This article shows how to display changes in Git feature branches that are not yet merged into master.
We are going to find a simple answer for that simple question, starting off with basic git diff commands.
Let’s say you have built up the following commit tree:
---x---A---B master
\
C feature
There are two branches, master and feature. While they have two commits in common, x and A, each of them introduces another commit, B and C.
What’s on the feature branch that’s not yet merged into master?
If we look at the tree from above, we notice it’s the changes introduced by commit C we’re interested in. What’s the right Git command for showing those?
Let’s try git diff feature master first. This returns a bit more than we would like to see though: it also shows changes from the master branch.
That’s obvious if we think about it: The actual difference we’d like to see is between commit C and A, not between C and B!
This leads us to git diff C A, which yields the expected result. Unfortunately, real commit hashes are not as easy to remember as single letters, such as A and C.
In this simple example, we know that A is the parent of C, so we might replace A by feature~. In practice though, commit trees tend to look more like this one:
---x---A---o---o---o---o---o---B master
\
o---o---o---o---o---o---C feature
There’s another Git command that determines the common ancestor of two commits, which is exactly what we want to know here.
The command git merge-base master feature conveniently returns A (the commit hash).
This is everything we need to answer the question from above. Let’s combine the two git diff commands we’ve worked out so far:
git diff $(git merge-base master feature) feature
Good news is that Git offers a shorthand for exactly this, the triple-dot diff:
git diff master...feature
Note that the log sub command has a slightly different semantic. We’re going to cover that here in a follow-up article.