jj/README.md

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. It does not indicate any commitment or direction from Google.

Introduction

I started the project mostly in order to test the viability of some UX ideas in practice. I continue to use it for that, but my short-term goal now is to make it useful as an alternative CLI for Git repos.

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" (I initially called it "Jujube", but changed since jujutsu is more well-known).

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).

• 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.

Status

The tool is quite feature-complete. I have almost exclusively used jj to develop the project itself since early January 2021. However, there will be changes to workflows and backward-incompatible changes to the on-disk formats (I'll try to provide upgrade commands if requested). It's also likely that workflows and setups different from what I personally use are not well supported.

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

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.