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 Unix 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 like `/bin', allowing users or groups to establish custom environments.
A shell allows execution of Unix 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,
and the shell allows control over the contents of their
environment. Unix shells also provide a small set of built-in
commands (builtins) implementing functionality impossible
(e.g., cd, break, continue, and
exec), or inconvenient (history, getopts,
kill, or pwd, for example) to obtain via separate
utilities. Shells may be used interactively or
non-interactively: they accept input typed from the keyboard or
from a file. 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 have begun offering 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.
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