/dev/zero

All kinds of articles on the Internet tell you how to revert a range of git commits in one massive, squashed-together revert commit. But to split them up into separate revert commits you have to pass a list of commits to git revert, and that list has to go in reverse order to avoid conflicts.

git rev-list --reverse ${last_good_commit}.. | xargs git revert

We’ve all been there, running git merge only to have it spew CONFLICT all over the place and quit partway through. But if we already know that every time there is a conflict then all of “our” code that we are merging changes into should take precedence over “their” changes that we are pulling in, git 1.7.3 provides a handy shortcut for that:

% git merge -X ours theirbranch

If “their” changes that we are pulling in should always take precedence instead then things are equally easy:

% git merge -X theirs theirbranch

If “ours” or “theirs” style merges will work, but only for certain files, things get a bit more complicated. But with a bit of background knowledge of how git’s index works we can save ourselves a bunch of work.

The index stores what stage a file is in. Normally, a file’s stage is 0.

% git ls-files -s myfile
100644 4b48deed3a433909bfd6b6ab3d4b91348b6af464 0       myfile

A file with a merge conflict is different because the index actually has three different versions of it: the version that the two branches we are merging most recently had in common, the version on “our” side of the merge, and the version on “their” side of the merge. In git these correspond to stages 1, 2, and 3. This is one reason one has to run git add on every conflicting file before completing a merge.

% git ls-files -s myfile
100644 4b48deed3a433909bfd6b6ab3d4b91348b6af464 1       myfile
100644 5be4a414b32cf4204f889469942986d3d783da84 2       myfile
100644 39c5733494077ae8bc45c45c15a708ffe9871966 3       myfile

Rather than opening up the conflicting file and clearing out the parts that clash by hand, we can instead simply copy the version we want from the index into our working tree:

% git checkout-index -f --stage 3 myfile
% git add myfile
% git commit

Today I found myself needing to move some commits between two repositories. In general the best way to do this is by pulling changes from one into the other, but in this case the repositories did not have direct access to each other. Rather than copying an entire repository from one machine to another or mucking about with a pile of patches, we can save time by performing the sending and receiving sides of the network-enabled git fetch command by hand.

In the source repository, add the changes we want to move to a bundle that we can copy to a USB stick:

% git bundle create changes.bundle master..mybranch
Counting objects: 5, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 313 bytes, done.
Total 3 (delta 2), reused 0 (delta 0)
% cp changes.bundle /media/usbstick

In the destination repository, ensure that we have the commits necessary to use the bundle and then tell git fetch to grab the changes from it:

% git bundle verify /media/usbstick/changes.bundle
The bundle contains 1 ref
7a1d2087f10e6db33e6b4a28e2c427b65238a62c refs/heads/mybranch
The bundle requires these 1 ref
6f5fced94ef76f1b46e259db72ad6fc39c49ba72 
/media/usbstick/changes.bundle is okay
% git fetch /media/usbstick/changes.bundle mybranch
Receiving objects: 100% (3/3), done.
Resolving deltas: 100% (2/2), completed with 2 local objects.
From /media/usbstick/changes.bundle
 * branch            mybranch   -> FETCH_HEAD
% git merge FETCH_HEAD
Updating 6f5fced..7a1d208
Fast-forward
 README |    2 ++
 1 file changed, 2 insertions(+)

Today I did some work on a branch in git only to discover that I based it on some unstable code rather than the stable code that I usually want to use as a baseline.

You can fix this by using a bit of git’s rebase magic:

git rebase --onto master testing

This moves the entire current branch onto another.