Question

I have created a object tag

git tag -a tag_1 -m "test tag"

But the following command reports a fatal error:

git cat-file -t tag_1^{tag}
fatal: Not a valid object name tag_1^{tag}

However, the following one works fine:

git cat-file -t master^{commit}
commit

Why does it report fatal?

Answer 1

The syntax used to specify an object in git cat-file is the one specified in gitrevisions.

<object>

The name of the object to show.
For a more complete list of ways to spell object names, see the “SPECIFYING REVISIONS” section in gitrevisions(7).

And it applies to a <rev>.

<rev>^{<type>}, e.g. v0.99.8^{commit}

A suffix ^ followed by an object type name enclosed in brace pair means the object could be a tag, and dereference the tag recursively until an object of that type is found or the object cannot be dereferenced anymore (in which case, barf).
<rev>^0 is a short-hand for <rev>^{commit}.

In this case, I am not sure it can dereference tag_1 “until an object of that type is found” (since it is already a tag).