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.
$ type cd cd is a shell builtin $ type mkdir mkdir is /bin/mkdir
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.
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