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This has been my 20% for at least a year (dedicating most of my Wednesdays to it), so let's give Google some credit for that in the README. |
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Jujutsu
Disclaimer
This is not a Google product. It is an experimental version-control system (VCS). It was written by me, Martin von Zweigbergk (martinvonz@google.com). It is my personal hobby project and my 20% project at Google. It does not indicate any commitment or direction from Google.
Introduction
Jujutsu is a Git-compatible
DVCS. It combines
features from Git (data model,
speed), Mercurial (anonymous
branching, simple CLI free from "the index",
revsets, powerful history-rewriting), and Pijul/Darcs
(first-class conflicts), with features not found in either
of them (working-copy-as-a-commit,
undo functionality, automatic rebase,
safe replication via rsync, Dropbox, or distributed file
system).
The command-line tool is called jj for now because it's easy to type and easy
to replace (rare in English). The project is called "Jujutsu" because it matches
"jj".
Features
Compatible with Git
Jujutsu has two backends. One of them is a Git backend (the other is a native one). This lets you use Jujutsu as an alternative interface to Git. The commits you create will look like regular Git commits. You can always switch back to Git.
The working copy is automatically committed
Most Jujutsu commands automatically commit the working copy. This leads to a simpler and more powerful interface, since all commands work the same way on the working copy or any other commit. It also means that you can always check out a different commit without first explicitly committing the working copy changes (you can even check out a different commit while resolving merge conflicts).
Operations update the repo first, then possibly the working copy
The working copy is only updated at the end of an operation, after all other
changes have already been recorded. This means that you can run any command
(such as jj rebase) even if the working copy is dirty.
Entire repo is under version control
All operations you perform in the repo are recorded, along with a snapshot of the repo state after the operation. This means that you can easily revert to an earlier repo state, or to simply undo a particular operation (which does not necessarily have to be the most recent operation).
Conflicts can be recorded in commits
If an operation results in conflicts, information about those conflicts will be recorded in the commit(s). The operation will succeed. You can then resolve the conflicts later. One consequence of this design is that there's no need to continue interrupted operations. Instead, you get a single workflow for resolving conflicts, regardless of which command caused them. This design also lets Jujutsu rebase merge commits correctly (unlike both Git and Mercurial).
Basic conflict resolution:
Juggling conflicts:
Automatic rebase
Whenever you modify a commit, any descendants of the old commit will be rebased onto the new commit. Thanks to the conflict design described above, that can be done even if there are conflicts. Branches pointing to rebased commits will be updated. So will the working copy if it points to a rebased commit.
Comprehensive support for rewriting history
Besides the usual rebase command, there's jj describe for editing the
description (commit message) of an arbitrary commit. There's also jj edit,
which lets you edit the changes in a commit without checking it out. To split
a commit into two, use jj split. You can even move part of the changes in a
commit to any other commit using jj move.
Status
The tool is quite feature-complete, but some important features like (the
equivalent of) git blame and git log <paths> are not yet supported. There
are also several performance bugs. It's also likely that workflows and setups
different from what I personally use are not well supported. For example,
pull-request workflows currently require too many manual steps.
I have almost exclusively used jj to develop the project itself since early
January 2021. I haven't had to re-clone from source (I don't think I've even had
to restore from backup).
There will be changes to workflows and backward-incompatible changes to the
on-disk formats before version 1.0.0. Even the binary's name may change (i.e.
away from jj). For any format changes, I'll try to implement transparent
upgrades (as I've done with recent changes), or provide upgrade commands or
scripts if requested.
Installation
# We need the "nightly" Rust toolchain. This command installs that without
# changing your default.
$ rustup install nightly
$ cargo +nightly install --git https://github.com/martinvonz/jj.git
To set up command-line completion, source the output of
jj debug completion --bash/--zsh/--fish. For example, if you use Bash:
$ source <(jj debug completion) # --bash is the default
You may also want to configure your name and email so commits are made in your
name. Create a ~/.jjconfig file and make it look something like this:
$ cat ~/.jjconfig
[user]
name = "Martin von Zweigbergk"
email = "martinvonz@google.com"
Getting started
The best way to get started is probably to go through
the tutorial. Also see the
Git comparison, which includes a table of
jj vs. git commands.
Related work
There are several tools trying to solve similar problems as Jujutsu. See related work for details.