Every package has a version number, in its Version control file field.
dpkg
imposes an ordering on version numbers, so that it can tell
whether packages are being up- or downgraded and so that dselect
can tell whether a package it finds available is newer than the one installed
on the system. The version number format has the most significant parts (as
far as comparison is concerned) at the beginning.
The version number format is: [epoch/:]upstream-version[-/debian-revision].
The three components here are:
It is provided to allow mistakes in the version numbers of older versions of a package, and also a package's previous version numbering schemes, to be left behind.
dpkg
will not usually display the epoch unless it is essential
(non-zero, or if the upstream-version contains a colon);
dselect
does not display epochs at all in the main part of the
package selection display.
dpkg
's format and comparison scheme.
The comparison behaviour of dpkg
with respect to the
upstream-version is described below. The
upstream-version portion of the version number is mandatory.
The upstream-version may contain only alphanumerics and the characters . + - : (full stop, plus, hyphen, colon) and should start with a digit. If there is no debian-revision then hyphens are not allowed; if there is no epoch then colons are not allowed.
dpkg
compares it in
the same way.
It is optional; if it isn't present then the upstream-version may not contain a hyphen. This format represents the case where a piece of software was written specifically to be turned into a Debian binary package, and so there is only one `debianization' of it and therefore no revision indication is required.
It is conventional to restart the debian-revision at 1 each time the upstream-version is increased.
dpkg
will break the upstream-version and
debian-revision apart at the last hyphen in the string. The absence
of a debian-revision compares earlier than the presence of one (but
note that the debian-revision is the least significant part of the
version number).
The debian-revision may contain only alphanumerics and the characters + and . (plus and full stop).
The upstream-version and debian-revision parts are
compared by dpkg
using the same algorithm:
The strings are compared from left to right.
First the initial part of each string consisting entirely of non-digit characters is determined. These two parts (one of which may be empty) are compared lexically. If a difference is found it is returned. The lexical comparison is a comparison of ASCII values modified so that all the letters sort earlier than all the non-letters.
Then the initial part of the remainder of each string which consists entirely of digit characters is determined. The numerical values of these two parts are compared, and any difference found is returned as the result of the comparison. For these purposes an empty string (which can only occur at the end of one or both version strings being compared) counts as zero.
These two steps are repeated (chopping initial non-digit strings and initial digit strings off from the start) until a difference is found or both strings are exhausted.
Note that the purpose of epochs is to allow us to leave behind mistakes in
version numbering, and to cope with situations where the version numbering
changes. It is not there to cope with version numbers containing
strings of letters which dpkg
cannot interpret (such as
ALPHA or pre-), or with silly orderings (the author
of this manual has heard of a package whose versions went 1.1,
1.2, 1.3, 1, 2.1,
2.2, 2 and so forth).
If an upstream package has problematic version numbers they should be converted to a sane form for use in the Version field.
If you need to compare version numbers in a script, you may use dpkg --compare-versions .... Type dpkg --help --> --for details on arguments.
In general, Debian packages should use the same version numbers as the upstream sources.
However, in some cases where the upstream version number is based on a date (e.g., a development `snapshot' release) dpkg cannot handle these version numbers currently, without epochs. For example, dpkg will consider `96May01' to be greater than `96Dec24'.
To prevent having to use epochs for every new upstream version, the version number should be changed to the following format in such cases: `19960501', `19961224'. It is up to the maintainer whether he/she wants to bother the upstream maintainer to change the version numbers upstream, too.
Note, that other version formats based on dates which are parsed correctly by dpkg should not be changed.
Native Debian packages (i.e., packages which have been written especially for Debian) whose version numbers include dates should always use the `YYYYMMDD' format.
ijackson@gnu.ai.mit.edu
bweaver@debian.org
schwarz@debian.org
srivasta@debian.org
debian-policy@lists.debian.org