s/docs/GRAMMAR

# GRAMMAR

<command> ::= <token>+

<pipe> ::= <command> '|' <pipe>
         | <command>

<conj> ::= <pipe> '&&' <conj>
         | <pipe>

<disj> ::= <conj> '||' <disj>
         | <conj>

<line> ::= <disj> ('<' <token>)? ('>' <token>)? '&'?

or in words:

A command is a sequence of tokens like: grep -v foo
A pipe is a sequence of commands with | between: ps aux | grep watch | grep -v grep
A disj is a sequence of pipes with || in between, like:  mount /proc || exit -1
note: | binds tighter than && which binds tighter than ||
And each line can optionally be set to run in the background/in parallel using &
you can also (optionally) choose to redirect stdin/out

# AST

So the abstract syntax tree ends up being a right associative tree of '||' nodes,
containing right associative trees of '&&' nodes, containing ... '|' nodes which
contain command nodes.

# VARIABLES

After tokenization variables are expanded.

The names of variables are made from these charactors: [A-Z0-9_]

"Environment variable names used by the utilities in the Shell and Utilities volume of IEEE Std 1003.1-2001 consist solely of uppercase letters, digits, and the '_' (underscore) from the characters defined in Portable Character Set and do not begin with a digit."

* http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html

The syntax is $VAR or ${VAR}. The reason we have ${VAR} is to allow things like ${FOO}BAR.

# SYNTAX

builtins
'cd' <directory>