First, the good news -- There's almost certainly a way to recover. The bad news is that I don't exactly how to do so in this case. I'll make some suggestions below on what I think will work, but I don't run Proxmox, so I can't test directly.
But first, I'm going to make a suggestion:
- When installing software, use the official docs first, rather than a "Technical" site such as the one you referenced above.
The article you linked to above is just plain wrong and will (did) result in system corruption. The correct process is documented by the Fish team in:
With apologies, a bit of a rant -- Sites like the one you linked to are "paid content sites". They pay for articles from "freelance" (albeit "vetted") writers. Unfortunately, the content can vary wildly in quality. To give you the worst example I've seen, last year a new visitor came into a Subreddit I frequent on a technical topic. The person wrote (paraphrasing, but accurate):
I have no knowledge of [topic] and have never used it, but I've been asked to write an article about how to do [xyz] with [topic]. Can anyone give me some guidance?
Note that I do not know which site they were writing for.
Of course, not every article on every paid-content site is like that, but the article you refer to above is bad. Any documentation that will absolutely, no-questions-asked, corrupt your system is bad. The main problem is in:
Step 4 ("Make fish shell as default shell") in that doc reads:
sudo chsh -s /usr/local/bin/fish
That's just multiple mistakes wrapped up into one!
First, and worst -- That's the incorrect path for Fish under Ubuntu. That is, I believe, where Fish would be installed under macOS. Under Ubuntu, it's (typically) /usr/bin/fish, but as the Fish docs point out, you should always confirm the location on your system.
By using sudo chsh without a username, it changes the shell for the root user. This is bad, and makes recovery so much more difficult.
If you wanted to change the shell for the current user using sudo, the syntax would be:
sudo chsh -s <path/to/shell> $USER
But even that's a bad idea, as chsh as the root user (a.k.a. sudo chsh) will allow you to set a "bad shell" path. It will warn you, but it will still change /etc/passwd to use the bad shell path you told it to.
sudo chsh -s /this/is/bad $USER
grep $USER /etc/passwd
You will see the wrong path is set.
Side-note: I'm able to try these things out on my system without fear of corruption since I'm using Windows Subsystem for Linux, a virtual environment where I know I can easily recover any of these mistakes. Proxmox containers should actually be very similar to the technology that WSL uses "under-the-hood", so I'm assuming there's just as easy a recovery mechanism there.
The correct command for changing your shell to Fish under Ubuntu is simply:
chsh -s /usr/bin/fish
As long as you don't use sudo, it will change your user's shell, and it will validate that the shell is a good one.
Recovery Plan
You mention that you are using Ubuntu as a container in Proxmox. That's good, I believe. If you were using a VM, you would need to boot into Ubuntu under Proxmox using a valid shell. That would still be possible.
Again, the bad news here is that I'm not quite sure how to do this, and since Proxmox doesn't run under Ubuntu, you may need a different site (such as the Proxmox forums) to answer this. In general, you're going to need to:
Access the Proxmox commandline.
I'm assuming from the Proxmox doc that you will need to use the pct tool to manage the container.
Access the Ubuntu container with a valid shell. You might try pct exec <vmid> /usr/bin/bash.
You'll need to be root inside the Ubuntu container, which I believe will be the default for pct.
Once you have access to the Ubuntu container in a valid shell, as root:
chsh -s /usr/bin/bash
There's a chance this won't work since the shell for the current user is invalid. However, I believe that root will still be able to reset the shell (a normal user definitely would not). However, if you run into problems, the other alternative is:
vipw
This will allow you to edit your /etc/passwd directly, and change the /usr/local/bin/fish to /usr/bin/bash.
This will reset your root user's shell back to a good one. For root, I would recommend leaving the shell as Bash. Change your shell to Fish for your normal, non-root user as described in the Fish docs.
