shell - Why does "git submodule add ..." write to stderr rather than stdout?

Question

The message

Cloning into 'sub-mod'...
done.

after a git submodule add... command is written to stderr. I expected the message to be written to stdout since I don't think it indicates something went wrong with the command.

I can reproduce this with the following sequence of commands:

rm   -rf /tmp/repo /tmp/module
mkdir    /tmp/repo /tmp/module

cd /tmp/module

git init  > /dev/null
echo "foo" > foo;
git add foo > /dev/null
git commit . -m "+ foo" > /dev/null


cd /tmp/repo

git init > /dev/null
git submodule add /tmp/module/ sub-mod 1> /dev/null

If I change the redirection in the last command to ... 2> /dev/null, nothing is printed.

Answer 1

This is not limited to submodules, as noted here:

The registration of the submodule will be reported to stderr, as that is consistent with the rest of progress reporting within Git.

This helps us in a later patch when we want to reuse the init_submodule function in update_clone whose stdout will be piped to shell which reads parameters off stdout in a very specific way.

You can see it also in this recent patch:

Reroute the output of stdout to stderr as it is just informative messages, not to be consumed by machines.

We want to init submodules from the helper for submodule update in a later patch and the stdout output of said helper is consumed by the parts of submodule update which are still written in shell.

So we have to be careful which messages are on stdout.