The working copy is where the current checkout's files are written so you can interact with them. It also where files are read from in order to create new commits (though there are many other ways of creating new commits).
Unlike most other VCSs, Jujutsu will automatically create commits from the
working-copy contents when they have changed. Most jj commands you run will
commit the working-copy changes if they have changed. The resulting revision
will replace the previous working-copy revision.
Also unlike most other VCSs, added files are implicitly tracked. That means that
if you add a new file to the working copy, it will be automatically committed
once you run e.g. jj st. Similarly, if you remove a file from the working
copy, it will implicitly be untracked. To untrack a file while keeping it in
the working copy, first make sure it's ignored and then run
jj untrack <path>.
This section only applies if you have set ui.enable-open-commits = true
in your config.
As described in the introduction, Jujutsu automatically rewrites the current
checkout with any changes from the working copy. That works well while you're
developing that revision. On the other hand, if you check out some existing
revision, you generally don't want changes to the working copy to automatically
rewrite that revision. Jujutsu has a concept of "open" and "closed" revisions to
solve this. When you check out a closed revision, Jujutsu will actually create a
new, open revision on top of it and check that out. The checked-out revision
is thus always open. When you are done making changes to the currently
checked-out revision, you close it by running jj close. That command then
updates to the rewritten revision (as most jj commands do), and since the
rewritten revision is now closed, it creates a new open revision on top. If you
check out a closed revision and make changes on top of it that you want to go
into the revision, use jj squash.
When you check out a commit with conflicts, those conflicts need to be represented in the working copy somehow. However, the file system doesn't understand conflicts. Jujutsu's solution is to add conflict markers to conflicted files when it writes them to the working copy. It also keeps track of the (typically 3) different parts involved in the conflict. Whenever it scans the working copy thereafter, it parses the conflict markers and recreates the conflict state from them. You can resolve conflicts by replacing the conflict markers by the resolved text. You don't need to resolve all conflicts at once. You can even resolve part of a conflict by updating the different parts of the conflict marker.
If the commit with conflicts was closed, your conflict resolution would be in
the working-copy commit. Once you have resolved the conflicts, you would then
typically use jj squash to move the conflict resolutions into the conflicted
commit.
If the commit with conflicts was open, it would simply not have conflicts
anymore once you have resolved them. If you prefer to do the conflict resolution
in a separate commit, you can use jj new before you resolve the conflicts and
then jj squash once you're done.
There's not yet a way of resolving conflicts in an external merge tool
(martinvonz#18). There's also no good way of
resolving conflicts between directories, files, and symlinks
(martinvonz#19). You can use jj restore to
choose one side of the conflict, but there's no way to even see where the
involved parts came from.
You probably don't want build outputs and temporary files to be under version
control. You can tell Jujutsu to not automatically track certain files by using
.gitignore files (there's no such thing as .jjignore yet).
See https://git-scm.com/docs/gitignore for details about the format.
.gitignore files are supported in any directory in the working copy, as well
as in $HOME/.gitignore. However, $GIT_DIR/info/exclude or equivalent way
(maybe .jj/gitignore) of specifying per-clone ignores is not yet supported.
You can have multiple working copies backed by a single repo. Use
jj workspace add to create a new working copy. The working copy will have a
.jj/ directory linked to the main repo. The working copy and the .jj/
directory together is called a "workspace". Each workspace can have a different
commit checked out.
Having multiple workspaces can be useful for running long-running tests in a one while you continue developing in another, for example.
When you're done using a workspace, use jj workspace forget to make the repo
forget about it. The files can be deleted from disk separately (either before or
after).