Why does git-describe prefix the commit ID with the letter 'g'?

Question

Typical output of git-describe looks like

some-tag-32-gf31f980 

where some-tag is a tag name, 32 means that the described commit is 32 commits after the commit with that tag, and gf31f980 means that the commit ID is uniquely abbreviated to f31f980.

My question is about the g in gf31f980. Why is it there? My first thought was that it is inserted to disambiguate parsing of the output of git-describe. But I can't think of any case in which it actually helps to have it. For example, the 32 component might be omitted, and there is no way to know that the output above describes a commit 32 commits after the tag some-tag, rather than the commit at the tag some-tag-32. But the g doesn't help with this.

A regex match to extract just the commit ID can search for /-g([0-9a-f]+)$/. There is no easy way to simplify this; you can't do /-g(.*)$/ for example because that might erroneously match a g in the tag name. Without the g you could still do /-([0-9a-f]+)$/, so the g isn't helping you there. Non-regex parsing processes will behave similarly.

The g is generated explicitly; the relevant source code (around line 240 of builtin/describe.c) is:

static void show_suffix(int depth, const unsigned char *sha1) {         printf("-%d-g%s", depth, find_unique_abbrev(sha1, abbrev)); } 

It's hard to search for information about this, because the relevant term, g, is a stopword.

What's the g for?

Answer 1

Jesse Luehrs immediately pointed out on Twitter that this question is answered in the git-describe man page:

The "g" prefix stands for "git" and is used to allow describing the version of a software depending on the SCM the software is managed with.