[bash] The simple grammar for mini shell

after extracts specified special characters and expands environments, wildcard, we need to split all strings to token and translate to symbols. for translation, writing new grammar is necessary.
In our case, pipe(|), logical operators(&&, ||), redirections(<, >, <<, >>) are considered only.
for going not to deep and not to far, we preprocess the token by identifying type depends on location.

The grammar symbols

LBRACE	RBRACE	PIPE
(		)		|

IN_RID	OUT_RID	IN_HEREDOC	OUT_HEREDOC  (with filepath)
<		>		<<			>>	

AND_IF	OR_IF	CMD			OPTION							ARG
&&		||		cmd(str)	option(str started with '-')	argument(str)

The Grammar

%start  complete_command
%%
complete_command : and_or
and_or			: 	<pipeline>
					| <and_or> OR_IF <pipeline>
					| <and_or> AND_IF <pipeline>
pipeline		: <simple_command>
				 	| <pipeline> PIPE <simple_command>
simple_command	: <command>
					| <brace_group>
					| <redirect_list> <command>
brace_group		: LBRACE <and_or> RBRACE
command			: CMD
					| CMD <options>
options 		: <argument>
					| OPTION <argument> 
redirect_list	: <redirect>
					| <redirect_list> <redirect>
argument		: ARG
					| <argument> ARG
redirect		: OUT_HEREDOC
					| IN_HEREDOC
					| IN_DIR
					| OUT_DIR

as long as new character is not to be added, this grammar is enoughly deep.

Example

• complete command : echo -n "hi"

1. pass extracting, expanding because of no existance of problem to tokenizing.
2. tokenize(using link list.)

str : echo	->	str : -n		->	str: "hi"
type: cmd		type : option		type : argument

3. make tree using grammar.
	
        root : echo
  	       		  \ 
                   -n
                     \
                     "hi"

• complete command : << end >outfile cat

1. separate redirection and filepath and split them.
{"<<", "end", ">", "outfile", "cat"}

2. tokenize.

str : <<			-> str : end			->	str : >			->	str : outfile	-> str : cat
type : in_heredoc	   type : filepath			type : out_rid		type : filepath	   type : cmd

3. make tree using grammar.

		root : cat
    		/  
          << - end	
          	\
             > - outfile

• complete command : echo "1" && echo "2" || echo "3"

1. pass.
2. tokenize.

str : echo	-> str : "1"		-> str : echo 	-> str: "2"				-> str : echo 	-> str: "3"
type : cmd	   type : argument	   type : cmd	   type : argument 	   type : cmd		type : argument

3. make tree using grammar.

			root : ||
                 /   \
               &&    echo
             /   \ 		\
          echo  echo    "3"
            \      \
            "1"    "2"

📖 reference
https://pubs.opengroup.org/onlinepubs/009604499/utilities/xcu_chap02.html#tag_02_10_02