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 the Jujutsu Git repo using jj:

# Note the "git" before "clone" (there is no support for cloning native jj
# repos yet)
$ jj git clone https://github.com/martinvonz/jj.git
Fetching into new repo in "<dir>/jj"
Working copy now at: 265ecf5cab2d (no description set)
Added 98 files, modified 0 files, removed 0 files
$ cd jj

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

$ jj st
Parent commit: 723ebb380971 cleanup: restructure escaped newlines to make new rustc happy
Working copy : 265ecf5cab2d (no description set)
The working copy is clean

We can see from the output above that our working copy has a commit ID (265ecf5cab2d in the example).

Let's check out a particular commit, so we get more predicable output:

$ jj co 080a9b37ff7e
Working copy now at: 608c179a60df
Added 7 files, modified 65 files, removed 21 files
$ jj st
Parent commit: 080a9b37ff7e cli: make `jj st` show parent commit before working copy commit
Working copy : 608c179a60df (no description set)
The working copy is clean

You might have noticed that even though we asked to check out some commit (080a9b37ff7e), our working-copy commit ended up being another commit (608c179a60df). That is because jj co (short for jj checkout) creates a new commit on top of the commit you asked it to check out. The new commit is for the working-copy changes.

Creating our first change

Now let's say we want to edit the README.md file in the repo to say that Jujutsu is ready for use. 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` by default). Enter something like
# "Jujutsu is ready!" in the editor and then close it.
$ jj describe
Working copy now at: b2985d68096d Jujutsu is ready!

Now make the change in the README:

# Adjust as necessary for compatibility with your flavor of `sed`
$ sed -i 's/not ready/ready/' README.md
$ jj st
Parent commit: 080a9b37ff7e cli: make `jj st` show parent commit before working copy commit
Working copy : 5f80190c44b9 Jujutsu is ready!
Working copy changes:
M README.md

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.md b/README.md
index aa9b9e31a8...c30897997c 100644
--- a/README.md
+++ b/README.md
@@ -4,7 +4,7 @@
 ## Disclaimer

 This is not a Google product. It is an experimental version-control system
-(VCS). It is not ready for use. It was written by me, Martin von Zweigbergk
+(VCS). It is ready for use. 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.

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 visible 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. That may remind you of what we said earlier that jj checkout does; jj checkout is in fact 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: 192b456b024b (no description set)
$ jj st
Parent commit: fb563a4c6d26 Jujutsu is ready!
Working copy : 192b456b024b (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. 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, "revsets", and aliases

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

$ jj log
@ f39aeb1a0200 martinvonz@google.com 2021-05-23 23:10:27.000 -07:00 192b456b024b
| (no description set)
o f63e76f175b9 martinvonz@google.com 2021-05-23 22:13:45.000 -07:00 fb563a4c6d26
| Jujutsu is ready!
o 6a91b4ba16c7 martinvonz@google.com 2021-05-23 22:08:37.000 -07:00 main 080a9b37ff7e
~ cli: make `jj st` show parent commit before working copy commit

The @ indicates the working-copy commit. The first hash on a line is the "change ID", which is an ID that follows the commit as it's rewritten (similar to Gerrit's Change-Id). The second hash is the commit ID, which changes when you rewrite the commit. You can give either hash 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 -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()'
@ f39aeb1a0200 martinvonz@google.com 2021-05-23 23:10:27.000 -07:00 192b456b024b
: (no description set)
o 6a91b4ba16c7 martinvonz@google.com 2021-05-23 22:08:37.000 -07:00 main 080a9b37ff7e
: cli: make `jj st` show parent commit before working copy commit
o 000000000000  1970-01-01 00:00:00.000 +00:00 000000000000
  (no description set)

The 000000000000 commit is a virtual commit that's called the "root commit". It's the root commit of every repo. The root symbol 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). There are also a few more functions, such as heads(<set>), which filters out revisions in the input set if they're ancestors of other revisions in the set.

Conflicts

Now let's see how Jujutsu deals with merge conflicts. We'll start by making some commits:

# Start creating a chain of commits off of the grandparent of the working copy
$ jj new @-- -m A; echo a > file1
Working copy now at: 9164f1d6a011 A
Added 0 files, modified 1 files, removed 0 files
$ jj new -m B1; echo b1 > file1
Working copy now at: 5be91b2b5b69 B1
$ jj new -m B2; echo b2 > file1
Working copy now at: fd571967346e B2
$ jj new -m C; echo c > file2
Working copy now at: 4ae1e0587eef C
$ jj log
@ 8e6178b84ffb martinvonz@google.com 2021-05-26 12:39:35.000 -07:00 1769bdaa8d6d
| C
o 5548374c0794 martinvonz@google.com 2021-05-26 12:39:30.000 -07:00 de5690380f40
| B2
o ce619d39bd96 martinvonz@google.com 2021-05-26 12:39:20.000 -07:00 47e336632333
| B1
o cf49e6bec410 martinvonz@google.com 2021-05-26 12:39:12.000 -07:00 661432c51c08
~ A

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:

$ jj rebase -s 5548374c0794 -d cf49e6bec410
Rebased 2 commits
Working copy now at: 9195b6d2e8dc C
Added 0 files, modified 1 files, removed 0 files
$ jj log
@ 8e6178b84ffb martinvonz@google.com 2021-05-26 12:39:35.000 -07:00 66274d5a7d2d conflict
| C
o 5548374c0794 martinvonz@google.com 2021-05-26 12:39:30.000 -07:00 0c305a9e6b27 conflict
| B2
| o ce619d39bd96 martinvonz@google.com 2021-05-26 12:39:20.000 -07:00 47e336632333
|/  B1
o cf49e6bec410 martinvonz@google.com 2021-05-26 12:39:12.000 -07:00 661432c51c08
~ A

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 5548374c0794  # Replace the hash by what you have for B2
Working copy now at: 619f58d8a988 (no description set)
Added 0 files, modified 1 files, removed 0 files
$ jj st
Parent commit: 5548374c0794 B2
Working copy : 619f58d8a988 (no description set)
The working copy is clean
There are unresolved conflicts at these paths:
file1
$ cat file1
<<<<<<<
%%%%%%%
-b1
+a
+++++++
b2
>>>>>>>
$ echo resolved > file1
$ jj squash
Rebased 1 descendant commits
Working copy now at: e659edc4a9fc (no description set)
$ jj log
@ 461f38324592 martinvonz@google.com 2021-05-26 12:53:08.000 -07:00 e659edc4a9fc
| (no description set)
| o 8e6178b84ffb martinvonz@google.com 2021-05-26 12:39:35.000 -07:00 69dbcf76642a
|/  C
o 5548374c0794 martinvonz@google.com 2021-05-26 12:39:30.000 -07:00 576d647acf36
| B2
| o ce619d39bd96 martinvonz@google.com 2021-05-26 12:39:20.000 -07:00 47e336632333
|/  B1
o cf49e6bec410 martinvonz@google.com 2021-05-26 12:39:12.000 -07:00 661432c51c08
~ A

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 47e336632333. 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
@ 5bd384507342 martinvonz@<hostname> 2021-05-26 12:53:08.339 -07:00 - 2021-05-26 12:53:08.350 -07:00
| squash commit 41f0d2289b568bfcdcf35f73d4f70f3ab6696398
| args: jj squash
o 2fd266a8a2e0 martinvonz@<hostname> 2021-05-26 12:53:08.335 -07:00 - 2021-05-26 12:53:08.338 -07:00
| commit working copy
o 1e6dd15305a3 martinvonz@<hostname> 2021-05-26 12:52:39.374 -07:00 - 2021-05-26 12:52:39.382 -07:00
| check out commit 0c305a9e6b274bc09b2bca85635299dcfdc6811c
| args: jj co 0c305a9e6b27
o 401652a2f61e martinvonz@<hostname> 2021-05-26 12:44:51.872 -07:00 - 2021-05-26 12:44:51.882 -07:00
| rebase commit de5690380f40f3f7fc6b7d66d43a4f68ee606228 and descendants
| args: jj rebase -s de5690380f40 -d 661432c51c08
[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: 41f0d2289b56
$ jj log
@ b1e3a4afde5e martinvonz@google.com 2021-05-26 12:52:39.000 -07:00 41f0d2289b56
| (no description set)
| o 8e6178b84ffb martinvonz@google.com 2021-05-26 12:39:35.000 -07:00 66274d5a7d2d conflict
|/  C
o 5548374c0794 martinvonz@google.com 2021-05-26 12:39:30.000 -07:00 0c305a9e6b27 conflict
| B2
| o ce619d39bd96 martinvonz@google.com 2021-05-26 12:39:20.000 -07:00 47e336632333
|/  B1
o cf49e6bec410 martinvonz@google.com 2021-05-26 12:39:12.000 -07:00 661432c51c08
~ A

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=401652a2f61e 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 vimdiff, add this to your ~/.jjconfig.toml file:

[ui]
diff-editor = "vimdiff"

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

$ jj new origin/main -m abc; printf 'a\nb\nc\n' > file
Working copy now at: 61b0efa09dbe abc
Added 0 files, modified 0 files, removed 1 files
$ jj new -m ABC; printf 'A\nB\nc\n' > file
Working copy now at: 9d97c5018b23 ABC
$ jj new -m ABCD; printf 'A\nB\nC\nD\n' > file
Working copy now at: c5a985bc3f41 ABCD
$ jj log
@ 874f2d307594 martinvonz@google.com 2021-05-26 14:36:38.000 -07:00 687009839bae 
| ABCD
o 2bbc0c1eb382 martinvonz@google.com 2021-05-26 14:36:26.000 -07:00 ad9b1ce3b5d0 
| ABC
o 3680117711f5 martinvonz@google.com 2021-05-26 14:36:05.000 -07:00 a355fb177b21 
~ abc

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 -i (short for jj squash --interactive) 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
Working copy now at: 4b4c714b36aa 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 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 @-
Created 2423c134ea70 ABC
Rebased 1 descendant commits
Working copy now at: d31c52e8ca41 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 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).