When SSHing to Vcenter Appliance it then displayed the message "Shell is disabled." when trying to launch the bash shell. Entering the command "shell.set --enabled true" will temporarily enable it again.

Command> shell
Shell is disabled.
Command> shell.set --enabled true
Command> shell
    ---------- !!!! WARNING WARNING WARNING !!!! ----------

Your use of "pi shell" has been logged!

The "pi shell" is intended for advanced troubleshooting operations and while
supported in this release, is a deprecated interface, and may be removed in a
future version of the product.  For alternative commands, exit the "pi shell"
and run the "help" command.

The "pi shell" command launches a root bash shell.  Commands within the shell
are not audited, and improper use of this command can severely harm the
system.

Help us improve the product!  If your scenario requires "pi shell," please
submit a Service Request, or post your scenario to the
https://communities.vmware.com/community/vmtn/vcenter/vc forum and add
"appliance" tag.