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dpkg technical manual - Chapter 1
Quick summary of dpkg's external interface
The basic dpkg package control file supports the following major features:-
The "dpkg status area" is the term used to refer to the directory where dpkg
keeps its various status files (GNU would have you call it the dpkg shared
state directory). This is always, on Debian systems, /var/lib/dpkg. However,
the default directory name should not be hard-coded, but #define'd, so that
alteration is possible (it is available via configure in dpkg 1.4.0.9 and
above). Of course, in a library, code should be allowed to override the
default directory, but the default should be part of the library (so that
the user may change the dpkg admin dir simply by replacing the library).
Dpkg keeps a variety of files in its status area. These are discussed later
on in this document, but a quick summary of the files is here:-
- available - this file contains a concatenation of control information
from all the packages which dpkg knows about. This is updated using the dpkg
commands "--update-avail <file>", "--merge-avail <file>", and
"--clear-avail".
status - this file contains information on the following things for
every package:-
- Whether it is installed, not installed, unpacked, removed,
failed configuration, or half-installed (deconfigured in
favour of another package).
- Whether it is selected as install, hold, remove, or purge.
- If it is "ok" (no installation problems), or "not-ok".
- It usually also contains the section and priority (so that
dselect may classify packages not in available)
- For packages which did not initially appear in the "available"
file when they were installed, the other control information
for them.
The exact format for the "Status:" field is:
Status: Want Flag Status
Where Want may be one of unknown, install,
hold, deinstall, purge. Flag
may be one of ok, reinstreq, hold,
hold-reinstreq.
Status may be one of not-installed, unpacked,
half-configured, installed, half-installed
config-files, post-inst-failed, removal-failed.
The states are as follows:-
- not-installed
- No files are installed from the package, it has no config files
left, it uninstalled cleanly if it ever was installed.
- unpacked
- The basic files have been unpacked (and are listed in
/var/lib/dpkg/info/[package].list. There are config files present,
but the postinst script has _NOT_ been run.
- half-configured
- The package was installed and unpacked, but the postinst script
failed in some way.
- installed
- All files for the package are installed, and the configuration
was also successful.
- half-installed
- An attempt was made to remove the packagem but there was a failure
in the prerm script.
- config-files
- The package was "removed", not "purged". The config files are left,
but nothing else.
- post-inst-failed
- Old name for half-configured. Do not use.
- removal-failed
- Old name for half-installed. Do not use.
The two last items are only left in dpkg for compatibility - they are
understood by it, but never written out in this form.
Please see the dpkg source code, lib/parshelp.c,
statusinfos, eflaginfos and wantinfos for more
details.
- info - this directory contains files from the control archive of every
package currently installed. They are installed with a prefix of "<packagename>.".
In addition to this, it also contains a file called <package>.list for every
package, which contains a list of files. Note also that the control file is
not copied into here; it is instead found as part of status or available.
- methods - this directory is reserved for "method"-specific files - each
"method" has a subdirectory underneath this directory (or at least, it can
have). In addition, there is another subdirectory "mnt", where misc.
filesystems (floppies, CDROMs, etc.) are mounted.
- alternatives - directory used by the "update-alternatives" program. It
contains one file for each "alternatives" interface, which contains information
about all the needed symlinked files for each alternative.
- diversions - file used by the "dpkg-divert" program. Each diversion takes
three lines. The first is the package name (or ":" for user diversion), the
second the original filename, and the third the diverted filename.
- updates - directory used internally by dpkg. This is discussed later,
in the section Updates, section 3.1.
- parts - temporary directory used by dpkg-split
These files are installed under /usr/lib/dpkg (usually), but
/usr/local/lib/dpkg is also a possibility (as Debian policy dictates). Under
this directory, there is a "methods" subdirectory. The methods subdirectory
in turn contains any number of subdirectories for each general method
processor (note that one set of method scripts can, and is, used for more than
one of the methods listed under dselect).
The following files may be found in each of these subdirectories:-
- names - One line per method, two-digit priority to appear on menu
at beginning, followed by a space, the name, and then another space and the
short description.
- desc.<name> - Contains the long description displayed by dselect
when the cursor is put over the <name> method.
- setup - Script or program which sets up the initial values to be used
by this method. Called with first argument as the status area directory
(/var/lib/dpkg), second argument as the name of the method (as in the directory
name), and the third argument as the option (as in the names file).
- install - Script/program called when the "install" option of dselect is
run with this method. Same arguments as for setup.
- update - Script/program called when the "update" option of dselect is
run. Same arguments as for setup/install.
As yet unwritten. You can refer to the other manuals for now. See
dpkg(8)
.
The dpkg utility itself is required for quite a number of packages, even if
they have been installed with a tool totally separate from dpkg. The reason for
this is that some packages, in their pre-installation scripts, check that your
version of dpkg supports certain features. This was broken from the start, and
it should have actually been a control file header "Dpkg-requires", or similar.
What happens is that the configuration scripts will abort or continue according
to the exit code of a call to dpkg, which will stop them from being wrongly
configured.
These special command-line options, which simply return as true or false are
all prefixed with "--assert-". Here is a list of them (without the prefix):-
- support-predepends - Returns success or failure according to whether
a version of dpkg which supports predepends properly (1.1.0 or above) is
installed, according to the database.
- working-epoch - Return success or failure according to whether a version
of dpkg which supports epochs in version properly (1.4.0.7 or above) is
installed, according to the database.
Both these options check the status database to see what version of the "dpkg"
package is installed, and check it against a known working version.
This strange option is described as follows in the source code:
/* Print a single package which:
* (a) is the target of one or more relevant predependencies.
* (b) has itself no unsatisfied pre-dependencies.
* If such a package is present output is the Packages file entry,
* which can be massaged as appropriate.
* Exit status:
* 0 = a package printed, OK
* 1 = no suitable package available
* 2 = error
*/
On further inspection of the source code, it appears that what is does is
this:-
- Looks at the packages in the database which are selected as "install",
and are installed.
- It then looks at the Pre-Depends information for each of these packages
from the available file. When it find a package for which any of the
pre-dependencies are not satisfied, it breaks from the loop through the packages.
- It then looks through the unsatisfied pre-dependencies, and looks for
packages which would satisfy this pre-dependency, stopping on the first it
finds. If it finds none, it bombs out with an error.
- It then continues this for every dependency of the initial package.
Eventually, it writes out the record of all the packages to satisfy the
pre-dependencies. This is used by the disk method to make sure that its
dependency ordering is correct. What happens is that all pre-depending
packages are first installed, then it runs dpkg -iGROEB on the directory,
which installs in the order package files are found. Since pre-dependencies
mean that a package may not even be unpacked unless they are satisfied, it is
necessary to do this (usually, since all the package files are unpacked in one
phase, the configured in another, this is not needed).
[Abstract]
[Copyright Notice]
[Contents]
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dpkg technical manual
$Id: dpkg-tech.sgml,v 1.1 1998/07/02 02:58:12 jgg Exp $
Tom Lees tom@lpsg.demon.co.uk