How to determine if Git merge is in process

Question

Is there a Git command that can be used to determine if a merge is in-process (i.e. uncommitted)? I know I can simply check for .git/MERGE_HEAD, but is this pr

Answer 1

You can safely check for the existence of MERGE_HEAD in your git directory[1] to determine whether there's a merge in progress. The MERGE_HEAD file contains the commit ids that are being merged in, that will be the nth parents of the committed merge. For example, if I try to merge branch br1 which is at c085e0c:

> git merge --no-ff --no-commit br1
Automatic merge went well; stopping before committing as requested

> cat .git/MERGE_HEAD
c085e0caa2598064bfde9cc6318bd2c73fee2371

If you are doing an octopus merge, then you will see more than one commit id in this file (newline separated):

> git merge --no-ff --no-commit br2 br3
Fast-forwarding to: br2
Trying simple merge with br3
Automatic merge went well; stopping before committing as requested

> cat .git/MERGE_HEAD
69b0169b1cba60de86161b375f013c59ad9581d8
99dad0187351d61ae8e88d49c96d7383f9e8fa6d

Using MERGE_HEAD is actually how git clients determine whether there's a merge in progress. Some provide this insight through leaky abstractions:

> git merge --abort
fatal: There is no merge to abort (MERGE_HEAD missing).

So checking for the presence of MERGE_HEAD is perfectly acceptable and is is as future proof as anything in software - meaning that any changes to git-merge would have to take compatibility with other git clients into account and would (hopefully!) gracefully transition and/or provide backward compatibility.

[1] You can trust the existence of MERGE_HEAD much more than you can trust that .git is underneath your working directory. Somebody could use the --git-dir flag or the GIT_DIR environment variable.