Archive for January, 2007

Debian Packaging 101 (Part 1)

Making packages for Debian derivatives (like Ubuntu) isn’t really hard. It just required some dedication to learn how the packaging system work. Yet, most users don’t know how to make packages for their distribution. In this series, I will try to give a brief introduction packaging.

First, I would like to tell you that, if you never compiled a program before, this short guide will be useless for you. Therefore, I assume that you know and that you did compile a program before. Also, I will use the term “package” to refer to the compiled program package ready to installed on Debian or its derivatives. And, I will use the term “program” to refer to the software, like “GNU Emacs”.

A package consists of two things: the source code of a program and a debian/ directory which contains information how to build the program. There is different methods to specify the packaging information. The most common one, and the one I will discuss here, is using a tool called debhelper. This tool makes the packaging process easier by abstracting the common packaging tasks into little scripts, run at the build time. The typical directory structure of a package looks like this:

gnu-hello/
  debian/
    changelog
    compat
    control
    copyright
    postinst
    prerm
    rules
  doc/
  man/
  src/
  AUTHORS
  ChangeLog
  configure
  COPYING
  INSTALL
  README

Obviously, this example is simplified, but you get the idea. That are the interesting things for a packager. By the way, GNU Hello a good example of package to study. You get it with:

$ apt-get source hello-debhelper

Here, a quick description of the files in the debian/ directory:

  • changelog: The history of the package’s changes.
  • compat: A file that contains the debhelper version that is used.
  • control: The description of the package and the list of dependencies.
  • copyright: A copy of the licence the program uses.
  • postinst: A post-installation script used to setup things after the package has been unpacked.
  • prerm: Another script that is run before the removal of the package. It usually used to undo the things postinst has done.
  • rules: The instructions how to build the package. This is simply a Makefile.

In more complicated programs, there is usually other files. However, I won’t talk about them in this introduction. Anyway, I am out of time. In the second part of this series, I will explain the tools used to build packages.

Web Design Tips

Today, I been impressed how high my comment has scored on Reddit. So, I taught it would be useful to repost it, here, on my blog for future references.

First, learn how to write proper HTML without any IDE, like Dreamweaver. Or if you do, only use the code view. After that, embrace the web standards, but without becoming a fanatic. Finally, learn how to separate the structure of your web pages from their design with CSS.

In summary, to improve your website you should:

  • Rewrite it or do a heavy clean up job, without Dreamweaver’s design view.
  • Make it pass this test.
  • Fix all the links.
  • Learn how to use the <title /> tag.
  • Remove any animated GIF image and Flash applet (unless it helps to the comprehension of the content).
  • Fix the fonts. Try to use (or at least fallback to) either “web core fonts” or serif/sans-serif.
  • Replace the <font /> tags with their CSS equivalents.
  • Use tags properly (ie. only use <table /> if you really want to make a data table).

References

Pretty Emacs

Update: If you are using Ubuntu 8.04 LTS “Hardy Heron” or Ubuntu 8.10 “Intrepid Ibex”, use the packages in the PPA of the Ubuntu Emacs Lisp team, instead of the packages referenced here. For Ubuntu 9.04 “Jaunty Jackalope” and newer, use the packages in Ubuntu repositories.

Emacs is my editor of choice. In fact, I should say it’s my framework of choice, but that’s for another post. Until recently, I disliked the poor font backend of Emacs. So, I was always using Emacs within a terminal window to get a decent looking interface. However, this grungy font era is over, since Emacs’s hackers added recently to my favorite editor a XFont backend, thus making possible to use good looking fonts, like Bitstream Vera Sans Mono.

Screenshot of Emacs with XFT support

I made a package that makes the installation, as painless as possible. So, feel free to use it. However, please note that this is an alpha release of Emacs, therefore it should only be used for testing. (From my experience, it’s rock solid.)

Still interested? Then, here the instructions. First, add my repository into your software source list, by adding the following lines to /etc/apt/sources.list:

deb     http://ppa.launchpad.net/avassalotti/ubuntu feisty main
deb-src http://ppa.launchpad.net/avassalotti/ubuntu feisty main

If you are running Ubuntu 6.10 (Edgy Eft) or the current development version of Ubuntu (Gutsy Gibbon), change feisty for edgy or gutsy.

Finally, run either apt-get or aptitude to fetch and install the packages:

sudo aptitude update
sudo aptitude install emacs-snapshot emacs-snapshot-el

Now, you need to specify the font you want to use in your Xresources file.

echo "Emacs.font: Monospace-10" >> ~/.Xresources
xrdb -merge ~/.Xresources

Here, I use the default monospace font, but any other monospaced font should work too. For example, if you want to use Lucida Sans Typewriter instead, change Monospace-10 for Lucida Sans Typewriter-10 in the above command.

And that’s it! Now, launch Emacs and enjoy the good looking fonts.

If you need support with the package, just email me at alexandre@peadrop.com.

Update: Il y a, maintenant, une version en fran├žais de ce guide sur le wiki de Ubuntu-fr.

The secret of Ubuntu success

Ubuntu has been my first distribution of Linux. I learned everything I know about Linux, on Ubuntu. However, there is a question that has been bothering me for a while: “Why Ubuntu is so successful?”. It took me some time to find the answer. The answer is simple, we are nice to new comers. We don’t try to prove anything to them; we just help them. (Of course, being the best distribution out there helps :) )

The slogan of Ubuntu, “Linux for Human Being”, took all its meaning when I finally understood that. We are the humans. We aren’t supposed to work for the computers. They are the ones supposed to do the work.

That brings me to the second reason of Ubuntu’s success. We are making Linux works out of the box. Users shouldn’t need to have to read a 10,000-pages manual just to use Linux. Not because I read the manuals, everyone should. Of course, we need all those friendly howtos and guides. They make Ubuntu so much more fun to use.

I think we need to continue in that direction. Sure, translucent windows are cool, but we should always remember the roots of our success.

Understanding Linux File Permissions

File permissions are probably one of the biggest difference between Windows and Unix-style operating systems. They make Linux much more secure when they are well used. However, they can also cause nightmare to the casual Linux administrator.

The first thing you need to know is that a Linux system has two way of classifying users. There is, of course, the user name, but there is also groups. Groups are, strictly speaking, only a way to share permissions between the users. For example, all the member of the admin group on your system is able to use the command sudo. As you probably know, sudo allows you to run a command as another user (by default, the root user).

Let me introduce you to your command-line friends that will help you to manage the permissions of your system.

  • adduser: This command let you add new user on your system. It can also add a user into a group.
  • addgroup: Its name says it all. This command let you add new group on your system.
  • chmod: I believe this is the most widely known Unix command. It is even a verb in the world of server-side technology, like PHP. This command let you alter the permissions of a file. It is a swiss-army knife. Learn it, and use it well.
  • chown: Also a very important command, chown can change the user and group ownership of a file.
  • chgrp: This is chown‘s little brother. Unlike chown, this command can only change the group ownership of a file.
  • groups: Somehow less important but still useful, groups shows you the groups you are a member of.
  • whoami: Don’t know why, but I love the name of this command. Anyway, this command tells you who you are.
  • who: This command shows you who is login on your system. I never use it, since I find w more useful for my usage.
  • w: And here our last little friend, the w command. It displays a list of the logged users like who, but also display their attached process and the uptime of the machine you’re on.

Obviously if you want to learn to use those commands well, you will need to do some homework and read their respective manual pages (with man <command>).

So, how permissions work? First, we need an example:

alex@helios /etc % ls -l
total 1548
-rw-r--r--  1 root   root      2584 2006-11-29 08:40 adduser.conf
drwxr-xr-x  4 root   root      4096 2006-12-13 10:46 apt
drwxr-xr-x  2 root   root      4096 2006-12-17 00:15 cron.d
drwxr-sr-t  5 cupsys lp        4096 2006-11-29 08:51 cups
-rw-r--r--  1 root   root       817 2006-11-29 08:39 fstab
-rw-r--r--  1 root   root       806 2006-12-17 00:15 group
-rw-r--r--  1 root   root      1430 2006-12-17 00:15 passwd
lrwxrwxrwx  1 root   root        13 2006-11-29 08:40 motd -> /var/run/motd
drwxr-xr-x  2 root   root      4096 2006-12-22 23:36 rc0.d
drwxr-xr-x  2 root   root      4096 2006-12-19 12:06 rc1.d
drwxr-xr-x  2 root   root      4096 2006-12-19 12:06 rc2.d
drwxr-xr-x  2 root   root      4096 2006-12-19 12:06 rc3.d
drwxr-xr-x  2 root   root      4096 2006-12-19 12:06 rc4.d
drwxr-xr-x  2 root   root      4096 2006-12-19 12:06 rc5.d
drwxr-xr-x  2 root   root      4096 2006-12-22 23:36 rc6.d
-rwxr-xr-x  1 root   root       306 2006-11-29 08:40 rc.local
-rwxr-xr-x  1 root   root       306 2006-11-29 08:40 rc.local
-rw-r-----  1 root   shadow     873 2006-12-17 00:15 shadow
-rw-r--r--  1 root   root       214 2006-12-02 13:27 shell
-r--r-----  1 root   root       403 2006-11-29 09:10 sudoers

Only the first, third and fourth column are interesting for us, right now. The first column gives us information about the file permissions. The third is the owner of the file and the fourth is the group.

So, what all this mess means? File permissions are like little switches you turn on and off. There is three types of permission: read, write, and execute. There’s also three types of ownership: owner (or user), group, and other. So, 3 times 3 equals 9 switches you can control.

That is exactly what we see in the first column. The first element of this column is the type of the file. A - means it’s a normal file; a d is for a directory and l is for a link pointing to a file. There is several other types of file, but they are much less useful to know for the casual Linux system administrator.

You probably figured that the rest are the permissions. Here a legend of the symbol I will use for the rest of this post:

u - owner
g - group
o - others

r - read
w - write
x - execute

t - file type

As you will see, there is nothing complicated about the first column in the output of the ls -l command. It’s a simple representation of the switches I mentioned earlier. So, let’s decrypt it:

tuuugggooo

That’s it. Just read it out loud: type, owner, group and others. So, if you see something like -rwxr-xr-x, you can read it as: “a normal file which the owner has the read, write and execute permission and which its group and others has the read and execute permission.” That is extremely verbose, but correct.

You can change the permissions with the chmod command:

alex@helios ~ % ls -l file
-rw-r--r-- 1 alex alex 0 2007-01-01 23:58 file
alex@helios ~ % chmod og+rw file
alex@helios ~ % ls -l file
-rw-rw-rw- 1 alex alex 0 2007-01-01 23:58 file

I won’t go in details here, because it’s quite simple to understand. If you want to know more, The info page of chmod is a great source of information (info coreutils ls).

If you already knew what are permissions, you are probably 1) rolling on the floor laughing, how I gone into the great details of that simple thing, or 2) grumbling that you want a refund because I wasted your bandwidth. So, hold on here the more advanced stuff.

You probably saw numerical (or should I say octal) permissions, like 777. But, do you actually know how to read them? For example, what 645 means? Hopefully, you aren’t trying to remember all of them. I going to give a trick.

As you probably know, each digit represents the permissions of one type of ownership (owner, group and other). One thing you need to know is they are not decimal digits; they are octal digits. So, something like 855 is not a valid permission.

Now, here one interesting property of octal digit: you can write them all as three bits (binary digits) number. Here the full list:

Octal   Binary
0       000
1       001
2       010
3       011
4       100
5       101
6       110
7       111

As you may know, bits are like switches you flip on and off. Sound familiar? Right, they are exactly like permissions. Now imagine that instead of letters, the permissions in the ls -l were shown as binary numbers:

alex@helios ~ % ls -l file
-110100100 1 alex alex 0 2007-01-01 23:58 file

110100100 is a perfectly legit binary number and in octal it is 644. So, what happens if we chmod our file to 644? You certainly deduced it:

alex@helios ~ % chmod 644 file
-rw-r--r-- 1 alex alex 0 2007-01-01 23:58 file

Pretty nice, eh? You been working with the binary system without knowing it. Back to our problem, you need to change the permissions of a file to 645. So, how do you calculate what it means? That is simple, now that you know it’s just a binary number:

Binary  Octal  English
100     4      read
010     2      write
001     1      execute

Therefore:

owner: 6 = 4+2 = read+write
group: 4 = 4   = read
other: 5 = 4+1 = read+execute

So, let’s check if we were right:

alex@helios ~ % chmod 645 file
alex@helios ~ % ls -l file
-rw-r--r-x 1 alex alex 0 2007-01-01 23:58 file

I bet you didn’t know it was that simple. Now, you can show to your geek friends how good you are by calculating any octal permissions in your head.

There is also some special permissions you can use too, like setuid, setgid, and sticky. I won’t cover them here, because they are pretty useless to the casual Linux system administrator.

I hope you enjoyed this introduction, because that’s all folks!