How can I get a git submodule's associated commit ID from a past commit in the parent clone?

Is there a way, short of actually checking out the parent commit, to determine a submodule's SHA-1 commit ID based on a commit ID in the parent clone? I know I can find the currently associated SHA-1 with git submodule.

Here's an example:

• I have a clone with a single submodule foo that has changed several times in the last month.
• I have a tag in the parent clone that is a few weeks old called released-1.2.3. I want to find out what the associated SHA-1 of foo was for this tagged commit.
• I could simply check out released-1.2.3 and use git submodule to see, but I'm wondering if there's a way to do this without affecting the working tree, as I want to script it.

I want to do this because I want to construct a script to do a 'diff' on all changes within a submodule between two commits within the parent repository - i.e. "tell me what files changed within the submodule foo between these two commits in the parent."

Solution 1:

You may use git-ls-tree to see what the SHA-1 id of a given path was during a given commit:

$ git ls-tree released-1.2.3 foo
160000 commit c0f065504bb0e8cfa2b107e975bb9dc5a34b0398  foo

(My first thought was git show released-1.2.3 foo, but that fails with "fatal: bad object".)

Since you are scripting the output, you will probably want to get just the SHA-1 id by itself, e.g.:

$ git ls-tree released-1.2.3 foo | awk '{print $3}'
c0f065504bb0e8cfa2b107e975bb9dc5a34b0398

Also: When writing scripts around git, try to stick to the plumbing commands, as described in the manual. They have a more stable interface, while the more familiar “porcelain” commands will possibly change in incompatible ways.

Solution 2:

This one worked for me:

$ git rev-parse released-1.2.3^{commit}:foo
<SHA1>

Perhaps also quite easy to use in script.

Solution 3:

I did find one promising avenue:

$ git log --raw <since>..<until> --submodule -- <path/to/submodule>

With the --raw option, this does print out the (abbreviated) SHA-1 IDs corresponding to the submodule's associated commits. Unfortunately the output is very verbose and will take some work to process in a script.

What I really need is a git facility that, given a parent commit ID, gives me the latest change to a submodule prior to that commit. I.e. which submodule SHA-1 'git submodule update' would check out if I were to actually checkout that parent commit.