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Re: Help reviewing a new section of APT-Howto

On Thu, Aug 30, 2001 at 09:33:21PM -0300, Gustavo Noronha Silva wrote:

[translation edited in place]

Also, I have a few questions; they are embedded as comments.

<sect id="pin">How to keep specific versions of packages installed
You may have occasion to modify something in a package and don't have
time or don't want to port those changes to a new version of the
program.  Or, for instance, you may have just upgraded your Debian
distribution to 3.0, but want to continue with the version of a certain
package from Debian 2.2.  You can "pin" the version you have installed
so that it will not be upgraded.
Using this resource is simple.  You just need to edit the file
The format is simple:
Package: &lt;package&gt;
Pin: &lt;pin definition&gt;
Pin-Priority: &lt;pin's priority&gt;
For example, to keep package <package>sylpheed</package> that I
have modified to use "reply-to-list" at version 0.4.99, I add:
Package: sylpheed
Pin: version 0.4.99*
Note that I used an <tt>*</tt> (asterisk).  This is a "wildcard"; it say
that I want that this "pin" to be valid for all versions beginning with
0.4.99.  This is because Debian versions its packages with a "Debian
revision" and I don't want to avoid the installation of these revisions.
So, for instance, versions 0.4.99-1 and 0.4.99-10 will be installed as
soon as they are made available.
<!-- I think this is a poor example of wildcard usage because if
somebody modifies a file owned by a package, won't it get clobbered by
*any* upgrade? -->
The <code>Pin-Priority</code> field is optional; if not specified, it
defaults to a value corresponding to the current state of the package.
<!-- is this true??? -->
<!-- This doesn't help me understand a very important point.  What *is*
the default pin priority?  0?  100?  989?  999?  1000?  If no pin for a
package is specified in the preferences file, does it have an effective
pin-priority, or do pin priorities only enter the picture if a pin is
declared for a package? -->
Let's take a look at how pin priorities work.  A priority lower than 0
indicates that the package should never be installed.  Priorities 0 to
100 denote packages that are not installed and that have no available
versions.  These won't come into the version-choosing process.  Priority
100 is the priority assigned to an installed package - for the installed
version of a package to be replaced by a different version, the
replacement must have a priority greater than 100.
Priorities above 100 indicate that a package should be installed.
Typically, the installed version of a package is changed only to upgrade
it to a newer version.  Any priority between 100 and 1000 (inclusive)
indicates this typical behavior.  A package with such a priority will
not downgrade to an available version with a lower version number.  For
instance, if I have sylpheed 0.5.3 installed and define a pin on
sylpheed 0.4.99 with priority 999, package 0.4.99 will <em>not</em> be
installed to satisfy the pin.  To make a package "downgradable", to
satisfy the pin, it needs possess a priority greater than 1000.
This concept of downgrading can have powerful applications; suppose that
you just upgraded your <tt>stable</tt> version of Debian to the
<tt>testing</tt> one but wanted to retract that decision - perhaps
<tt>testing</tt> just wasn't "tested" enough to suit you.  You can
define a default pin, using a wildcard for the package name, to come
back to stable.  For example:
Package: *
Pin: release a=stable
Pin-Priority: 1001
After that, a <tt>apt-get -u dist-upgrade</tt> will downgrade your
system to the stable version.
A pin can be specified on a package's <tt>version</tt>, <tt>release</tt>
or <tt>origin</tt>.
Pinning on a <tt>version</tt>, as we have seen, supports literal
version numbers as well as wildcards to specify several
versions at one time.
<!-- what's available?  standard shell globs?  ?*[]   or more? -->
Option <tt>release</tt> depends on the Release file from an APT
repository or from a CD.  This option may be of no use at all if you're
using package repositories that don't provide this file.  You may see
the contents of the Release files that you have on
<tt>/var/lib/apt/lists/</tt>.  The paramters for a release are:
<tt>a</tt> (archive), <tt>c</tt> (components), <tt>v</tt> (version),
<tt>o</tt> (origin) and <tt>l</tt> (label).
An example:
Package: *
Pin: release v=2.2*,a=stable,c=main,o=Debian,l=Debian
Priority: 1001
In this example, we chose version 2.2* of Debian (which can be 2.2r2,
2.2r3 -- this accomodates "point releases" that typically include
security fixes and other very important updates), the <tt>stable</tt>
repository, section <tt>main</tt> (as opposed to <tt>contrib</tt> or
<tt>non-free</tt>) and origin and label Debian.  Origin (o=) defines who
produced that Release file, the label (l=) defines the name of the
distribution: Debian for Debian itself and Progeny for Progeny, for
example.  A sample Release file:
$ cat /var/lib/apt/lists/ftp.debian.org.br_debian_dists_potato_main_binary-i386_Release
Archive: stable
Version: 2.2r3
Component: main
Origin: Debian
Label: Debian
Architecture: i386

G. Branden Robinson                |     Exercise your freedom of religion.
Debian GNU/Linux                   |     Set fire to a church of your
branden@debian.org                 |     choice.
http://people.debian.org/~branden/ |

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