In Unix, a shell builtin is a command or a function, called from a shell, that is executed directly in the shell itself, instead of an external executable program which the shell would load and execute.

How can I tell if the command I am using is a shell builtin rather than an external program?

• type <command_name>

$ type cd
cd is a shell builtin

$ type mkdir
mkdir is /bin/mkdir

But I want to use an external program rather than a shell-builtin.

If a command specified to the shell contains a slash /, the shell will not execute a builtin command. Thus, while specifying echo causes a builtin command to be executed under shells that support the builtin echo command, specifying /bin/echo does not.

Why can I not use sudo <shell_builtin> ?

A shell builtin is not an executable file so sudo does not know about it. You will need to use the command sudo -s to get a new shell with root privileges or alternatively you can do sudo bash -c "<shell_builtin>" args?.