Packages generally contain all of the files necessary to implement a set of related commands or features. There are two types of Debian packages:
dpkg;
details are given in its manual page.
.dsc file
describing the source package (including the names of the following
files), a .orig.tar.gz file that contains the original
unmodified source in gzip-compressed tar format and usually a
.diff.gz file that contains the Debian-specific changes to the
original source. The utility dpkg-source packs and unpacks
Debian source archives; details are provided in its manual page.
Installation of software by the package system uses
"dependencies" which are carefully designed by
the package maintainers. These dependencies are documented
in the control file associated
with each package. For example, the package containing the
GNU C compiler (gcc) "depends" on the package
binutils which includes the linker and assembler.
If a user attempts to install gcc without having first installed
binutils,
Debian's package system will send an error message that it also needs
binutils, and will install gcc only if the user agrees
to install binutils first. (However, this facility can be
overridden by the insistent user.) See more
details on package dependencies below.
Debian's packaging tools can be used to:
A Debian "package", or a Debian archive file, contains the
executable files, libraries, and documentation associated with a particular
suite of program or set of related programs. Normally, a Debian archive
file has a filename that ends in .deb.
The internals of this Debian binary packages format are described in
the deb(5) manual page. This internal format is subject
to change, therefore always use dpkg-deb(8) for
manipulating .deb files.
The Debian binary package file names conform to the following convention: <foo>_<VersionNumber>-<DebianRevisionNumber>.deb
Note that foo is supposed to be the package name.
As a check, one can learn the package name associated with a
particular Debian archive file (.deb file) in one of these ways:
dpkg --info foo_VVV-RRR.deb. This sends
a message to STDOUT which gives, among other things, the
Package Name corresponding to the archive file being unpacked.
The VVV component is the version number specified by the
upstream developer. There are no standards in place here, so the version
number may have formats as different as
"960428" and "2.7.2.l.3".
The RRR component is the Debian revision number, and is
specified by the
Debian developer (or an individual user if he chooses to build the package
himself). This number corresponds to the revision level of the Debian
package (which includes the Debian-specific Makefile, called
debian/rules, as well as the Debian control file, usually called
debian/control). Thus, a new revision level usually signifies
changes in the Debian Makefile, the Debian control file, the installation
or removal scripts, or in the configuration files used with the package.
Specifics regarding the contents of a Debian control file are provided in the dpkg programmer's manual. Briefly, a sample control file is shown below for the Debian package hello:
Package: hello
Version: 1.3-13
Architecture: i386
Depends: libc5 (>= 5.2.18)
Installed-Size: 31
Maintainer: Ian Jackson <ian@chiark.greenend.org.uk>
Description: The classic greeting, and a good example
The GNU hello program produces a familiar, friendly greeting. It
allows nonprogrammers to use a classic computer science tool which
would otherwise be unavailable to them.
.
Seriously, though: this is an example of how to do a Debian package.
It is the Debian version of the GNU Project's `hello world' program
(which is itself an example for the GNU Project).
The Package field gives the package name. This is the name by which the package can be manipulated by the package tools, and usually similar to but not necessarily the same as the first component string in the Debian archive file name.
The version field gives both the upstream developer's version number and (in the last component) the revision level of the Debian package of this program as explained in pkgname .
The Architecture field specifies the chip for which this particular binary was compiled.
The Depends field gives a list of packages that have to be installed in order to install this package successfully.
The Installed-Size indicates how much disk space the installed package will consume. This is intended to be used by installation front-ends in order to show whether there is enough disk space available to install the program .
The Maintainer field gives the e-mail address of the person who is currently responsible for maintaining this package.
The Description field gives a brief summary of the package's features.
Conffiles are listings of configuration files, usually placed in
/etc, that the package management system will not overwrite
when a package is upgraded.
This ensures that local values for the contents of these files
will be preserved, and is a critical feature enabling the in-place upgrade
of packages on a running system.
To determine exactly which files are preserved during an upgrade, run dpkg --status package.
These files are executable scripts which are automatically run before
or after a package is installed.
Along with a file named control, all of these files are part
of the "control" section of a Debian archive file.
The individual files are:
This script executes before that package will be unpacked from its Debian archive (".deb") file. Many 'preinst' scripts stop services for packages which are being upgraded until their installation or upgrade is completed (following the successful execution of the 'postinst' script).
This script typically completes any required
configuration of the package foo once foo has been unpacked
from its Debian archive (".deb") file.
Often, 'postinst' scripts ask the
user for input, and/or warn the user that if he accepts default values,
he should remember to go back and re-configure that package as the
situation warrants. Many 'postinst' scripts then execute any commands
necessary to start or restart a service once a new package has been
installed or upgraded. It is a good idea to check the contents of
the 'postinst' script for any configuration advice, when trying out a
package for the first time.
This script typically stops any daemons which are associated with a package. It is executed before the removal of files associated with the package.
This script typically modifies links or other
files associated with foo. (See notes on
Virtual packages.)
Currently all of the control files can be found in /var/lib/dpkg/info.
The files relevant to package foo begin with the name
"foo" and have file extensions of "preinst",
"postinst", etc., as appropriate. The file foo.list
in that directory
lists all of the files that were installed with the package foo.
(Note that the location of these files is a dpkg internal; you should not
rely on it.)
Each Debian package is assigned a priority by the distribution maintainers, as an aid to the package management system. The priorities are:
A virtual package is a generic name that applies to any one of a group
of packages, all of which provide similar basic functionality.
For example, both the tin and trn programs
are both news readers, and should therefore satisfy any dependency of
a program that required a news reader on a system in order to work
or to be useful.
They are therefore both said to provide the "virtual package"
called news-reader.
Similarly, smail and sendmail both provide the
functionality of a mail transport agent. They are therefore said to
provide the virtual package, "mail transport agent".
If either one is installed, then any program depending on the
installation of a mail-transport-agent will be satisfied by
the existence of this virtual package.
Debian provides a mechanism so that, if more than one package which
provide the same virtual package is installed on a system,
then system administrators can set one as the preferred package.
The relevant command is update-alternatives, and is
described further in the section on
diversions.
The Debian package system has a range of package "dependencies" which are designed to indicate (in a single flag) the level at which Program A can operate independently of the existence of Program B on a given system:
More detailed information on the use of these terms can be found in the Debian programmer's manual.
"Pre-Depends" is a special dependency.
In the case of most packages, dpkg will unpack its archive
file (i.e., its .deb file) independently of whether or not the
files on which it depends exist on the system. Simplistically,
unpacking means that dpkg will extract the files from the archive
file that were meant to be installed on your filesystem, and put them
in place. If those packages depend on the existence of some
other packages on your system, dpkg will refuse to complete the
installation by executing its "configure" action until the
other packages are installed.
However, for some packages, dpkg will refuse even to unpack
them until certain dependencies are resolved. Such packages are said
to "Pre-depend" on the presence of some other packages.
The Debian project provided this mechanism to support the safe upgrading
of systems from a.out format to ELF format, where
the order in which packages were unpacked was critical.
More detailed information on the use of these terms can be found in the Debian programmer's manual.
These "want" flags tell what the user wanted to do with
a package (as indicated either by the user's actions in
the "Select" section of dselect, or by the user's
direct invocations of dpkg). Their meanings are:
See the dpkg programmer's manual and the Debian policy manual