git merge-index [-o] [-q] <merge-program> (-a | [--] <file>*)
This looks up the <file>(s) in the index and, if there are any merge entries, passes the SHA-1 hash for those files as arguments 1, 2, 3 (empty argument if no file), and <file> as argument 4. File modes for the three files are passed as arguments 5, 6 and 7.
If git merge-index is called with multiple <file>s (or -a) then it processes them in turn only stopping if merge returns a non-zero exit code.
Typically this is run with a script calling Git's imitation of the merge command from the RCS package.
A sample script called git merge-one-file is included in the distribution.
ALERT ALERT ALERT! The Git "merge object order" is different from the RCS merge program merge object order. In the above ordering, the original is first. But the argument order to the 3-way merge program merge is to have the original in the middle. Don't ask me why.
torvalds@ppc970:~/merge-test> git merge-index cat MM This is MM from the original tree. # original This is modified MM in the branch A. # merge1 This is modified MM in the branch B. # merge2 This is modified MM in the branch B. # current contents
torvalds@ppc970:~/merge-test> git merge-index cat AA MM cat: : No such file or directory This is added AA in the branch A. This is added AA in the branch B. This is added AA in the branch B. fatal: merge program failed
where the latter example shows how git merge-index will stop trying to merge once anything has returned an error (i.e., cat returned an error for the AA file, because it didn't exist in the original, and thus git merge-index didn't even try to merge the MM thing).
Part of the git(1) suite