Vagrant: The guest operating system of the machine could not be detected!

TLDR; check that the shell defined by your config.ssh.shell is installed

Edit: Reported as https://github.com/mitchellh/vagrant/issues/3040 and got immediate reaction, cool.

Today I encountered following error with Vagrant when executing "vagrant up":

The guest operating system of the machine could not be detected!
Vagrant requires this knowledge to perform specific tasks such
as mounting shared folders and configuring networks. Please add
the ability to detect this guest operating system to Vagrant
by creating a plugin or reporting a bug.

This box had worked before, and it was based on Ubuntu 13.10, so I was pretty confused.

After enabling debug logging for vagrant (well, after updating vagrant and trying zillion different things :)), following log was given:

.... (a lot of log)
DEBUG ssh: stderr: bash: /bin/zsh: No such file or directory

DEBUG ssh: Exit status: 127
ERROR warden: Error occurred: The guest operating system of the machine could not be detected!
.... (a lot of log)
ERROR vagrant: The guest operating system of the machine could not be detected!
Vagrant requires this knowledge to perform specific tasks such
as mounting shared folders and configuring networks. Please add
the ability to detect this guest operating system to Vagrant
by creating a plugin or reporting a bug.
ERROR vagrant: /opt/vagrant/embedded/gems/gems/vagrant-1.4.3/lib/vagrant/guest.rb:48:in `detect!'
/opt/vagrant/embedded/gems/gems/vagrant-1.4.3/lib/vagrant/machine.rb:182:in `guest'

After seeing this, the reason was obvious: I had added zsh as shell with

config.ssh.shell = "/bin/zsh"

And it was not installed to box.