4 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 (see section 3.2.1 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 describes 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 (see section 7.2 Job Control Builtins), the directory stack
(see section 6.8.1 Directory Stack Builtins), the command history
(see section 9.2 Bash History Builtins), and the programmable completion
facilities (see section 8.7 Programmable Completion Builtins).
Many of the builtins have been extended by POSIX or Bash.