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Transcript of Richard Stallman on GPLv3 in Brussels, Belgium; 1st of April 2007
Transcript of Richard Stallman on GPLv3 in Brussels, Belgium; 1st of April 2007

Transcript released 4th of April 2007. Background and related documents:
Recording thanks to Sean Daly. Transcription by Ciarán O’Riordan. Please support work such as this by joining the Fellowship of FSFE, by donating to FSFE, and by encouraging others to do each. The speech was given in English.

Table of contents
The Presentation

(go to menu) [Section: What is Free Software?]

In order to understand the GNU General Public License, first you have to understand what free software means. Free Software means software that respects the users’ freedom. Software that’s not free is proprietary software. Non-free software, user subjugating software, is distributed in a social system that keeps the users the divided and helpless. Divided because every user is forbidden to share the program with anyone else and helpless because the users don’t have the source code, so they can’t change it, they have no control over it, and they can’t even verify independently what it’s really doing. It may have malicious features, quite often it does, and the users may not even be able to tell what malicious features it has.

Free Software respects the users’ freedom. That’s why it’s important when we say free software, we’re talking about freedom, not price, we don’t mean gratis software. It’s not an issue of price at all, price is merely a detail. A secondary detail, not an ethical issue. To understand the term Free Software correctly, think of free speech, not free beer.

Specifically, Free Software means that the user has four essential freedoms:
- Freedom zero is the freedom to run the program as you wish.
- Freedom one is the freedom to study the source code and change it so that the program does what you wish when you run it.
- Freedom two is the freedom to help your neighbour; that is, the freedom to distribute exact copies to others, when you wish.
- Freedom three is the freedom to contribute to your community; that’s the freedom to distribute copies of your modified versions, when you wish.
Each of these freedoms is the freedom to do something if you wish, when you wish. It’s not an obligation, it’s not a requirement. You’re not required to run the program. You’re not required to study it or change it. You’re not required to distribute exact copies. And you’re not required to distribute modified versions. But, you are free to do those things, when you wish.

If this were a speech to introduce Free Software I would explain the reasons why those freedoms are essential, but in this brief introductions I will have to omit that. Suffice it to say that if a program respects these four freedoms, then the social system of its distribution and use is an ethical system. Which means that, in this regard at least, the software is ethically legitimate. But if one of these freedoms is substantially missing, then the program is proprietary software, meaning that the social system of its distribution and use is unethical and regardless of what the program does, it shouldn’t be distributed in this way.

Developing a non-free program is no contribution to society. It’s an attack on freedom and social solidarity. Such attacks ought to be discouraged, and I hope to see the day when they no longer occur. That is the goal of the free software movement; that all computer users should have these four freedoms, for all the software that they use. That subjugation of others, through the software that they run, should no longer occur.

(go to menu) [Section: The GNU project]

To bring this about, or at least start, in 1984 I began developing an operating system whose purpose was to be entirely Free Software. It’s name is GNU, which is a recursive acronym for “GNU’s Not Unix”. This system is, technically speaking, compatible with Unix, but the most important thing about it is that it is not Unix. Because Unix is proprietary software and GNU, because it’s not Unix, can be made free by us, it’s developers, and that’s the whole point – to give you an operating system that you can run without ceding your freedom to anyone.

There are now tens of millions, perhaps a hundred million users of a variant of the GNU system. Most of them don’t know that it’s a variant of the GNU system because they think it’s Linux. Linux is actually one component of the system they use. A component that does an essential job and that was developed in 1991, and liberated by its developer in 1992. The GNU system plus Linux, which is the program we call the kernel, made a complete free operating system in 1992. With the GNU plus Linux combination it became possible for the first time to use a PC compatible computer in freedom.

(go to menu) [Section: Free Software licences]

The way that Linux was liberated was by rereleasing it under the GNU General Public License. The GNU General Public License is the Free Software licence that I wrote for use in the components of the GNU system. When we wrote these components, we made them Free Software by releasing them under the GNU GPL.

But what does that mean? A program is legally considered a literary work, and it’s subject to copyright. Everything that you write is automatically copyrighted – which is somewhat of an absurd law, but that’s the way it is – and copyright law, by default, forbids copying, distribution, modification of a work. So, how can any program ever be free software? The only way is if the copyright holders put on a notice saying to the users “you have the four freedoms, because we don’t object”. That notice is a free software licence. Any notice which has that effect is a free software licence.

There are actually dozens of different free software licences but the GNU GPL is the most popular one. It’s used for about 70% of all free software packages. The main difference between the GNU GPL and most Free Software licences is that the GNU GPL is a copyleft licence.

Copyleft is a technique that I invented, a way of using copyright law to defend the freedom of other people. In order to be a free software licence, the licence has to recognise the four freedoms, if you get a program under any free software licence, you have the four freedoms. The licence gives you them. And that includes freedom number three, the freedom to distribute copies of modified versions, and freedom number two, the freedom to distribute copies without modification, but when you do that, when you distribute those copies to other people, will they have the four freedoms?

Maybe yes, maybe no, it depends how you do it. For instance, even in 1984, I could see there was a danger that you might distribute a binary of the program without providing the source code, and that would deny the subsequent recipients freedom number one. The freedom to study the source code and change it so the program does what they wish.

So if you could distribute copies without source code, people would get it from you and they wouldn’t have the four freedoms. Not all of them.

Another thing you might conceivably do is put on additional restrictions. When you distribute it, perhaps you would put on a different licence and that licence might be restrictive, it might not respect the four freedoms for other people. If you could do that, they who got the program from you would not have Free Software. Many Free Software licences permit these things, so they respect the four freedoms but they don’t defend the four freedoms. A middle man could strip off the freedom from the program and then pass on the program to you and you would get the code but you wouldn’t get freedom. You might just get a binary, or you might get a binary with a licence that says you’re not allowed to copy it. Or you might even get source code, but it would say you’re not allowed to copy it. Various things might happen which would result in your not getting Free Software.

My goal, in developing the GNU operating system, was specifically to give freedom to all computer users. If it were possible for middle men to make the software non-free – if, for instance, by changing it, that they had an excuse to make the software non-free, the goal would be defeated. We would fail to provide freedom to the users.

So I designed a Free Software licence specifically to make sure that everyone who gets any version of the program gets it as free software, with the four freedoms. In version one of the GNU GPL, which was published in 1989, it was designed to block the two kinds of attack that I just described to you.

One being: don’t distribute the source code, release only a binary. Well, GPL version one said a condition of distributing binaries was that you make source code available also, to the same people. That blocked that one attack.

The other attack was to put on additional licence terms, to change the licence. Well, GPL version one said you have permission to distribute copies but it has to be under this licence, no more and no less. Any other way, you’re not allowed to distribute. So, no adding other requirements. No removing the requirements that protect peoples’ freedom. Every copy that everyone gets, legally must be distributed under the same licence: the GNU GPL.

In 1991, I published version two of the GNU GPL, and this blocked another possible method of attacking users’ freedom. A method I didn’t know about in 1989 but which I became aware of afterward. This would be that a patent holder might sue a redistributor, and in the settlement the redistributor might agree “when we redistribute this program, we will put certain restrictions on the users”.

What could we do about that? Software patents are weapons that should not exist. They attack the freedom of computer users, they sabotage software development, they’re only good for the megacorporations. It’s the megacorporations that lobby for them, with the help of their pet government in Washington.

The fact is, in countries foolish enough to authorise software patents, a patent holder can stop the distribution of any program that implements the patented idea, and there’s nothing we can do in the licence of the program to prevent it from being suppressed in this way. Because the free software licence is just permission to use the copyright holder’s copyrighted work, it has nothing to do with somebody else’s patent.

We can’t prevent a patent holder from killing the program, but, we can hope to save the program from a fate worse than death, which is, to be made non-free, to be turned into an instrument of subjugation. Better that our software should cease to exist, at least for 20 years, the duration of a patent, than it become an instrument for subjugation. And that, I found a way to do. So in GPL version two, section seven says that if any conditions are imposed on you or you agree to any conditions that forbid you to distribute the program with all the freedoms granted by the GPL, then you can’t distribute at all.

So anyone who tries to sue or threaten you, and tries to settle this in a way that would turn the program into an instrument of subjugation, discovers that all he has achieved is to kill it. …which at least avoids the fate worse than death for the program. And in fact, this gives our community a certain measure of safety because very often the patent holder will not find it particularly advantageous to kill the program. On the other hand, if the patent could make it effectively non-free and charge people for permission to run it, that would be advantageous. So if the patent holder could inflict a fate worse than death, that would be more tempting than merely to kill it off. So we actually make it less likely that any bad thing will happen, by standing firm.

GPL version two has been used now for sixteen years. Over the years, we saw various reasons to change it. Two years ago, we decided it was time to start seriously working on this and get out a new version. We realised it would take time. I set aside several months to work on GPL version three in 2005, so as to prepare the first draft that was released in January 2006.

We have just published the third discussion draft and we’ve decided to wait for sixty days and then prepare what we hope will be the final draft for the last bit of comments before the final draft.

So what are the changes we’ve made?

People often ask for a simple summary of these changes, but there can’t be one, and the reason is that the changes are all in specifics because the basic idea is the same: defend freedom for all users. That will never change. So all the changes are in specific areas and details. Let me tell you about the most important ones. One of them is internationalisation.

(go to menu) [Section: Internationalisation]

We have changed the language to avoid certain terms whose meanings vary more than necessary between countries. Terms like “distribute”. In GPL version three, we will not use the word “distribute”. We’ve formulated two new terms, and defined them in ways that get results that are as uniform worldwide as possible. The first term is “propagate”. That means basically copying, or anything else like it, that requires permission under copyright law.

Anything except just to modify one copy or run it is to propagate the work. And then the second term is “convey”. “Convey” basically replaces “distribute to others” but it’s not defined in terms of the word “distribute”, it’s defined in terms of propagate in such a way that others may get copies. So we avoid the word “distribution”, and then, in the bulk of the licence, we set terms for propagating the work and terms for conveying the work. And this way the resulting conditions are as uniform as possible worldwide. Now, they will never be exactly uniform, as long as some details of copyright law vary.

(go to menu) [Section: Tivoisation]

Another major change consists of a new form attacking a user’s freedom that we’ve seen in the past few years. It’s called tivoisation. This is the practice of designing a machine so that if the user installs a modified version of a program, the machine refuses to run it.

It’s named after the first product I heard of which did this, which is called the Tivo. The Tivo contains Free Software released under version two, and they provide the source code, so the user of the Tivo can modify the program and compile it, and install the modified version in his machine, whereupon the machine won’t run at all because it notices that this is a modified version. This means that in some nominal sense, the user has freedom number one, but really, in practical terms it has been taken away, it has been turned into a sham. And this happens systematically, and it makes a systematic threat to users’ freedom. So we’ve decided to block this, and the way we block it is as part of the conditions for distributing binaries, we say that if you distribute in, or for use in, a certain product, then you must provide whatever the user needs in order to install her own modified version and have them function the same way, unless her changes in the code change the function. But the point is that it’s not just the user has to be able to install it and has to be able to run, but it has to be able to do the same job, despite having been modified.

If the mere fact that the program is modified is detected and prevents the program from doing the same job, that’s also a violation, that means that the installation information is insufficient, and that is what would happen under Microsoft’s scheme that it used to call Palladium. The idea was that files would be encrypted so that only a particular program could ever read them. If you got the source code of that program and you modified it and you compiled it, the checksum of your version would be different. So your version would be unable to read the files encrypted for the original version. And the original version would be unable to read any files encrypted for your version except that there aren’t any. They would pretend that this is just a symmetrical situation and they haven’t done anything to shaft you, but our conditions say they have to give you what it takes so that you can make your modified version do the same job, and the mere fact that it’s been modified may make it fail to do that job.

(go to menu) [Section: Tivoisation – the limits in draft 3]

In version three we’ve decided to limit this requirement somewhat. It’s limited to a category that we call “user products”, which include consumer products and anything that’s going to be built into a house. This is to exclude products that are made specifically for businesses and are not normally used by consumers at all.

The reason we did this is because the big danger is in the area of user products, and this way we were able to get at least a partial agreement of businesses that at least we hope will be able to help us actually put an end to this threat.

The products that do tivoisation typically do it because there is some other malicious feature in the product. For instance, the Tivo spies on the user and it implements Digital Restrictions Management. That’s why they want to stop you from changing the program. Digital Restrictions Management is unethical. It shouldn’t ever be tolerated. And spying on the user is unethical also, but we have not put anything in the GPL to say that you can’t make the software spy on the user or that it has to be able to copy things. There is no restriction at all on what functionality your modified version of the program can have.

The anti-tivoisation requirement doesn’t limit you in that regard. You can implement the nastiest features you can think of and release a modified version with those nasty features. We’re only trying to make sure that the users who get the program from you will have the freedom to remove those nasty features and fill in whatever useful features you took out. They should be free to modify it just as you were free to modify it. That’s what the anti-tivoisation provisions do.

(go to menu) [Section: Deflating the EUCD and DMCA]

We also have provisions in section three to try to defeat unjust laws such as the Digital Millennium Copyright Act in the US and the EU Copyright Directive and any similar laws that implement that WIPO copyright directive – a treaty which no government representing its citizens would ever sign.

It actually has two paragraphs. One which is aimed at the DMCA and similar laws, and another which is aimed at the EU Copyright Directive, but they both achieve the same goal. The goal they achieve is, if somebody uses GPL covered software as part of a scheme to encode or decode works, then he has waived any possibility of claiming that some other software which decodes those works is a violation of these laws. This is simply a way of trying to prevent people being forbidden to distributed software, perhaps modified versions of the same software or perhaps not but either way, we’re making sure that these laws can’t be used to suppress the modified versions that people may wish to distribute under the GPL.

[26:30]

(go to menu) [Section: Novell, Microsoft, and patents]

There’s another form of attack on Free Software’s freedoms that we found out about last November with the Novell-Microsoft deal. What happened was that Novell made a deal with Microsoft where Novell pays for distributing copies of GPL-covered software and Microsoft gives the customers of Novell a very limited patent licence which is conditional on their not exercising many of the rights that the GPL gives them.

This is a big threat, so we’ve gone at it from two directions. One is aimed at Microsoft’s role in that deal. We say: if you make a deal to procure someone else’s distribution of a program under GPL version three and you provide any sort of patent licence to anybody in connection with that, then it extends to anybody who gets it. So if Novell were to distribute software under GPL version three under this deal, then this affects Microsoft because they’re procuring distribution through this deal.

The other paragraph, and these are both in section [11], is aimed at the Novell side in the deal, which is, it says that if you distribute the program under an arrangement you made with someone else, to gain promises of patent safety for your customers in a discriminatory way, then you’re violating the licence and you lose your right to distribute.

This actually has a few more conditions because we were trying to avoid covering certain other things, for instance, consider a patent parasite, one of those companies that has only one business which is to go around threatening people with patent law suits and making them pay. When this happens, the businesses that are attacked often have no choice but to pay them off. We don’t want to put them in a position of being GPL violators as a result. So we put in a condition: “this paragraph applies only if the patent holder makes a business of distributing software”. Patent parasites don’t. As a result, the victim of the patent parasites is not put in violation by this paragraph.

We also did something to exclude blanket cross-licences and various other practices which are not threatening in the same way.

[29:38]

This section [11], which these paragraphs are included in, does somewhat more. We decided in GPL version three that distributors should explicitly give patent licences when they distribute the software. We do this in two ways. Those that contribute to the development of the program, that make any changes at all, give an affirmative patent licence to all downstream recipients. Those who just pass along copies, without changing it, they are bound not to sue any of those downstream recipients or their licence terminates.

This is a sort of compromise that is designed to encourage lots of companies to participate in the distribution, and thus, we hope, make our community somewhat safer.

In previous versions of the GPL, we didn’t have any kind of explicit patent licence. We took for granted that if you distribute someone a program under the GPL and you say that he’s allowed to do certain things, that you can’t sue him for doing those things. And in the US, that’s true, within certain bounds, but it’s not true worldwide. So this is another aspect of internationalisation – getting the same result around the World.

(go to menu) [Section: Termination]

Another change that we have made is in termination. With GPL version two, if you violate the licence, you have lost your rights to do anything with the program. And the only way to get them back is to go to all the copyright holders and beg for forgiveness. Which in most cases they will give you, if you show that you are going to follow the licence in the future, but their may be lots and lots of copyright holders.

This could be a gigantic amount of work. It could be totally unfeasible. With GPL version three, your licence is not terminated automatically. Rather, the copyright holder can write to you can say “I’m putting you on notice”, and after that point, the copyright holder can terminate your licence, if he wants to. But he doesn’t have to. He could just forgive you instead if he sees that you’re going to comply.

The result is, suppose you do something that violates the licence, and you distribute an entire GNU/Linux distribution with thousands of programs in it, and you fail to follow the requirements. And suppose ten copyright holders contact you and say you violated the licence, and you say “Oops, I better do this right” and then sixty days go by and maybe, say, five more copyright holders will contact you within those sixty days then those are the only ones that can terminate your licence. But the idea is that you show those fifteen developers that you are complying now and you promise to keep doing so. And they say “Ok, we forgive you”.

You don’t have to find all the other thousands of copyright holders and ask them for forgiveness. So, as long as you keep on with your violating practice, all the copyright holders still can put you on notice when the word gets around to them, but when you stop violating the licence, then they have sixty more days when they can still complain, and then they can’t complain anymore. So only the ones who complain within that time are the ones you have to deal with.

[34:00]

We’ve also put in an automatic cure period for first time violators. If you violate the licence, and it’s the first time you violated the licence for a certain copyright holder and you correct it within 30 days, things are automatically restored. You’re ok. But that doesn’t apply the second time.

We’re trying to make it easy for accidental violators, those who are willing to clean up their act, while still facilitating enforcement against those who don’t clean up their act.

[34:55]

Another change we’ve made, has to do… …what we’ve done that’s new in section 10 is we’ve dealt with kinds of situations that arise when a company subdivides or sells a division to another company, what happens then? We want to make sure this is clear. It shouldn’t be very controversial, but at least now it’s well defined.

(go to menu) [Section: Formalising added permissions]

Another thing that we’ve done is that we’ve formalised what it means to give added permission. There’s a practice used with GPL version two which is common enough which is that you say “This is released under version 2 of the GPL or later, and in addition we say you can do X, Y, or Z”. Added special permission as an exception.

And this is perfectly fine since what it means to release a program under a free software licence is simply you give the permission stated in the licence and you can also give other permissions.

But in order to make it clearer what this means, we’ve formalised it. So we’ve explained that you can give additional permissions for part of the code that you introduce into the GPL covered program, where you have the right to give those permission. Because the program is available under the GPL terms, when somebody modifies it, she can take off the added permissions. The GPL gives her permission to release her version under the GPL. She doesn’t have to give any added permissions for her version. But she could also pass on those added permissions because she got those added permissions to you and when she modifies those parts, she can also give those same added permissions for those changes when she wants.

So when a program is released under GPL plus added permissions, everyone who passes it along has a choice. Preserve the added permissions or remove them. Section 7 explains this. It also explains that certain kinds of explicit requirements found in other free software licences are, under our understanding, compatible with the GPL and that’s in fact our practice with GPL version two but again, it’s good to make it explicit.

And they are basically trivial ones. Like, this copyright notice can’t be removed. Or different statements of warranty disclaimers. They’re not substantive conditions.

In draft 2, this section, section 7, was more complicated. We set it up so that there were a few specific kinds of substantive requirements that you could add. And so we’d need a lot more complicated mechanism to keep track of them. A lot of people didn’t like the complexity of this. So we got rid of that and we dealt with those issues in other ways. For instance, there’s a section in the latest draft which says when a program is released under GPL version three, you can link it with code that is under the Affero GPL. In a previous draft, we said that the added requirement in the Affero GPL was one of those requirements that you could add to parts of a program that you contribute. Now, it isn’t. Now, the GPL is purely and simply the GPL and it only allows linking with other code that was released by its developers under the Affero GPL.

The Affero GPL, for those who don’t know, is basically GNU GPL version two, plus one additional requirement which says if you use the software publicly on a website, then you’ve got to give the users of the site a way to download the source code you’re running. The source code corresponding to the version they are talking through.

The basic idea is that for those developing software meant for use in this way, on servers, they want those who publicly deploy modified versions to contribute their code back to the community.

When we advised Affero on releasing this licence, we had the intention of making a future GPL version compatible with it and this is how we’re going to achieve it.

(go to menu) [Section: BitTorrent]

Another change that we’ve made is for support of things like BitTorrent. BitTorrent is peculiar because when you receive copies, you automatically end up distributing copies to other people and you don’t even know it. This is so bizarre, that I never imagined any such thing back in 1991. And in fact, distributing a program under GPL version two using BitTorrent does things that violate the GPL.

All these people are redistributing copies and they don’t know it, and they are failing to carry out their responsibilities under the GPL. So we changed it so they won’t have any problem. Obviously, distribution using BitTorrent should be OK.

[42:00]

The problem has to do with binaries. The GPL’s idea is that if you distribute binaries, you’ve got to make source available. But if somebody’s participating in a torrent for the binaries, well, he may not know where the source code is, and he didn’t set it up anyway, and we don’t want to make him responsible for dealing with the source code, as a mere user trying to get a copy through the torrent.

Another change, that will make things simpler for a lot of people, is that you will now be permitted to distribute physical copies of binaries and put the source code on a server for people to get. The reason that wasn’t allowed in the past was that for a lot of users in the past, downloading the corresponding source code might have been prohibitively slow and expensive.

In a lot of parts of the World, certainly, sixteen years ago, people wouldn’t have had broadband. All they would have had is dial-up connections. You could give somebody a CD of an entire system, and if you could say to him “to get the source code, just pull it down through your phone line”, that would have been ridiculous. So we said you have to offer them to mail them a source CD, or other source medium, which would have been a lot cheaper and much more feasible than getting the whole thing through a phone line. But nowadays in fact there are services which which download anything for you and mail it to you on a CD anywhere in the World and they don’t charge very much. So the problem we were trying to protect against, which is that lots of users would find it unfeasible to get the source, isn’t a problem anymore. These services charge less than a mail order service required by GPL version two is likely to cost. So we can just say, we presume people will use those services if they need them and we don’t have to put that requirement on every distributor.

[44:44]

I think this is basically it. There are many little details, and I certainly have forgotten about some of them right now.

We took out the paragraph in GPL version two about putting on a geographical limit saying that you can’t distribute the program in a certain country if something like patents in that country have effectively made the program non-free. We think we’ve defended against that pretty well and nobody ever used that anyway.

So, in about 90 days…

[interruption for tape change]

(go to menu) [Section: Patent retaliation and the Apache licence]

Previous drafts had a couple of provisions regarding retaliation against aggression using software patents. First of all, there was a very limited kind of retaliation directly in the GPL itself which said that if somebody started using a modified version of a program and then sued somebody else for patent infringement for making similar improvements in his own version, that this would cause termination of a specific right, namely the right to continue modifying the program.

We took that out because it didn’t seem like it would be terribly effective. We also, in section 7, where it allowed putting on additional requirements, one of them was a stronger kind of patent retaliation and there were two kinds that were allowed. The latest draft essentially does one of them itself, and so we don’t need to make that another kind of requirement you can add. And the other kind, it looked like nobody’s actually doing it, so just for simplicity we took out that option.

The reason that we allowed people to add these two kinds of patent retaliation clauses was for compatibility, and one of them is used by the Apache licence. We hoped to make GPL version three compatible with the Apache licence and we thought we had. We were focusing on the Apache licence’s patent retaliation clause, and in previous drafts we said it was ok to add that requirement to the code you contribute. Now we just have that requirement, later on in the GPL, so it doesn’t even need to be added.

So we are compatible with that clause in the Apache licence but a few months ago we noticed that there was another clause in the Apache licence requiring indemnity in certain cases, and there’s no way we can be compatible with that. So we’re not going to achieve that goal of making GPL version three compatible with the existing Apache licence. I regret that.

(go to menu) [Section: The bracketed, dated clause]

There’s one bracketed clause in the current draft and that concerns a cut-off date in the paragraph that forbids making deals to get patent safety for your customers alone. We haven’t decided whether that will apply to deals that have already been signed, or only deals that are signed after the release of this draft. And that mainly depends on how good a job we find we have done drawing a line between these pernicious deals and other kinds of deals that we don’t want to cover. We’ve come up with some criteria that seemed to do the job, and we hope to find out whether we’ve really done the job.

We think that we have addressed the specific deal between Novell and Microsoft sufficiently well with the other requirement, the one that is aimed at Microsoft’s role in that deal. Even if we post-date the effect of the second paragraph, the one that aims at the Novell role, such that it doesn’t apply to Novell, we’ve dealt with that particular deal. But if we have drawn the lines well enough, or if we can, then we’ll make it apply both to existing and to new deals. It’s a decision we haven’t made yet.

[52:00, end]

[Note: for more information on GPLv3, see FSFE’s GPLv3 page and the background and related documents linked at the top of this page]
13 11 月, 2015
Tivoisation and the open source debate

http://www.lexology.com/library/detail.aspx?g=af98fe0c-83ae-4710-9caa-4ab2688ca98a

Germany, Global, USA June 2 2008

What is Tivoisation?

Tivoisation is the creation of a computer system that incorporates open source software but uses technical methods to prevent users from running modified versions of the software on that hardware. The term arose as a result of TiVo‘s use of open source software that had been licensed under the terms of the Free Software Foundation’s GPL licensing arrangements on the TiVo brand digital video recorders. In general terms if a product is “TiVo’d” then:

the manufacturer puts a chip in the computer which checks any software before it is run and which will only allow authorised software to be run;
the chip can recognise authorised software by, for example, comparing a checksum (like a fingerprint) to a list of authorised checksums, or by checking for an encrypted signature; and
the manfucturer withholds the information which the user would need in order to make software authorised.

By doing this, the manufacturer can still publish new versions of the software for download to the device. The manufacturer embeds the encrypted signature in the new version of the software, or sends a remote command which would add the checksum of the new version to the list of authorised checksums.

However, if a user tries to use a modified version of the software, or tries to run some third-party software, the device will refuse to function fully, or will simply not run the software at all.

Conflict with elements of the Open Source Movement

Some of the proponents of open source software, and in particular the Free Software Foundation (with Richard Stallman leading the debate), believe that Tivoisation is inconsistent with open source principles by denying users key elements of the freedoms that the GNU General Public Licence (GPL) was designed to promote.

TiVo’s software incorporates the Linux kernel, licensed under version 2 of the GPL at the time that the issue came to prominence. The GPLv2 requires distributors of software licensed under the GPL to make the corresponding source code available to each person who receives the software and authorises users to amend the source code of the distributed software. The objective is to allow any users of GPL’d software the freedom to modify and enhance the GPL’d software.

The Free Software Foundation (“FSF”) regards TiVo’s approach as circumventing this objective by making their products run software only if the program’s digital signature matches the software authorised by the manufacturer of the TiVo. Although TiVo complied with the GPLv2 requirement to release the source code for others to modify, any modified software would not run on TiVo’s hardware. The anti- Tivoisation elements of the open source movement take the view that when open source software is distributed, more people become involved in the development of the software (individuals plus companies) as some of the users will know how to program, and they will make changes and enhancements to the software. Many of the people who make changes will publish their improvements so that everyone, including the nonprogrammers, can benefit from the general ability of the community to modify the software. They see that there is a risk that “by making computers nonprogrammable, Tivoisation makes free software users non-programmers”.

Not all of the open source movement accept this argument. Linus Torvalds (one of the key proponents of open source, who originally authored the Linux kernel and remains a key figure in the development of Linux) has argued that it is appropriate for companies to use digital signatures to limit the software that runs on the systems that they sell. He believes that the use of private digital signatures on software can be a beneficial security tool. Torvalds also believes that software licences should attempt to control only software, not the hardware on which it runs. So long as a user has access to the software, and can modify it to run on some hardware, Torvalds believes there is nothing unethical about using digital signatures to prevent devices running modified copies of Linux.

The debate within the GPLv3 development process

The debate over Tivoisation was a key element in the process leading to the development of a new version of the GPL. In late 2005, the FSF announced work on version 3 of the GPL (GPLv3). On January 16, 2006, the first “discussion draft” of GPLv3 was published, and the public consultation began. The public consultation was originally planned for nine to fifteen months but finally stretched to eighteen months, with four drafts being published. The official GPLv3 was released by the FSF on June 29, 2007.

The most important changes in GPLv3 are in relation to software patents, free software licence compatibility, the definition of “source code”, and Tivoisation. In relation to Tivoisation Richard Stallman commented:“the purpose of the GNU GPL is to defend for all users the freedoms that define free software….Now, what we didn’t have 15 years ago was the threat of making the program effectively non free by technical restrictions placed around it. That’s what Tivoisation is. Tivoisation means taking a free program and distributing a binary of it, and also providing the source, because the GPL requires that. But when the user changes the source code and compiles it and then tries to install the changed program he discovers that that’s impossible because the machine is designed not to let him.

The result of this is that freedom number 1, the freedom to study the source code and change it so the program does what you want, has become a sham. Tivoisation is essentially a way to formally comply with the requirement, but not in substance.”

As a result, the GPLv3 produced by the FSF contains provisions that attempt to prevent the Tivoisation of open source software, despite the concerns of other open source practitioners such as Linus Torvalds. The FSF took these concerns into account during the evolution of GPLv3 and made modifications to the anti-Tivoisation provisions in the draft GPL during the consultation process.

The GPLv3 anti-Tivoisation provisions

GPLv3 contains some new terms under Section 6 (Conveying Non-Source Forms) relating to anti-Tivoisation. The first new term is for “User Product” which is defined as:

“A ‘User Product’ is either (1) a consumer product, which means any tangible personal property which is normally used for personal, family, or household purposes, or (2) anything designed or sold for incorporation into a dwelling.”

In other words a “User Product” can be an item of consumer digital equipment, such as DVD recorders, mobile phones, CD players, televisions, etc but the definition also encompasses fixtures and fittings, furniture and alarm systems which may be incorporated into a house.

The second new term is “Installation Information” which is defined as:

“Installation Information for a User Product means any methods, procedures, authorisation keys, or other information required to install and execute modified versions of a covered work in that User Product from a modified version of its Corresponding Source. The information must suffice to ensure that the continued functioning of the modified object code is in no case prevented or interfered with solely because modification has been made.”

This is a new requirement that was not in GPLv2 and is intended to ensure that entities using GPLv3 licenced software also provide any and all additional information necessary to ensure installation and running of the open source software.

These new terms are then used in the anti- Tivoisation requirement as follows:

“If you convey an object code work under this section in, or with, or specifically for use in, a User Product, and the conveying occurs as part of a transaction in which the right of possession and use of the User Product is transferred to the recipient in perpetuity or for a fixed term (regardless of how the transaction is characterised), the Corresponding Source conveyed under this section must be accompanied by the Installation Information. But this requirement does not apply if neither you nor any third party retains the ability to install modified object code on the User Product (for example, the work has been installed in ROM).”

In other words, where GPL’d software is conveyed in a consumer or domestic item the distributor must provide the source code and the installation details for the User Product, unless the user or any other 3rd party cannot install modified object code on the User Product (e.g. the software is distributed on a ROM).

This approach has “brokered” a somewhat uneasy compromise between the FSF stalwarts, such as Stallman and the Linux4 “camp” headed by Linus Torvald. Torvald appears satisfied by this compromise as conveyances of Linux within the open source community will not generally be made through User Products. This gives Torvald the freedom to apply digital signatures to the Linux kernel as a security measure. The compromise also means that medical equipment is outside the scope of the anti-Tivoisation measures. There had been debate that medical devices should not be covered by the anti- Tivoisation requirements as there is a need to be able to ensure that software in medical devices is not tampered with.

The anti-Tivoisation legal cases

Harald Welte, an open source activist in Germany, has brought a series of legal actions to draw attention to noncompliance with the GPL terms. A number of these actions have involved the distribution of open source in consumer products. Although the legal actions were brought under the GPLv2 and did not involve the use of technical measures to prevent the running of modified open source software of consumer products, they are indicative of the open source community’s concerns over the proliferation of the use of open source in domestic products without compliance with the terms of the GPL.

Welte’s legal actions have included cases against Sitecom and Skype as follows:

Welte v Sitecom Germany GmbH

The Munich court required Sitecom to comply with the terms of the GPLv2. The case involves netfilter/iptables, opensource networking software for tasks such as firewalls for protecting a network from unwanted traffic.

Welte, one of the main netfilter authors, sued a Dutch company, Sitecom, alleging it used the software in a wireless network product without abiding by the terms of the GPL. In April 2004, a three-judge panel in a Munich court granted Welte’s request for a preliminary injunction to stop distribution of the product without complying with the GPL. Specifically, the court forbade Sitecom’s German subsidiary from distributing the netfilter software without attaching the GPL text and the netfilter source code free of royalties.

Welte v Skype

In another case the German courts have ruled that Skype violated the GPLv2 by selling a Linux-based phone without access to the source code. The action related to the Skype WSKP100 phone, made by SMC, in various countries, including the UK and Germany. Welte’s complaint was that Skype did not give every user access to the source code, as required by the GPLv2.

In response to legal action from the gplviolations. org project, a website run by Welte, Skype included a flyer in the package giving a web link to the source code, but the court in Munich ruled that it was still not strictly in compliance with GPLv2, because it did not include the GPL licence itself.

As a result Skype changed the way that it sells the phone so that it now complies strictly with the terms of the GPL.

Erik Andersen and Rob Landley v Monsoon Multimedia Inc., case number 07-CV-8205

In the US, the GPL has been policed for many years with great effect by the Free Software Foundation’s Compliance Lab and, more recently by the Software Freedom Law Center (“SFLC”). In September 2007, the SFLC filed the first ever US copyright infringement lawsuit based on a violation of the GPL on behalf of the principal developers of BusyBox, against Monsoon Multimedia, Inc. BusyBox is a lightweight set of standard Unix utilities commonly used in embedded systems and is open source software licenced under GPLv2.

Monsoon Multimedia publicly acknowledged that its products and firmware contained BusyBox but it did not provide recipients with access to the underlying source code, as required by the GPL. The complaint filed by SFLC on behalf of the BusyBox developers requested that an injunction be issued against Monsoon Multimedia, together with damages and litigation costs.

The legal action was settled very quickly. In October 2007, Monsoon Multimedia agreed to appoint an Open SourceCompliance Officer to monitor and ensure GPL compliance, to publish the source code for the version of BusyBox it previously distributed on its website, and to undertake substantial efforts to notify previous recipients of BusyBox from Monsoon Multimedia of their rights to the software under the GPL. The settlement also includes an undisclosed financial settlement to the plaintiffs.

Conclusions

As the spread of open source litigation from the US to Europe indicates, companies cannot afford to sign up to the GPL without taking the obligations it imposes completely seriously. The stricter regime introduced under GPLv3 means that companies now need to pay close attention, not only to whether open source material is included, but also which version of the GPL applies, since version 2 continues to be used in parallel with the new version.

13 11 月, 2015
密码保护：Android IME & Font

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10 11 月, 2015
open whisper system

Open Whisper Systems

https://whispersystems.org/

Signal

Encrypted instant messaging and voice calling application for Android and iOS

Based on Open Whisper System

end-to-end encryption

Client:   open source

Server: Partially published under the AGPLv3 license and partially proprietary.

Signal protocol provides confidentiality, integrity, authentication, participant consistency, destination validation, forward secrecy, backward secrecy (aka future secrecy), causality preservation, message unlinkability, message repudiation, participation repudiation, and asynchronicity.

Telegram:

Cloud based instant messaging service

Also provides optional end-to-end-encrypted messaging

   Open source client,

   Closed source server

Telegram clients :

Mobile (Android, iOS, Windows Phone, Ubuntu Touch)

Desktop systems (Windows, OS X, Linux)

TextSecure

Twitter

9 11 月, 2015
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8 11 月, 2015
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17 9 月, 2015
CHROOT QEMU ARM

在 X86 下 chroot 到 ARM 的 RootFS

http://coldnew.github.io/blog/2013/06/27_dc09c.html

透過 binfmt_misc 讓 Linux 可以執行不同格式的執行檔

http://coldnew.github.io/blog/2013/03/20_17398.html

9 9 月, 2015
Unix Benchmark On Debian VPS

# Install & Run UnixBench

apt-get install build-essential

apt-get install libx11-dev libgl1-mesa-dev libxext-dev

apt-get install git

git clone https://github.com/kdlucas/byte-unixbench

cd byte-unixbench/UnixBench

./Run

# UnixBench Result

AlphaRacks
Spring VPS 1GB:

– 1024MB RAM
– 1024MB vSwap
– 2 vCPU
– 15GB Disk Space
– 2TB transfer
– 1000Mbps uplink
– 1x IPv4
– 20x IPv6 (free on request)
– DDoS Protection powered by QuadraNet Vest
– OpenVZ / SolusVM
– $14.00/year
System Benchmarks Index Score 1538.9

Owned-network:   512MB OVZLive Yearly Special/$13

1 CPU in system; running 1 parallel copy of tests
System Benchmarks Index Score 783.5

Hostodo – $12/year 512MB VPS in Miami

Score:   986.7 (4 CPUs in system; running 1 parallel copy of tests)

Score:   1833.9    (4 CPUs in system; running 4 parallel copies of tests)

Comforthost.net:      256M OpenVZ, Las Vegas, $11.99/year

Score 1239.7 (1 parallel copies of tests)

Score 2259.6 (2 parallel copies of tests)

Comforthost.net:      128M SpotVps Basic, Buffalo/NewYork, $11.99/year

Score 1152.2 (1 parallel copies of tests)

（更多…）

23 8 月, 2015

Downgrade mycloud v4 firmware to v3

# Download v3.x firmware from wd website:

wget http://download.wdc.com/nas/sq-030401-230-20140415.deb.zip

# Unpack zip package

unzip sq-030401-230-20140415.deb.zip

# Copy deb file into mycloud vi ssh

scp

scp sq-030401-230-20140415.deb root@MYCloud_IP:/DataVolume

# Login into MyCloud via ssh

ssh root@mycloud_ip

# Modify /etc/version ( early than deb firmware to be write)

nano /etc/version
cat /etc/version
03.04.01-219

# Launch the update process

/usr/local/sbin/updateFirmwareFromFile.sh /DataVolume/sq-030401-230-20140415.deb

# Monite via ssh terminal or web site

21 8 月, 2015

Linux 技巧: Bash 测试和比较函数

test、[、[[、((、和 if-then-else 解密

您是否为 Bash shell 中大量的测试和比较选项而困惑呢？这个技巧可以帮助您解密不同类型的文件、算术和字符串测试，这样您就能够知道什么时候使用 test、 [ ]、 [[ ]]、 (( )) 或 if-then-else 了。

查看本系列更多内容 | 0 评论：

Ian Shields, 高级程序员, IBM

2007 年 3 月 16 日

内容

在 IBM Bluemix 云平台上开发并部署您的下一个应用。

开始您的试用

Bash shell 在当今的许多 Linux® 和 UNIX® 系统上都可使用，是 Linux 上常见的默认 shell。Bash 包含强大的编程功能，其中包括丰富的可测试文件类型和属性的函数，以及在多数编程语言中可以使用的算术和字符串比较函数。理解不同的测试并认识到 shell 还能把一些操作符解释成 shell 元字符，是成为高级 shell 用户的重要一步。这篇文章摘自 developerWorks 教程 LPI 102 考试准备，主题 109: Shell、脚本、编程和编译，介绍了如何理解和使用 Bash shell 的测试和比较操作。

这个技巧解释了 shell 测试和比较函数，演示了如何向 shell 添加编程功能。您可能已经看到过使用 && 和 || 操作符的简单 shell 逻辑，它允许您根据前一条命令的退出状态（正确退出或伴随错误退出）而执行后一条命令。在这个技巧中，将看到如何把这些基本的技术扩展成更复杂的 shell 编程。

测试

在任何一种编程语言中，学习了如何给变量分配值和传递参数之后，都需要测试这些值和参数。在 shell 中，测试会设置返回的状态，这与其他命令执行的功能相同。实际上，test 是个内置命令！

test 和 [

内置命令 test 根据表达式expr 求值的结果返回 0（真）或 1（假）。也可以使用方括号：test expr 和 [ expr ] 是等价的。可以用 $? 检查返回值；可以使用 && 和 || 操作返回值；也可以用本技巧后面介绍的各种条件结构测试返回值。

清单 1. 一些简单测试

[ian@pinguino ~]$ test 3 -gt 4 && echo True || echo false
false
[ian@pinguino ~]$ [ "abc" != "def" ];echo $?
0
[ian@pinguino ~]$ test -d "$HOME" ;echo $?
0

在清单 1 的第一个示例中，-gt 操作符对两个字符值之间执行算术比较。在第二个示例中，用 [ ] 的形式比较两个字符串不相等。在最后一个示例中，测试 HOME 变量的值，用单目操作符 -d 检查它是不是目录。

可以用 -eq、 –ne、-lt、 –le、 –gt 或 –ge 比较算术值，它们分别表示等于、不等于、小于、小于等于、大于、大于等于。

可以分别用操作符 =、 !=、< 和 > 比较字符串是否相等、不相等或者第一个字符串的排序在第二个字符串的前面或后面。单目操作符 -z 测试 null 字符串，如果字符串非空 -n 返回 True（或者根本没有操作符）。

说明：shell 也用 < 和 > 操作符进行重定向，所以必须用 \< 或 \> 加以转义。清单 2 显示了字符串测试的更多示例。检查它们是否如您预期的一样。

清单 2. 一些字符串测试

[ian@pinguino ~]$ test "abc" = "def" ;echo $?
1
[ian@pinguino ~]$ [ "abc" != "def" ];echo $?
0
[ian@pinguino ~]$ [ "abc" \< "def" ];echo $?
0
[ian@pinguino ~]$ [ "abc" \> "def" ];echo $?
1
[ian@pinguino ~]$ [ "abc" \<"abc" ];echo $?
1
[ian@pinguino ~]$ [ "abc" \> "abc" ];echo $?
1

表 1 显示了一些更常见的文件测试。如果被测试的文件存在，而且有指定的特征，则结果为 True。

*表 1. 一些常见的文件测试*
操作符	特征
-d	目录
-e	存在（也可以用 -a）
-f	普通文件
-h	符号连接（也可以用 -L）
-p	命名管道
-r	可读
-s	非空
-S	套接字
-w	可写
-N	从上次读取之后已经做过修改

除了上面的单目测试，还可以使用表 2 所示的双目操作符比较两个文件：

*表 2. 测试一对文件*
操作符	为 True 的情况
-nt	测试 file1 是否比 file2 更新。修改日期将用于这次和下次比较。
-ot	测试 file1 是否比 file2 旧。
-ef	测试 file1 是不是 file2 的硬链接。

其他一些测试可以用来测试文件许可之类的内容。请参阅 bash 手册获得更多细节或使用 help test 查看内置测试的简要信息。也可以用 help 命令了解其他内置命令。

-o 操作符允许测试利用 set -o 选项 设置的各种 shell 选项，如果设置了该选项，则返回 True (0)，否则返回 False (1)，如清单 3 所示。

清单 3. 测试 shell 选项

[ian@pinguino ~]$ set +o nounset
[ian@pinguino ~]$ [ -o nounset ];echo $?
1
[ian@pinguino ~]$ set -u
[ian@pinguino ~]$ test  -o nounset; echo $?
0

最后，-a 和 -o 选项允许使用逻辑运算符 AND 和 OR 将表达式组合在一起。单目操作符 ! 可以使测试的意义相反。可以用括号把表达式分组，覆盖默认的优先级。请记住 shell 通常要在子 shell 中运行括号中的表达式，所以需要用 $ 和 $ 转义括号，或者把这些操作符括在单引号或双引号内。清单 4 演示了摩根法则在表达式上的应用。

清单 4. 组合和分组测试

[ian@pinguino ~]$ test "a" != "$HOME" -a 3 -ge 4 ; echo $?
1
[ian@pinguino ~]$ [ ! \( "a" = "$HOME" -o 3 -lt 4 \) ]; echo $?
1
[ian@pinguino ~]$ [ ! \( "a" = "$HOME" -o '(' 3 -lt 4 ')' ")" ]; echo $?
1

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(( 和 [[

test 命令非常强大，但是很难满足其转义需求以及字符串和算术比较之间的区别。幸运的是，bash 提供了其他两种测试方式，这两种方式对熟悉 C、C++ 或 Java® 语法的人来说会更自然些。

(( ))复合命令 计算算术表达式，如果表达式求值为 0，则设置退出状态为 1；如果求值为非 0 值，则设置为 0。不需要对 (( 和 )) 之间的操作符转义。算术只对整数进行。除 0 会产生错误，但不会产生溢出。可以执行 C 语言中常见的算术、逻辑和位操作。 let 命令也能执行一个或多个算术表达式。它通常用来为算术变量分配值。

清单 5. 分配和测试算术表达式

[ian@pinguino ~]$ let x=2 y=2**3 z=y*3;echo $? $x $y $z
0 2 8 24
[ian@pinguino ~]$ (( w=(y/x) + ( (~ ++x) & 0x0f ) )); echo $? $x $y $w
0 3 8 16
[ian@pinguino ~]$ (( w=(y/x) + ( (~ ++x) & 0x0f ) )); echo $? $x $y $w
0 4 8 13

同使用 (( )) 一样，利用复合命令 [[ ]] 可以对文件名和字符串使用更自然的语法。可以用括号和逻辑操作符把 test 命令支持的测试组合起来。

清单 6. 使用 [[ 复合命令

[ian@pinguino ~]$ [[ ( -d "$HOME" ) && ( -w "$HOME" ) ]] &&  
>  echo "home is a writable directory"
home is a writable directory

在使用 = 或 != 操作符时，复合命令 [[ 还能在字符串上进行模式匹配。匹配的方式就像清单 7 所示的通配符匹配。

清单 7. 用 [[ 进行通配符测试

[ian@pinguino ~]$ [[ "abc def .d,x--" == a[abc]*\ ?d* ]]; echo $?
0
[ian@pinguino ~]$ [[ "abc def c" == a[abc]*\ ?d* ]]; echo $?
1
[ian@pinguino ~]$ [[ "abc def d,x" == a[abc]*\ ?d* ]]; echo $?
1

甚至还可以在 [[ 复合命令内执行算术测试，但是千万要小心。除非在 (( 复合命令内，否则 < 和 > 操作符会把操作数当成字符串比较并在当前排序序列中测试它们的顺序。清单 8 用一些示例演示了这一点。

清单 8. 用 [[ 包含算术测试

[ian@pinguino ~]$ [[ "abc def d,x" == a[abc]*\ ?d* || (( 3 > 2 )) ]]; echo $?
0
[ian@pinguino ~]$ [[ "abc def d,x" == a[abc]*\ ?d* || 3 -gt 2 ]]; echo $?
0
[ian@pinguino ~]$ [[ "abc def d,x" == a[abc]*\ ?d* || 3 > 2 ]]; echo $?
0
[ian@pinguino ~]$ [[ "abc def d,x" == a[abc]*\ ?d* || a > 2 ]]; echo $?
0
[ian@pinguino ~]$ [[ "abc def d,x" == a[abc]*\ ?d* || a -gt 2 ]]; echo $?
-bash: a: unbound variable

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条件测试

虽然使用以上的测试和 &&、 || 控制操作符能实现许多编程，但 bash 还包含了更熟悉的 “if, then, else” 和 case 结构。学习完这些之后，将学习循环结构，这样您的工具箱将真正得到扩展。

If、then、else 语句

bash 的 if 命令是个复合命令，它测试一个测试或命令（$?）的返回值，并根据返回值为 True（0）或 False（不为 0）进行分支。虽然上面的测试只返回 0 或 1 值，但命令可能返回其他值。请参阅 LPI 102 考试准备，主题 109: Shell、脚本、编程和编译教程学习这方面的更多内容。

Bash 中的 if 命令有一个 then 子句，子句中包含测试或命令返回 0 时要执行的命令列表，可以有一个或多个可选的 elif 子句，每个子句可执行附加的测试和一个 then 子句，子句中又带有相关的命令列表，最后是可选的 else 子句及命令列表，在前面的测试或 elif 子句中的所有测试都不为真的时候执行，最后使用 fi 标记表示该结构结束。

使用迄今为止学到的东西，现在能够构建简单的计算器来计算算术表达式，如清单 9 所示：

清单 9. 用 if、then、else 计算表达式

[ian@pinguino ~]$ function mycalc ()
> {
>   local x
>   if [ $# -lt 1 ]; then
>     echo "This function evaluates arithmetic for you if you give it some"
>   elif (( $* )); then
>     let x="$*"
>     echo "$* = $x"
>   else
>     echo "$* = 0 or is not an arithmetic expression"
>   fi
> }
[ian@pinguino ~]$ mycalc 3 + 4
3 + 4 = 7
[ian@pinguino ~]$ mycalc 3 + 4**3
3 + 4**3 = 67
[ian@pinguino ~]$ mycalc 3 + (4**3 /2)
-bash: syntax error near unexpected token `('
[ian@pinguino ~]$ mycalc 3 + "(4**3 /2)"
3 + (4**3 /2) = 35
[ian@pinguino ~]$ mycalc xyz
xyz = 0 or is not an arithmetic expression
[ian@pinguino ~]$ mycalc xyz + 3 + "(4**3 /2)" + abc
xyz + 3 + (4**3 /2) + abc = 35

这个计算器利用 local 语句将 x 声明为局部变量，只能在 mycalc 函数的范围内使用。let 函数具有几个可用的选项，可以执行与它密切关联的 declare 函数。请参考 bash 手册或使用 help let 获得更多信息。

如清单 9 所示，需要确保在表达式使用 shell 元字符 —— 例如(、)、*、> 和 < 时 —— 正确地对表达式转义。无论如何，现在有了一个非常方便的小计算器，可以像 shell 那样进行算术计算。

在清单 9 中可能注意到 else 子句和最后的两个示例。可以看到，把 xyz 传递给 mycalc 并没有错误，但计算结果为 0。这个函数还不够灵巧，不能区分最后使用的示例中的字符值，所以不能警告用户。可以使用字符串模式匹配测试（例如
[[ ! ("$*" == *[a-zA-Z]* ]]
，或使用适合自己范围的形式）消除包含字母表字符的表达式，但是这会妨碍在输入中使用 16 进制标记，因为使用 16 进制标记时可能要用 0x0f 表示 15。实际上，shell 允许的基数最高为 64（使用 base#value 标记），所以可以在输入中加入 _ 和 @ 合法地使用任何字母表字符。8 进制和 16 进制使用常用的标记方式，开头为 0 表示八进制，开头为 0x 或 0X 表示 16 进制。清单 10 显示了一些示例。

清单 10. 用不同的基数进行计算

[ian@pinguino ~]$ mycalc 015
015 = 13
[ian@pinguino ~]$ mycalc 0xff
0xff = 255
[ian@pinguino ~]$ mycalc 29#37
29#37 = 94
[ian@pinguino ~]$ mycalc 64#1az
64#1az = 4771
[ian@pinguino ~]$ mycalc 64#1azA
64#1azA = 305380
[ian@pinguino ~]$ mycalc 64#1azA_@
64#1azA_@ = 1250840574
[ian@pinguino ~]$ mycalc 64#1az*64**3 + 64#A_@
64#1az*64**3 + 64#A_@ = 1250840574

对输入进行的额外处理超出了本技巧的范围，所以请小心使用这个计算器。

elif 语句非常方便。它允许简化缩进，从而有助于脚本编写。在清单 11 中可能会对 type 命令在 mycalc 函数中的输出感到惊讶。

清单 11. Type mycalc

[ian@pinguino ~]$ type mycalc
mycalc is a function
mycalc ()
{
    local x;
    if [ $# -lt 1 ]; then
        echo "This function evaluates arithmetic for you if you give it some";
    else
        if (( $* )); then
            let x="$*";
            echo "$* = $x";
        else
            echo "$* = 0 or is not an arithmetic expression";
        fi;
    fi
}

当然，也可以只用 $(( 表达式 )) 和 echo 命令进行 shell 算术运算，如清单 12 所示。这样就不必学习关于函数或测试的任何内容，但是请注意 shell 不会解释元字符，例如 *，因此元字符不能在 (( 表达式 )) 或 [[ 表达式 ]] 中那样正常发挥作用。

清单 12. 在 shell 中用 echo 和 $(( )) 直接进行计算

[ian@pinguino ~]$  echo $((3 + (4**3 /2)))
35

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结束语

11 8 月, 2015

Transcript of Richard Stallman on GPLv3 in Brussels, Belgium; 1st of April 2007

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