Git tricks

When working with GHC, there are a lot of ways you can use Git to make your life easier. Below are some of them:

Ignoring unrecorded changes in submodules


  $ git config --global diff.ignoreSubmodules dirty

to stop Git in the ghc repo from checking for unrecorded changes in the submodules.

Locate commit by SHA1

You can use<sha1-prefix> or the convenient form below (also available at the front page of

Selectively record changes to commit

Do you miss Darcs? Do you hate it when a file contains a bugfix *and* a new feature, and you want to commit both separately? That's OK! Just run:

$ git add -p

This opens the interactive diff selector, which behaves a lot like darcs record. It will go through every change you have made in the working tree, asking if you want to git add it to the index, so you can commit it afterwords.

Nota bene: this only adds files to the index, it does not commit them. Afterwords, you may commit the result using git commit. Do not use git commit -a, or you will just add all the changes to the commit!

Selectively cherry-pick a commit from a branch

You still miss Darcs. One thing that would be great is if you could just 'pluck' one commit from a branch into your tree, but not the others. Sounds good - git cherry-pick to the rescue!

$ git checkout master
$ git cherry-pick <sha1 id>

this will checkout to master, and pull in only the commit you refer to. It does not create a merge, it's as if the commit had existed on this branch all along. This is wonderfully useful for selectively plucking changes from someone's Git tree, or branch.

Merge a branch into a Super Big Commit

Let's say you have a branch foo you would like to merge into master, but you have 10 small commits on foo, and you only want to make 1 Big Commit on master. Many times, we land features in a single 'big commit' to keep the history clean. This is easily doable with:

$ git checkout master
$ git merge --squash foo

and then you can commit your new, unstaged changes into a big commit after fixing any conflicts. --squash basically tells git to merge the changes, but not merge the commits. This is exactly what you want.

Basic rebases

What if you have a branch that's slightly out of date called foo, and you want to bring it up to date with master?

$ git checkout master
$ git pull origin master
$ git rebase master foo

This will:

  • Checkout to master.
  • Update master to the latest upstream version.
  • Rebase foo onto master.

Where rebasing includes:

  • Checkout to the branch foo.
  • Discard all the commits you have made on foo, temporarily
  • Bring foo up to date with master (by fast-forwarding the tree)
  • Replay all your previous commits from foo onto the New-And-Improved foo branch

This, in effect, will bring foo up to date with master, while preserving your commits.

Q: But there was a conflict! A: That's OK. If git rebase encounters a conflict while replaying your work, it will stop and tell you so. It will ask you to fix the conflict, and git add the conflicting files. Then you can continue using git rebase --continue.

Q: I started to rebase, but I confused myself and don't know how to get out! Help! A: You can always run git rebase --abort, which will abort the current rebase operation, and return you to your working tree.

Using the reflog

Eventually when working in the repository, you'll invariably do something on accident that will delete work. Or maybe not delete your work - perhaps you simply want to "undo that thing you just did a minute ago". If you have never committed the changes, then you're out of luck (commit often, commit early - even locally!) But have you ever done something like:

  • Accidentally lost a commit, by deleting a branch?
  • Accidentally lost a commit through rebasing?
  • Amended a commit (git commit --amend), only to find out you broke it, and you want to undo the amendment?
  • Accidentally overwrote a branch with dangerous operation, like git push --force?

While you may think all hope is lost, the reflog can save you from all of these, and more. In short, the reflog is a log that records every modification which Git tracks. To understand that, first understand this: despite its appearance, the Git data model has a core tenet: it is immutable - data is never deleted, only new copies can be made (the only exception is when garbage collection deletes nodes which have no outstanding references - much like our own GC!) Not even a rebase - which can rewrite the history - can actually delete old data.

Second, we need to understand an important part of git checkout: the purpose of checkout is not to switch branches. Checkout, roughly speaking, allows you to check out your tree to any state, revision, or copy in the history. You don't have to checkout to a branch: you can checkout to a commit from 3 weeks ago, a commit that does not exist on a branch, or a completely empty branch with nothing in common. You can checkout the entire tree, or you could checkout an individual file, or a single directory. The point being: checkout takes you to a state in the history.

So with that in mind, think of reflog like the audit log you can use to see what operations were performed on the immutable git history. Every operation is tracked. Let's look at an example, from Austin's validation tree he uses to push commits:

$ git reflog --date=relative # this will open an interactive pager
ad15c2b HEAD@{5 hours ago}: pull -tu origin master: Fast-forward
75a9664 HEAD@{27 hours ago}: merge amp: Fast-forward
1ef941a HEAD@{27 hours ago}: checkout: moving from amp to master
75a9664 HEAD@{27 hours ago}: commit (amend): Implement the AMP warning (#8004)
daa9a30 HEAD@{28 hours ago}: rebase -i (finish): returning to refs/heads/amp
daa9a30 HEAD@{28 hours ago}: rebase -i (pick): Implement the AMP warning (#8004)
b20cf4e HEAD@{28 hours ago}: rebase -i (pick): Fix AMP warnings.
1ef941a HEAD@{28 hours ago}: checkout: moving from amp to 1ef941a82eafb8f22c19e2643685679d2454c24a
3e8c33e HEAD@{28 hours ago}: commit: Fix AMP warnings.
70406bc HEAD@{28 hours ago}: reset: moving to HEAD~
d2afc83 HEAD@{28 hours ago}: cherry-pick: Fix most AMP warnings.
70406bc HEAD@{28 hours ago}: commit (amend): Implement the AMP warning (#8004)
697f9da HEAD@{28 hours ago}: cherry-pick: Implement the AMP warning (#8004)
1ef941a HEAD@{28 hours ago}: checkout: moving from master to amp

The most recent operations are first, and older operations appear chronologically. Let's note a few things:

  • The work you previously had still exists, and has a commit ID. It is on the far left.
  • The reflog tells you what operation resulted in the commit: in my history, we can see I did:
    • At one point, I reset my tree and undid my latest commit (in 70406bc, using git reset.) Then I kept working.
    • Several git cherry-pick operations.
    • Several commits, and some git commit --amend operations.
    • I checked out to master.
    • Then I did a merge of the amp branch, which was a fast-forward: my previous changes had rebased the amp branch.
    • Later on, I pulled my tree and I got some updates from upstream.
  • The reflog tells you what was modified; in this case it shows you the commits I changed.

With this information, I can now restore my tree to any of those partial states. For example, let's say I git commit --amend the AMP patch in 75a9664, and did some more stuff. But then it turns out I didn't want any of that, and I didn't want the amendment either. I can easily do:

$ git checkout -b temp daa9a30

Now, I am on the temp branch, and my HEAD commit points to the patch, without any amendments. I've essentially checked out to a point in the tree without any of those changes - because git never modifies the original data, this old copy still exists. Now that I am on the temp branch, I can do any number of things. Perhaps I can just delete the old amp branch, and merge the temp branch instead now.

As you can see, the reflog saved me here: I undid some nasty work in my personal tree, which otherwise might have been much more error prone or difficult to perform.

The reflog is not needed often, but it is often indispensable when you need it.

Intermediate Git tricks

See 25 Tips for Intermediate Git Users blog post.

Advanced Git tricks

Finally, there are some advanced tips, not for the faint of heart:

Interactive rebases

At a certain point of git usage, you'll want to rewrite history by rebasing interactively. This can be done by running:

$ git rebase -i <commit range>

For example:

$ git rebase -i HEAD~10

will allow you to interactively rebase the last 10 commits on your branch. This power allows you to:

  • Reorder patches, by reordering the entries in the rebase list. If two patches can be applied independently (or, as we would say in the Darcs world, the patches commute), you can always switch their order and everything will be OK.
  • Drop patches, and completely remove them from the history, by removing them from the list.
  • Squash commits, which will let you compress a series of commits into one.
  • Reword commits, which will let you rewrite the commit message for any commit in the list, without touching anything else. (This is one of the most common ones I - Austin Seipp - use.)
Last modified 3 years ago Last modified on Apr 5, 2017 1:07:31 AM