# Debian packaging manual - chapter 5
Version numbering

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:

epoch
  This is a single unsigned integer, which should usually be small. It may be omitted, in which case zero is assumed. If it is omitted then the upstream-version may not contain any colons.

  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.

upstream-version
  This is the main part of the version. It is usually version number of the original (`upstream') package of which the `.deb` file has been made, if this is applicable. Usually this will be in the same format as that specified by the upstream author(s); however, it may need to be reformatted to fit into `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.

debian-revision
  This part of the version represents the version of the modifications that were made to the package to make it a Debian binary package. It is in the same format as the upstream-version and `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 ina script, you may use `dpkg --compare-versions ...`. Type `dpkg --help` --> --for details on arguments.

Debian packaging manual - Copyright ©1996 Ian Jackson.
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