From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeff King <peff@peff.net>
Subject: Re: Trouble with cat-file on tags
Date: Fri, 1 Apr 2016 08:26:31 -0400
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To: Sebastian Schuberth <sschuberth@gmail.com>
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On Fri, Apr 01, 2016 at 12:00:44PM +0200, Sebastian Schuberth wrote:

> This means
> 
> $ git cat-file tag refs/tags/v0.1.2
> 
> displays the *contents* of the tag, not the tag itself.

Right. `cat-file` is about looking at object content.

> Which leads me to
> the next question: For a given name of an annotated tag, how to get the hash
> of the tag object? The solution I found for now:
> 
> $ git show-ref --tags -- v0.1.2
> 92b67e2b0626519ef8cd4e9cacb2bdafba6d53f0 refs/tags/v0.1.2

Here you just want to resolve the ref, without looking at the object.
The canonical way is:

  git rev-parse --verify v0.1.2

You can also use that to peel the tag to a commit, or a commit to a tree
(e.g., with "v0.1.2^{commit}").

-Peff