(bashref.info)Shell Builtin Commands

   Builtin commands are contained within the shell itself.  When the
name of a builtin command is used as the first word of a simple command
(Note: Simple Commands), the shell executes the command directly,
without invoking another program.  Builtin commands are necessary to
implement functionality impossible or inconvenient to obtain with
separate utilities.

   This section briefly the builtins which Bash inherits from the
Bourne Shell, as well as the builtin commands which are unique to or
have been extended in Bash.

   Several builtin commands are described in other chapters:  builtin
commands which provide the Bash interface to the job control facilities
(Note: Job Control Builtins), the directory stack (Note: Directory
Stack Builtins), the command history (Note: Bash History Builtins),
and the programmable completion facilities (Note: Programmable
Completion Builtins).

   Many of the builtins have been extended by POSIX or Bash.