The main shell used by the ubuntu system itself is
So try to run that. It has no features for interactive use like completion,
but it's a full shell in terms of basic shell features.
You already commented that
/bin/sh is missing, which is a symbolic link to
Either the link, or the
dash binary is missing.
But I would expect that Ubuntu can not do much without
dash, and it seems to be still up - so we can hope hope
/bin/dash is there.
Maybe some scripts in of the system are made to run with
dash, but specify to use
sh, which is now missing.
You could recreate the symbolic link
/bin/sh pointing to
sudo ln -s -r /bin/dash /bin/sh
The system itself does not need
bash - all scripts that are part of the Ubuntu system should run with the features of
Only interactive terminals run bash by default, and it's required by custom scripts when they start with the line
Other shells to try: