How can I preview a merge in git?

Solution 1:

• git log ..otherbranch
  • list of changes that will be merged into current branch.
• git diff ...otherbranch
  • diff from common ancestor (merge base) to the head of what will be merged. Note the three dots, which have a special meaning compared to two dots (see below).
• gitk ...otherbranch
  • graphical representation of the branches since they were merged last time.

Empty string implies HEAD, so that's why just ..otherbranch instead of HEAD..otherbranch.

The two vs. three dots have slightly different meaning for diff than for the commands that list revisions (log, gitk etc.). For log and others two dots (a..b) means everything that is in b but not a and three dots (a...b) means everything that is in only one of a or b. But diff works with two revisions and there the simpler case represented by two dots (a..b) is simple difference from a to b and three dots (a...b) mean difference between common ancestor and b (git diff $(git merge-base a b)..b).

Solution 2:

I've found that the solution the works best for me is to just perform the merge and abort it if there are conflicts. This particular syntax feels clean and simple to me. This is Strategy 2 below.

However, if you want to ensure you don't mess up your current branch, or you're just not ready to merge regardless of the existence of conflicts, simply create a new sub-branch off of it and merge that:

Strategy 1: The safe way – merge off a temporary branch:

git checkout mybranch
git checkout -b mynew-temporary-branch
git merge some-other-branch

That way you can simply throw away the temporary branch if you just want to see what the conflicts are. You don't need to bother "aborting" the merge, and you can go back to your work -- simply checkout 'mybranch' again and you won't have any merged code or merge conflicts in your branch.

This is basically a dry-run.

Strategy 2: When you definitely want to merge, but only if there aren't conflicts

git checkout mybranch
git merge some-other-branch

If git reports conflicts (and ONLY IF THERE ARE conflicts) you can then do:

git merge --abort

If the merge is successful, you cannot abort it (only reset).

If you're not ready to merge, use the safer way above.

[EDIT: 2016-Nov - I swapped strategy 1 for 2, because it seems to be that most people are looking for "the safe way". Strategy 2 is now more of a note that you can simply abort the merge if the merge has conflicts that you're not ready to deal with. Keep in mind if reading comments!]