== Comparing RPM versions in the shell

Via [[Planet Debian http://planet.debian.org/]] I recently read
[[Comparing version numbers in the shell
http://www.df7cb.de/blog/2014/Comparing_Version_Numbers_in_Shell.html]],
which sort of suggests using _sort -V_ to compare package version
numbers on RPM-derived systems. My reaction to this is kind of mixed,
because how good an idea this is depends on what you're trying to do
with this comparison.

If you just want to compare generic version numbers, '_sort -V_'
may do what you want. You should read the coreutils documentation
for _sort_ very carefully, especially [[Details about version sort
https://www.gnu.org/software/coreutils/manual/html_node/Details-about-version-sort.html]],
because how it works may not be quite what you expect or what you
want. You probably want to test it.

If you want to compare version numbers in the same way that RPM
does it, so that your idea of a 'newer version' specifically matches
what eg '_rpm -U_' or '_yum localupdate_' would conclude, you should
use the _rpmdev-vercmp_ command (it's part of the _rpmdevtools_
package).  This uses RPM's actual version comparison code (as exposed
through the RPM Python bindings). See '_rpmdev-vercmp -h_' for basic
usage and then experiment with it; note that it prints output and
has somewhat odd status returns. You may also want to see
'_rpmdev-sort_'.

(If you just care about version numbers, you can ignore the epoch
and the release parts of ordinary RPM version numbering; _rpmdev-vercmp_
doesn't require you to supply them.)

If you want to compare the versions of actual RPM packages as _rpm_
and _yum_ do it, you really have to use _rpmdev-vercmp_ and you
need to extract full RPM version information about both packages,
including their RPM epochs if any. It's a pity that there is no
easy command to print out the full version information that
_rpmdev-vercmp_ requires (it's a bit complex and annoying for reasons
covered [[here RPMEpochNumbers]], although for most purposes you
can apparently consider a missing epoch number to be equal to an
epoch number of '0').

As it turns out, the best approximation of quickly printing out
full RPM version information for _rpmdev-vercmp_ purposes is something
like:

>  rpm -q --qf '%{N} %{epochnum}:%{V}-%{R}\n' ...

Note that you should not accidentally give _rpmdev-vercmp_ strings
that have the package name in them; it will consider them to be the
version and then interpret the version number as the first part of
the release numbering (I think).

(I discovered _rpmdev-vercmp_ via [[this stackoverflow question and
its answer
http://stackoverflow.com/questions/2996279/how-does-yum-compare-versions]]
when I was researching this entry. Once again writing blog entries has
proven usefully educational.)