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(bashref.info)What is a shell?

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What is a shell?
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   At its base, a shell is simply a macro processor that executes
commands.  A Unix shell is both a command interpreter, which provides
the user interface to the rich set of GNU utilities, and a programming
language, allowing these utilitites to be combined.  Files containing
commands can be created, and become commands themselves.  These new
commands have the same status as system commands in directories such as
`/bin', allowing users or groups to establish custom environments.

   A shell allows execution of GNU commands, both synchronously and
asynchronously.  The shell waits for synchronous commands to complete
before accepting more input; asynchronous commands continue to execute
in parallel with the shell while it reads and executes additional
commands.  The "redirection" constructs permit fine-grained control of
the input and output of those commands.  Moreover, the shell allows
control over the contents of commands' environments.  Shells may be
used interactively or non-interactively: they accept input typed from
the keyboard or from a file.

   Shells also provide a small set of built-in commands ("builtins")
implementing functionality impossible or inconvenient to obtain via
separate utilities.  For example, `cd', `break', `continue', and
`exec') cannot be implemented outside of the shell because they
directly manipulate the shell itself.  The `history', `getopts',
`kill', or `pwd' builtins, among others, could be implemented in
separate utilities, but they are more convenient to use as builtin
commands.  All of the shell builtins are described in subsequent
sections.

   While executing commands is essential, most of the power (and
complexity) of shells is due to their embedded programming languages.
Like any high-level language, the shell provides variables, flow
control constructs, quoting, and functions.

   Shells offer features geared specifically for interactive use rather
than to augment the programming language.  These interactive features
include job control, command line editing, history and aliases.  Each
of these features is described in this manual.