.See /u/faculty/dick/cs320/math/comp.ascii
|- 	white_space::= SP | HT | ...
	doublequote::="\"", quote:="'", backquote::="`", backslash::="\\".

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 pipelined commands. 

pipeline::= command (input_redirection|) #("|" command) ("|" command output_redirction|).

input_redirection::= "<" file | "<<"string.
output_redirection::=( ">" | ">>" ) file.

command::=command_name #(separator argument),
Normally, separator::=whitespace #whitespace.

argument::=#(word | string | escaped_symbol ),
word::=#(value_shell_variable| value_of_argument | #(char~separator)),


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

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

built_in_structure::=assignment | selection | loop | other_command,
selection::=if_then_fi | case_esac,
loop::= for_do_done | while_do_done | ... .
other_command::=exit_statement | echo_statement |... .


A shell variable is identified by an identifier:
shell_variable::=identifier,
value_of_variable::= "$" variable| "${" identifier operator word "}",
	The "${...}" form is left undefined.

Arguments are numbered inside a shell script:
script_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,

double_quoted_string::= double_quote #( value_of_variable | value_of_argument | #(char~double_quote) | escaped_symbol) double_quote,
reverse_quoted_string::=backquote pipeline backquote,
single_quoted_string::=quote #(char~quote | escaped_symbol) quote.