Built-in POSIX commands are commands which the POSIX shell interprets and processes itself without, consequently, generating a new process. They differ from all the other “external” commands in the following ways:
As subprograms, they are components of the binary file /bin/sh. This means that you are not able to rename them.
You can change access permissions for the /bin/sh file only, not for the individual built-in POSIX commands.The POSIX shell prioritizes execution of built-in POSIX commands. If a built-in POSIX command has the same name as an external command then the POSIX shell always executes the built-in POSIX command when this name is specified. The external command is executed if the call name contains a slash (/), for example if it is called with the corresponding absolute path name.
Built-in POSIX commands are faster because they are executed by the POSIX shell itself.