"The bash shell is one of several shells available for
Linux. It is also called the Bourne-again shell, after Stephen
Bourne, the creator of an earlier shell (/bin/sh). Bash is
substantially compatible with sh, but it provides many improvements
in both function and programming capability. It incorporates
features from the Korn shell (ksh) and C shell (csh), and is
intended to be a POSIX-compliant shell.
"Before we delve deeper into bash, recall that a shell is a
program that accepts and executes commands. It also supports
programming constructs, allowing complex commands to be built from
smaller parts. These complex commands, or scripts, can be saved as
files to become new commands in their own right. Indeed, many
commands on a typical Linux system are scripts.
"Shells have some builtin commands, such as cd, break, and exec.
Other commands are external."