Working of .Deb Files , Compare with .exe files in Windows

[Andrew Farris]

On Wed, 2009-12-16 at 07:57 +0530, Jatin Davey wrote:
> Hi all
> 
> I am new to ubuntu. When i download any apps on the internet to be 
> installed on ubuntu i get to download .deb packages. I know that these 
> are the files that have the required application. I want to know the 
> following:

First of all, if you're installing programs, it's usually best to use
the software repositories (through USC or Synaptic). If what you want
isn't in there, you can also try to find a PPA (package repository) and
subscrie to it, provided you trust the package source (just google for
<application_name> PPA). Launchpad hosts lots of PPAs, and in my
experience they're perfectly safe, despite all the warnings. If you
absolutely can't get the software eitger of those ways, then a .deb is
the next best thing.

> 1. What are the contents of any .deb pacakage in general ?

.deb files are basically just an archive (they can actually be opened
with the standard archive manager). They typically contain 2 archives
(both gzipped), a data file, and a control archive. 

The control archive contains what's called a 'control file' that
basically has all the essential info about the package, such as it's
name, what service it provides, it's category, any programs it conflicts
with/depends on, the maintainer, etc... and scripts that can be run as
the program is installed/removed. I've also seen MD5 checksum files in
the control archives before.

The data archive contains all the program data (i.e. sources, binaries,
libraries, art, etc...) which is all arranged in a false directory
structure that mimics where everything is intended to be when the
install is done. I.e. if the program is going to store some data under
the shared folder, and have it's binary in the standard executable path,
then it may contain a directory structure something like this:
        (within data.tar.gz)
                ./usr/bin/executable
                ./usr/share/library.so
                .usr/share/icon.png
                etc...
                
There are tools that help with making these so you don't actually have
to archive everything yourself. Here's a link to a simple guide on how
to create .deb formatted packages... it's an interesting read if you
want to learn how the package is structured/built, even if you don't
plan on doing so yourself:
http://www.linuxfordevices.com/c/a/Linux-For-Devices-Articles/How-to-make-deb-packages/

> 2. What happens when it is installed ? (Where are the files installed , 
> Directories)

As would indicate from my last point above, the data file is basically
copied verbatim to the root directory (which is why you need root access
to install software). I believe it fails if there's conflicts with
preexisting data where it's trying to copy, but I'm not sure about that.

Either way, after that the package installer writes a little note in the
package database that you have a new application, and records the
locations of all the files you installed, and i believe it copies any
uninstaller scripts present to another place (dunno where). If you want
to view what files a particular package installed, you can go to
Synaptic, search for your package, then Right-Click > Properties >
Installed Files. Gdebi (the graphical package installer) will provide a
list for you when you when you attempt to install a .deb file by itself,
under the 'Included Files' tab.

> 3. Any application under linux is launched using a script which is 
> similar to an exe file in windows , based on this i want to know whether 
> an application can be installed anywhere and in any directory and it can 
> be launched from there without having to set environment variables.

this depends on your definition of 'script'... it also depends on where
you launch from. most every program I've ever seen on linux installs at
least a link to /usr/bin/ so you can launch anything from the terminal.
in fact, the .desktop launchers that are in the menus, and on your
panel, are actually just little text files that specify a terminal
command to run, along with an icon and other conveniences. Even so,
many/most programs are just directly-executed binaries.

As for running programs from any directory...it all depends on the
program. if the program is looking for files in explicitly named places
(such as /usr/lib/) that it placed there, but you installed it to a
non-standard place, then chances are it won't work. The bets thing to do
though is to try it. sometimes it will work just fine, sometimes you can
get it to work with a little fiddling, sometimes it won't work at all. 
If you're feeling adventurous, and the program is not a compiled binary,
you can sometimes simply modify the program to look in a different
location for files it's missing, but that's a little more difficult.

Anyway, I hope that helps...sorry for the long reply.

-- 
Andrew
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