You're viewing the documentation for the experimental version of Vagrant (the 1.x series). The current stable series is 1.0.x and the documentation can be found here. You can learn more about the Vagrant versioning scheme here.
Configuration key: config.ssh.shell
Default value: "bash"
Vagrant executes all SSH commands from within a login shell. This configuration parameter tells Vagrant what login shell to use. This shell must already be installed on the guest, and should be an absolute path if the command isn’t available by default on the $PATH.