How to check out old commits without modifying them?

[eproxus]

Sometimes one needs to check out and old commit in Git to view the repo at an historical time without the intention of modifying it. Is this possible in a nice way with Jujutsu, or at least with some Git command that doesn't break Jujutsu?

For example, if I run jj edit <old changeset> Jujutsu won't let me do that on pushed commits unless I add the flag --allow-immutable. In this case, I really want the commit to be immutable because I don't intend to change it, but I'd still like to see the repo at that point in time.

I guess I could do a git checkout <sha of old commit> but will that mess with Jujutsu's view of the world?

[bnjmnt4n]

You can use jj new <old revision>. This creates a new empty commit on top of the given revision, which means any accidental changes to the working copy won't mutate the immutable commit.

[eproxus]

So simple, thanks! Also seems that Jujutsu automatically abandons it if no modifications were made 👍

[sullyj3]

I guess I could do a git checkout but will that mess with Jujutsu's view of the world?

If you checkout an old commit with git, next time you run a jj command, it will notice and automatically do that jj new for you.

~/tmp/test_jj_repo
▷ jj git init --colocate
Initialized repo in "."

test_jj_repo on main
▷ jj commit -m 'first'
Working copy now at: xtmuqlsw 69c3fd74 (empty) (no description set)
Parent commit      : uspqxryy 0ac66fa9 (empty) first

test_jj_repo on HEAD
▷ jj commit -m 'second'
Working copy now at: ylwvtzxz 75477e84 (empty) (no description set)
Parent commit      : xtmuqlsw 088ff7ec (empty) second

test_jj_repo on HEAD
▷ jj log
@  ylwvtzxz [email protected] 2025-04-29 17:28:31 75477e84
│  (empty) (no description set)
○  xtmuqlsw [email protected] 2025-04-29 17:28:31 git_head() 088ff7ec
│  (empty) second
○  uspqxryy [email protected] 2025-04-29 17:28:18 0ac66fa9
│  (empty) first
◆  zzzzzzzz root() 00000000

test_jj_repo on HEAD
▷ git checkout 0ac66fa9
Previous HEAD position was 088ff7e second
HEAD is now at 0ac66fa first

test_jj_repo on HEAD
▷ jj log
Reset the working copy parent to the new Git HEAD.
@  rqpuqyvo [email protected] 2025-04-29 17:29:03 f67b27ab
│  (empty) (no description set)
│ ○  xtmuqlsw [email protected] 2025-04-29 17:28:31 088ff7ec
├─╯  (empty) second
○  uspqxryy [email protected] 2025-04-29 17:28:18 git_head() 0ac66fa9
│  (empty) first
◆  zzzzzzzz root() 00000000

[sullyj3]

I think you're focused on a different new commit to the one that's actually under discussion. We're not concerned with commit ylwvtzxz from the first example, that's not the one I was claiming was created by "the next time you run a jj command".

In the second example, no jj commands are run between the git checkout and the final jj log that would cause the new commit nlxkvqsl to be created on top of first. I don't see how it could be anything other than the jj log that brings that new commit into existence.

[joyously]

I think you're focused on a different new commit to the one that's actually under discussion.

No, but you are ignoring my example that doesn't have any extra commit. Your example shows an extra commit due to the commands you used. Mine shows a way to do the same thing without an extra commit, thereby proving that it isn't the "next command" that made an extra commit.

[sullyj3]

I'm still struggling to understand that example. At the point you do git checkout 1e1a8d3e (ie, first), is it not the case that 1e1a8d3e is already the git HEAD, since it is the parent of the current jj commit second? So therefore git's HEAD wouldn't change, the checkout is a no-op, and jj wouldn't detect a state change?

[joyously]

So therefore git's HEAD wouldn't change,

You are correct, sir. I apologize for my misunderstanding of how Git works (I rarely use it).
I still say that you are mistaking the new commit.
I extended my example to have a third commit, and then did the checkout of the first, and the result looks more like yours, with an empty commit for the working copy. However, mine has an empty commit before the first one where yours does not.
What I think you are misinterpreting is that your working copy empty commit is only empty because your repo is empty, but it represents second, and when you checkout first, it is abandoned (because it's empty) and a new working copy is created to hold first (but it's empty because first is empty). So maybe I was misreading your loose use of "next command will do a jj new for you".

[sullyj3]

No worries

mine has an empty commit before the first one where yours does not.

My understanding of what's happening is that as soon as you create the repo, the working copy is an empty revision on top of zzzzzzzz. In your example, this is wqqrwqnw, and in my first example, it's uspqxryy. When you do new -m from here, you skip describing that initial empty revision, creating "first" on top of it - your uvmrlnyo. Whereas when you create a repository and do desc then new (or equivalently, commit), you describe the initial working copy, then create an empty revision on top - my xtmuqlsw

What I think you are misinterpreting is that your working copy empty commit is only empty because your repo is empty, but it represents second, and when you checkout first, it is abandoned (because it's empty) and a new working copy is created to hold first (but it's empty because first is empty).

I'm struggling a bit to follow this, because it's not clear to me at which points in the sequence you mean. But let's try it with a nonempty repo:

~/tmp/test_jj_repo
▷ jj git init --colocate
Initialized repo in "."

test_jj_repo on main
▷ echo foo > foo

test_jj_repo on main [?]
▷ jj desc -m 'add foo'; jj new
Working copy  (@) now at: qltrqqpn 2ac5a120 add foo
Parent commit (@-)      : zzzzzzzz 00000000 (empty) (no description set)
Working copy  (@) now at: tlumtoys b492d816 (empty) (no description set)
Parent commit (@-)      : qltrqqpn 2ac5a120 add foo

test_jj_repo on HEAD
▷ echo bar > bar

test_jj_repo on HEAD [?]
▷ jj desc -m 'add bar'; jj new
Working copy  (@) now at: tlumtoys 1e8bbe9a add bar
Parent commit (@-)      : qltrqqpn 2ac5a120 add foo
Working copy  (@) now at: plypmpzx 07e97ed8 (empty) (no description set)
Parent commit (@-)      : tlumtoys 1e8bbe9a add bar

test_jj_repo on HEAD
▷ jj log
@  plypmpzx [email protected] 2025-05-04 10:38:25 07e97ed8
│  (empty) (no description set)
○  tlumtoys [email protected] 2025-05-04 10:38:25 git_head() 1e8bbe9a
│  add bar
○  qltrqqpn [email protected] 2025-05-04 10:38:00 2ac5a120
│  add foo
◆  zzzzzzzz root() 00000000

test_jj_repo on HEAD
▷ git checkout 2ac5a120
Previous HEAD position was 1e8bbe9 add bar
HEAD is now at 2ac5a12 add foo

test_jj_repo on HEAD
▷ jj log
Reset the working copy parent to the new Git HEAD.
@  oomyuusq [email protected] 2025-05-04 10:39:02 9f882e1e
│  (empty) (no description set)
│ ○  tlumtoys [email protected] 2025-05-04 10:38:25 1e8bbe9a
├─╯  add bar
○  qltrqqpn [email protected] 2025-05-04 10:38:00 git_head() 2ac5a120
│  add foo
◆  zzzzzzzz root() 00000000

test_jj_repo on HEAD
▷ ls
foo