first version
GPL v2	1991	Adopted by the Linux kernel. Used widely for the longest time
GPL v3	2007	Added a patent clause and anti-Tivoization

🔍 Going deeper: Tivoization comes from the digital video recorder company TiVo. TiVo released the Linux kernel source as the GPL requires, but made its devices refuse to run software lacking the manufacturer's signature. A situation where you got the source but couldn't run your modified version — GPL v3 added a clause to prevent this, and the Linux kernel did not agree to that change, so it stayed at v2.

Judging by Situation

Situation	Source-release obligation
You modify a GPL program and use it only in-house	None — it isn't distribution
You modify a GPL program and sell it to customers	Yes — you must give the source to the buyer
You embed GPL code in your program and distribute it	Yes — your whole program becomes GPL
You distribute a program that only links an LGPL library	None for your code (but yes for the library part if you modified it)
You modify an AGPL program and offer it only as a web service	Yes — you must release to users even without distributing
You embed MIT/BSD code and sell a commercial product	None — just keep the copyright notice and license text

Companies sometimes offer both the GPL and a commercial license (dual licensing). Customers who find the GPL terms burdensome pay for a commercial license. MySQL is the classic example.

Commonly Confused Pairs

Question	Answer
Is free software gratis?	No. Selling it for a fee is also possible
Can you sell a GPL program?	Yes. But you must release the source
If you use it only in-house, must you release it?	No. Without distribution there is no obligation
Can you put BSD code in a commercial product?	Yes. Just keep the copyright notice
What is the Linux kernel's license?	GPL v2

Tomorrow we look at the unit that bundles this kernel and these tools together for sale — the distribution.

📖 Glossary

Free software : Software that guarantees the four freedoms of running, studying, redistributing, and improving.
FSF : The Free Software Foundation. Founded by Richard Stallman in 1985.
OSI : The Open Source Initiative. Founded in 1998, it emphasizes the practicality of the development method.
Copyleft : A licensing approach that enforces the same freedoms on derived works. The GPL is the classic example.
Permissive : A lenient license with almost no conditions. BSD and MIT are the classic examples.
GPL : The GNU General Public License. The Linux kernel follows GPL v2.
LGPL : The GPL for libraries. It exempts you from the source-release obligation when you only link.
FOSS/FLOSS : A term covering both free software and open source.
Freeware : Software that can be used for free but whose source is not published and that carries no right to modify or redistribute.
Public domain : A state in which copyright has been waived or has expired and the work can be used with no conditions.

📝 Practice Questions

Click a choice to reveal the answer and explanation.

Question 1

Which of the following is the license adopted by the Linux kernel?

Question 2

Which of the following correctly describes free software?

Question 3

Which of the following best describes the GPL license?

Question 4

Which of the following is the license that relaxes the rules so that you don't have to release your own source code when you link and use a library?

Question 5

Which of the following groups consists of permissive licenses?

Official references

Linux Master 2 official exam syllabus(opens in a new tab)