jj/docs/tutorial.md

Tutorial

This text assumes that the reader is familiar with Git.

Preparation

If you haven't already, make sure you install and configure Jujutsu.

Cloning a Git repo

Let's start by cloning GitHub's Hello-World repo using jj:

# Note the "git" before "clone" (there is no support for cloning native jj
# repos yet)
$ jj git clone https://github.com/octocat/Hello-World
Fetching into new repo in "/tmp/tmp.O1DWMiaKd4/Hello-World"
Working copy now at: d7439b06fbef (no description set)
Added 1 files, modified 0 files, removed 0 files
$ cd Hello-World

Running jj st (short forjj status) now yields something like this:

$ jj st
Parent commit: 7fd1a60b01f9 Merge pull request #6 from Spaceghost/patch-1
Working copy : d7439b06fbef (no description set)
The working copy is clean

We can see from the output above that our working copy is a real commit with a commit ID (d7439b06fbef in the example). When you make a change in the working copy, the working-copy commit gets automatically amended by the next jj command.

Creating our first change

Now let's say we want to edit the README file in the repo to say "Goodbye" instead of "Hello". Let's start by describing the change (adding a commit message) so we don't forget what we're working on:

# This will bring up $EDITOR (or `pico` or `Notepad` by default). Enter
# something like "Say goodbye" in the editor and then save the file and close
# the editor.
$ jj describe
Working copy now at: e427edcfd0ba Say goodbye

Now make the change in the README:

# Adjust as necessary for compatibility with your flavor of `sed`
$ sed -i 's/Hello/Goodbye/' README
$ jj st
Parent commit: 7fd1a60b01f9 Merge pull request #6 from Spaceghost/patch-1
Working copy : 5d39e19dac36 Say goodbye
Working copy changes:
M README

Note that you didn't have to tell Jujutsu to add the change like you would with git add. You actually don't even need to tell it when you add new files or remove existing files. To untrack a path, add it to your .gitignore and run jj untrack <path>.

To see the diff, run jj diff:

$ jj diff --git  # Feel free to skip the `--git` flag
diff --git a/README b/README
index 980a0d5f19...1ce3f81130 100644
--- a/README
+++ b/README
@@ -1,1 +1,1 @@
-Hello World!
+Goodbye World!

Jujutsu's diff format currently defaults to inline coloring of the diff (like git diff --color-words), so we used --git above to make the diff readable in this tutorial.

As you may have noticed, the working-copy commit's ID changed both when we edited the description and when we edited the README. However, the parent commit stayed the same. Each change to the working-copy commit amends the previous version. So how do we tell Jujutsu that we are done amending the current change and want to start working on a new one? That is what jj new is for. That will create a new commit on top of your current working-copy commit. The new commit is for the working-copy changes. For familiarity for user coming from other VCSs, there is also a jj checkout/co command, which is practically a synonym for jj new (you can specify a destination for jj new as well).

So, let's say we're now done with this change, so we create a new change:

$ jj new
Working copy now at: aef4df99ea11 (no description set)
$ jj st
Parent commit: 5d39e19dac36 Say goodbye
Working copy : aef4df99ea11 (no description set)
The working copy is clean

If we later realize that we want to make further changes, we can make them in the working copy and then run jj squash. That command squashes the changes from a given commit into its parent commit. Like most commands, it acts on the working-copy commit by default. When run on the working-copy commit, it behaves very similar to git commit --amend, and jj amend is in fact an alias for jj squash.

Alternatively, we can use jj edit <commit> to resume editing a commit in the working copy. Any further changes in the working copy will then amend the commit. Whether you choose to checkout-and-squash or to edit typically depends on how done you are with the change; if the change is almost done, it makes sense to use jj checkout so you can easily review your adjustments with jj diff before running jj squash.

The log command and "revsets"

You're probably familiar with git log. Jujutsu has very similar functionality in its jj log command:

$ jj log
@  mpqrykypylvy martinvonz@google.com 2023-02-12 15:00:22.000 -08:00 aef4df99ea11
│  (empty) (no description set)
◉  kntqzsqtnspv martinvonz@google.com 2023-02-12 14:56:59.000 -08:00 5d39e19dac36
│  Say goodbye
◉  orrkosyozysx octocat@nowhere.com 2012-03-06 15:06:50.000 -08:00 master 7fd1a60b01f9
│  (empty) Merge pull request #6 from Spaceghost/patch-1
~

The @ indicates the working-copy commit. The first ID on a line (e.g. "mpqrykypylvy" above) is the "change ID", which is an ID that follows the commit as it's rewritten (similar to Gerrit's Change-Id). The second ID is the commit ID, which changes when you rewrite the commit. You can give either ID to commands that take revisions as arguments. We will generally prefer change IDs because they stay the same when the commit is rewritten.

By default, jj log lists your local commits, with some remote commits added for context. The ~ indicates that the commit has parents that are not included in the graph. We can use the --revisions/-r flag to select a different set of revisions to list. The flag accepts a "revset", which is an expression in a simple language for specifying revisions. For example, @ refers to the working-copy commit, root() refers to the root commit, branches() refers to all commits pointed to by branches. We can combine expressions with | for union, & for intersection and ~ for difference. For example:

$ jj log -r '@ | root() | branches()'
@  mpqrykypylvy martinvonz@google.com 2023-02-12 15:00:22.000 -08:00 aef4df99ea11
╷  (empty) (no description set)
╷ ◉  kowxouwzwxmv octocat@nowhere.com 2014-06-10 15:22:26.000 -07:00 test b3cbd5bbd7e8
╭─╯  Create CONTRIBUTING.md
│ ◉  tpstlustrvsn support+octocat@github.com 2018-05-10 12:55:19.000 -05:00 octocat-patch-1 b1b3f9723831
├─╯  sentence case
◉  orrkosyozysx octocat@nowhere.com 2012-03-06 15:06:50.000 -08:00 master 7fd1a60b01f9
╷  (empty) Merge pull request #6 from Spaceghost/patch-1
◉  zzzzzzzzzzzz 1970-01-01 00:00:00.000 +00:00 000000000000
   (empty) (no description set)

The 000000000000 commit (change ID zzzzzzzzzzzz) is a virtual commit that's called the "root commit". It's the root commit of every repo. The root() function in the revset matches it.

There are also operators for getting the parents (foo-), children (foo+), ancestors (::foo), descendants (foo::), DAG range (foo::bar, like git log --ancestry-path), range (foo..bar, same as Git's). See the revset documentation for all revset operators and functions.

Conflicts

Now let's see how Jujutsu deals with merge conflicts. We'll start by making some commits. We use jj new with the --message/-m option to set change descriptions (commit messages) right away.

# Start creating a chain of commits off of the `master` branch
$ jj new master -m A; echo a > file1
Working copy now at: 00a2aeed556a A
Added 0 files, modified 1 files, removed 0 files
$ jj new -m B1; echo b1 > file1
Working copy now at: 967d9f9fd288 B1
$ jj new -m B2; echo b2 > file1
Working copy now at: 8ebeaffa332b B2
$ jj new -m C; echo c > file2
Working copy now at: 62a3c6d315cd C
$ jj log
@  qzvqqupxlkot martinvonz@google.com 2023-02-12 15:07:41.946 -08:00 2370ddf3fa39
│  C
◉  puqltuttrvzp martinvonz@google.com 2023-02-12 15:07:33.000 -08:00 daa6ffd5a09a
│  B2
◉  ovknlmrokpkl martinvonz@google.com 2023-02-12 15:07:24.000 -08:00 7d7c6e6bd0b4
│  B1
◉  nuvyytnqlquo martinvonz@google.com 2023-02-12 15:07:05.000 -08:00 5dda2f097aa9
│  A
│ ◉  kntqzsqtnspv martinvonz@google.com 2023-02-12 14:56:59.000 -08:00 5d39e19dac36
├─╯  Say goodbye
◉  orrkosyozysx octocat@nowhere.com 2012-03-06 15:06:50.000 -08:00 master 7fd1a60b01f9
│  (empty) Merge pull request #6 from Spaceghost/patch-1
~

We now have a few commits, where A, B1, and B2 modify the same file, while C modifies a different file. Let's now rebase B2 directly onto A. We use the --source/-s option on the change ID of B2, and --destination/-d option on A.

$ jj rebase -s puqltuttrvzp -d nuvyytnqlquo
Rebased 2 commits
Working copy now at: 1978b53430cd C
Added 0 files, modified 1 files, removed 0 files
$ jj log
@  qzvqqupxlkot martinvonz@google.com 2023-02-12 15:08:33.000 -08:00 1978b53430cd conflict
│  C
◉  puqltuttrvzp martinvonz@google.com 2023-02-12 15:08:33.000 -08:00 f7fb5943ee41 conflict
│  B2
│ ◉  ovknlmrokpkl martinvonz@google.com 2023-02-12 15:07:24.000 -08:00 7d7c6e6bd0b4
├─╯  B1
◉  nuvyytnqlquo martinvonz@google.com 2023-02-12 15:07:05.000 -08:00 5dda2f097aa9
│  A
│ ◉  kntqzsqtnspv martinvonz@google.com 2023-02-12 14:56:59.000 -08:00 5d39e19dac36
├─╯  Say goodbye
◉  orrkosyozysx octocat@nowhere.com 2012-03-06 15:06:50.000 -08:00 master 7fd1a60b01f9
│  (empty) Merge pull request #6 from Spaceghost/patch-1
~

There are several things worth noting here. First, the jj rebase command said "Rebased 2 commits". That's because we asked it to rebase commit B2 with the -s option, which also rebases descendants (commit C in this case). Second, because B2 modified the same file (and word) as B1, rebasing it resulted in conflicts, as the jj log output indicates. Third, the conflicts did not prevent the rebase from completing successfully, nor did it prevent C from getting rebased on top.

Now let's resolve the conflict in B2. We'll do that by creating a new commit on top of B2. Once we've resolved the conflict, we'll squash the conflict resolution into the conflicted B2. That might look like this:

$ jj new puqltuttrvzp  # Replace the ID by what you have for B2
Working copy now at: c7068d1c23fd (no description set)
Added 0 files, modified 0 files, removed 1 files
$ jj st
Parent commit: f7fb5943ee41 B2
Working copy : c7068d1c23fd (no description set)
The working copy is clean
There are unresolved conflicts at these paths:
file1    2-sided conflict
$ cat file1
<<<<<<<
%%%%%%%
-b1
+a
+++++++
b2
>>>>>>>
$ echo resolved > file1
$ jj squash
Rebased 1 descendant commits
Working copy now at: e3c279cc2043 (no description set)
$ jj log
@  ntxxqymrlvxu martinvonz@google.com 2023-02-12 19:34:09.000 -08:00 e3c279cc2043
│  (empty) (no description set)
│ ◉  qzvqqupxlkot martinvonz@google.com 2023-02-12 19:34:09.000 -08:00 b9da9d28b26b
├─╯  C
◉  puqltuttrvzp martinvonz@google.com 2023-02-12 19:34:09.000 -08:00 2c7a658e2586
│  B2
│ ◉  ovknlmrokpkl martinvonz@google.com 2023-02-12 15:07:24.000 -08:00 7d7c6e6bd0b4
├─╯  B1
◉  nuvyytnqlquo martinvonz@google.com 2023-02-12 15:07:05.000 -08:00 5dda2f097aa9
│  A
│ ◉  kntqzsqtnspv martinvonz@google.com 2023-02-12 14:56:59.000 -08:00 5d39e19dac36
├─╯  Say goodbye
◉  orrkosyozysx octocat@nowhere.com 2012-03-06 15:06:50.000 -08:00 master 7fd1a60b01f9
│  (empty) Merge pull request #6 from Spaceghost/patch-1
~

Note that commit C automatically got rebased on top of the resolved B2, and that C is also resolved (since it modified only a different file).

By the way, if we want to get rid of B1 now, we can run jj abandon ovknlmrokpkl. That will hide the commit from the log output and will rebase any descendants to its parent.

The operation log

Jujutsu keeps a record of all changes you've made to the repo in what's called the "operation log". Use the jj op (short for jj operation) family of commands to interact with it. To list the operations, use jj op log:

$ jj op log
@  d3b77addea49 martinvonz@vonz.svl.corp.google.com 2023-02-12 19:34:09.549 -08:00 - 2023-02-12 19:34:09.552 -08:00
│  squash commit 63874fe6c4fba405ffc38b0dd926f03b715cf7ef
│  args: jj squash
◉  6fc1873c1180 martinvonz@vonz.svl.corp.google.com 2023-02-12 19:34:09.548 -08:00 - 2023-02-12 19:34:09.549 -08:00
│  snapshot working copy
◉  ed91f7bcc1fb martinvonz@vonz.svl.corp.google.com 2023-02-12 19:32:46.007 -08:00 - 2023-02-12 19:32:46.008 -08:00
│  new empty commit
│  args: jj new puqltuttrvzp
◉  367400773f87 martinvonz@vonz.svl.corp.google.com 2023-02-12 15:08:33.917 -08:00 - 2023-02-12 15:08:33.920 -08:00
│  rebase commit daa6ffd5a09a8a7d09a65796194e69b7ed0a566d and descendants
│  args: jj rebase -s puqltuttrvzp -d nuvyytnqlquo
[many more lines]

The most useful command is jj undo (alias for jj op undo), which will undo an operation. By default, it will undo the most recent operation. Let's try it:

$ jj undo
Working copy now at: 63874fe6c4fb (no description set)
$ jj log
@  zxoosnnpvvpn martinvonz@google.com 2023-02-12 19:34:09.000 -08:00 63874fe6c4fb
│  (no description set)
│ ◉  qzvqqupxlkot martinvonz@google.com 2023-02-12 15:08:33.000 -08:00 1978b53430cd conflict
├─╯  C
◉  puqltuttrvzp martinvonz@google.com 2023-02-12 15:08:33.000 -08:00 f7fb5943ee41 conflict
│  B2
│ ◉  ovknlmrokpkl martinvonz@google.com 2023-02-12 15:07:24.000 -08:00 7d7c6e6bd0b4
├─╯  B1
◉  nuvyytnqlquo martinvonz@google.com 2023-02-12 15:07:05.000 -08:00 5dda2f097aa9
│  A
│ ◉  kntqzsqtnspv martinvonz@google.com 2023-02-12 14:56:59.000 -08:00 5d39e19dac36
├─╯  Say goodbye
◉  orrkosyozysx octocat@nowhere.com 2012-03-06 15:06:50.000 -08:00 master 7fd1a60b01f9
│  (empty) Merge pull request #6 from Spaceghost/patch-1
~

As you can perhaps see, that undid the jj squash invocation we used for squashing the conflict resolution into commit B2 earlier. Notice that it also updated the working copy.

You can also view the repo the way it looked after some earlier operation. For example, if you want to see jj log output right after the jj rebase operation, try jj log --at-op=367400773f87 but use the hash from your own jj op log.

Moving content changes between commits

You have already seen how jj squash can combine the changes from two commits into one. There are several other commands for changing the contents of existing commits. These commands assume that you have meld installed. If you prefer a terminal-based diff editor, you can configure scm-diff-editor instead.

We'll need some more complex content to test these commands, so let's create a few more commits:

$ jj new master -m abc; printf 'a\nb\nc\n' > file
Working copy now at: f94e49cf2547 abc
Added 0 files, modified 0 files, removed 1 files
$ jj new -m ABC; printf 'A\nB\nc\n' > file
Working copy now at: 6f30cd1fb351 ABC
$ jj new -m ABCD; printf 'A\nB\nC\nD\n' > file
Working copy now at: a67491542e10 ABCD
$ jj log -r master::@
@  mrxqplykzpkw martinvonz@google.com 2023-02-12 19:38:21.000 -08:00 b98c607bf87f
│  ABCD
◉  kwtuwqnmqyqp martinvonz@google.com 2023-02-12 19:38:12.000 -08:00 30aecc0871ea
│  ABC
◉  ztqrpvnwqqnq martinvonz@google.com 2023-02-12 19:38:03.000 -08:00 510022615871
│  abc
◉  orrkosyozysx octocat@nowhere.com 2012-03-06 15:06:50.000 -08:00 master 7fd1a60b01f9
│  (empty) Merge pull request #6 from Spaceghost/patch-1
~

We "forgot" to capitalize "c" in the second commit when we capitalized the other letters. We then fixed that in the third commit when we also added "D". It would be cleaner to move the capitalization of "c" into the second commit. We can do that by running jj squash with the --interactive/-i option on the third commit. Remember that jj squash moves all the changes from one commit into its parent. jj squash -i moves only part of the changes into its parent. Now try that:

$ jj squash -i
Using default editor 'meld'; you can change this by setting ui.diff-editor
Working copy now at: 52a6c7fda1e3 ABCD

That will bring up Meld with a diff of the changes in the "ABCD" commit. Modify the right side of the diff to have the desired end state in "ABC" by removing the "D" line. Then save the changes and close Meld. If we look at the diff of the second commit, we now see that all three lines got capitalized:

$ jj diff -r @-
Modified regular file file:
   1    1: aA
   2    2: bB
   3    3: cC

The child change ("ABCD" in our case) will have the same content state after the jj squash command. That means that you can move any changes you want into the parent change, even if they touch the same word, and it won't cause any conflicts.

Let's try one final command for changing the contents of an exiting commit. That command is jj diffedit, which lets you edit the contents of a commit without checking it out.

$ jj diffedit -r @-
Using default editor 'meld'; you can change this by setting ui.diff-editor
Created 70985eaa924f ABC
Rebased 1 descendant commits
Working copy now at: 1c72cd50525d ABCD
Added 0 files, modified 1 files, removed 0 files

When Meld starts, edit the right side by e.g. adding something to the first line. Then save the changes and close Meld. You can now inspect the rewritten commit with jj diff -r @- again and you should see your addition to the first line. Unlike jj squash -i, which left the content state of the commit unchanged, jj diffedit (typically) results in a different state, which means that descendant commits may have conflicts.

Other commands for rewriting contents of existing commits are jj split, jj unsquash -i and jj move -i. Now that you've seen how jj squash -i and jj diffedit work, you can hopefully figure out how those work (with the help of the instructions in the diff).