Unexpected effect of log.showSignature on tformat:%H.

I had expected the output of git show -s --format='tformat:%H' to be always the same, and was surprised to find that scripts using this construction started to fail after I set log.showSignature true.

See filled out bugreport form below, including a repro script.

Regards,
/Niels Möller

Thank you for filling out a Git bug report!
Please answer the following questions to help us understand your issue.

What did you do before the bug happened? (Steps to reproduce your issue)

  git config --global log.showSignature true

I'm trying this out to get to see in git log which commits are signed.

What did you expect to happen? (Expected behavior)

That git show -s --format='tformat:%H' SOME_COMMIT should still output the commit hash and nothing more.

What happened instead? (Actual behavior)

I get an extra "No signature" line in the output (on stdout, not stderr), when looking at a signed commit.

What's different between what you expected and what actually happened?

The extra output line.

Anything else you want to add:

Here's a small script to reproduce the problem:

-----8<-----
#! /bin/sh

set -eu

export GIT_CONFIG_GLOBAL="$(pwd)/git.config"
git config --global log.showSignature true

git clone https://git.glasklar.is/glasklar/trust/audit-log.git
cd audit-log

hash=$(git show -s --format='tformat:%H' e36424e5688572c196d658a5df8c04c20de7606a)

echo "Hash is: |$hash|"
-----8<-----

This is what it looks when I run it:

-----8<-----

$ ./repro.sh 
Cloning into 'audit-log'...
remote: Enumerating objects: 143, done.
remote: Counting objects: 100% (8/8), done.
remote: Compressing objects: 100% (8/8), done.
remote: Total 143 (delta 1), reused 0 (delta 0), pack-reused 135 (from 1)
Receiving objects: 100% (143/143), 347.00 MiB | 107.22 MiB/s, done.
Resolving deltas: 100% (59/59), done.
error: gpg.ssh.allowedSignersFile needs to be configured and exist for ssh signature verification
Hash is: |No signature
e36424e5688572c196d658a5df8c04c20de7606a|
-----8<-----

I would have expected the output

Hash is: |e36424e5688572c196d658a5df8c04c20de7606a|

[System Info]
git version:
git version 2.39.5
cpu: x86_64
no commit associated with this build
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
uname: Linux 6.1.0-26-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.112-1 (2024-09-30) x86_64 compiler info: gnuc: 12.2
libc info: glibc: 2.40
$SHELL (typically, interactive shell): /bin/bash

[Enabled Hooks]
not run from a git repository - no hooks to show

It is a bit unexpected, but knowing how the command options evolved, it is not all that surprising X-<. If you are using --format, you are expected to use the %G placeholder and friends when you are interested in signatures. The --show-signature option is unaware of the other formatting options like --pretty={short,oneline,fuller, so it is understandable (not "expected") what you are seeing.

The thing is, I'm generally interested in signatures when using git interactively (that's why I enabled log.showSignature globally). But then I have a scripted usecase that has a tag name, and needs to query the corresponding commit hash, and in that context, I don't care about signatures at all. The docs for git show say that %H expands to the commit hash, which is exactly what I want. I didn't know about the %G formats, but after a quick look at the docs, I don't think they solves that problem.

I can work around this problem by using "export GIT_CONFIG_GLOBAL=/dev/null" in the script, which I guess might be generally good practice when using git in scripts? But I still think it would be useful if there were an easy and reliable way to get from tag to commit hash, regardless of the user's config settings.

Regards,
/Niels Möller

git show is a porcelain, and it can be affected by options, such as log.showSignature. If you want predictable behaviour, you need a plumbing command, whose behaviour isn't affected by configuration in this way. The equivalent to what you've requested is this:

    git rev-list --no-commit-header -1 -s --format='tformat:%H' SOME_COMMIT

I've confirmed that works correctly even if log.showSignature is set.

brian m. carlson (they/them or he/him)
Toronto, Ontario, CA

Thanks! It seems the way to understand this is that git show can display a bunch of different things, depending on configuration, and --tformat affects only one of potentially many output lines.

So git rev-list solves this problem. My script (which creates a file archive, in a reproducible fashion, from a list if git repos and (signed) tags) uses a few more git commands:

  git clone
  git ls-files
  git -c gpg.ssh.allowedSignersFile=... tag --verify
  git submodule

Of these, it appears only ls-files is listed as a "plumbing" command. I wonder if I still need to stick to GIT_CONFIG_GLOBAL=/dev/null to avoid surprises for the clone, tag and submodule commands?

Any advice appreciated.

Regards,
/Niels