SYNTAX

The Bourne shell a pure interpreter for a highly interactive, complex
and powerful programming language with syntax based loosely on
ALGOL 68.  The basic input (from a user or from a file) into the interpreter (sh)
is a sequence of commands. 

command::=command_name #(separator argument| redirection),
Normally, separator::=whitespace #whitespace.
	redirection::= ("<" | ">" | ">>" ) file | "<<"string.

A script is a sequence of commands and control structures separtaed by command separators

script::=(command | built_in_structure)#(command_separator (command | built_in_structure)),
command_separator::=EOLN #whitespace | semicolon #whitspace.

built_in_structure::=assignment | if_then_fi | case_esac | for_do_done | whil_do_done| exit_statement | echo_statement |... .


A shell variable is identified by an identifier:
variable::=identifier,
value_of_variable::= "$" variable,

Arguments are numbered:
	argument::= digit,
	value_of_argument::="$"digit.

There are several special variables/values:
	value::=value_of_argument | value_of_variable | 
		"$" ( "$" | "?" | "#" | "!" | ...).


string::= double_quoted_string |single_quoted_string|reverse_quoted_string,

escaped_symbol::=backslash char,

Using /home/dick/cs320/maths.ascii,
	white_space::= SP | HT | ...