Developing an interactive shell

Question

I have an application; to interact with it I have to develop an interactive shell. The shell will take input from user and dispatch it to the server, which will process and return a response.

In the past I have done such things, but they typically had the following format:

Shell>> COMMAND arg1 arg2 arg3 ..

and my implementation was in a manner similiar to:

/* Mock implementation */
while(true) {
   in = readline()
   tokens = in.split(' ') //split based on space
   switch(token[0])  {
      case "Command_a": processA(token[1], token[2])
      case "Command_b": processB(token[1], token[2], token[3])
   }
}

This time I have to work with more extensive grammar for user input. If I use my current approach, it will make things very difficult with lots of if , if-elseif-else, if-else, switch statements for both parsing and generating response.

How can I approach this problem in a manner that will make the interpreter modular and maintainable? What are some ways in which popular CLI-Interface are implemented? I will really appreciate some code examples.

PS: Language choices limited C++/Python

What is the "extensive grammar of user input"? It can be from "free-form English" (that takes serious AI support to parse) to exactly what you have had before (that takes the switch clause to parse). — Öö Tiib, Sep 05 '12 at 20:35

score 2 · Accepted Answer · answered Sep 05 '12 at 17:20

2

You can develop your own parser and interpreter using bison and flex, but probably wiser is to embed an existing interpreter like Python, Lua or Squirrel.

answered Sep 05 '12 at 17:20

piokuc

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But users will not be programmers – WeaklyTyped Sep 05 '12 at 17:20
But it is fairly easy syntax and not elaborate as a programming language. – WeaklyTyped Sep 05 '12 at 17:25
1

@WeaklyTyped Have you actually looked at the syntax to scheme (the default language behind Guile, the GNU extension language)? It's very, very simple. – Charles Duffy Sep 05 '12 at 17:33
I agree Scheme is great as an extension language. There are other good implementations suitable for embedding in C or C++ programs, for example http://code.google.com/p/chibi-scheme/ – piokuc Sep 05 '12 at 17:36
But Scheme's legendary parenthesis as well as the mere look and feel will create serious UX issues among users. – WeaklyTyped Sep 05 '12 at 17:39

score 1 · Answer 2 · answered Sep 05 '12 at 17:53

1

Take a look at boost::spirit, or just embed Lua as others suggested.

answered Sep 05 '12 at 17:53

mitchnull

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score 1 · Answer 3 · answered Sep 05 '12 at 18:04

1

The cmd module in Python, perhaps with the addition of Pyparsing should fit your needs perfectly.

answered Sep 05 '12 at 18:04

Chinmay Kanchi

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score 0 · Answer 4 · answered Sep 05 '12 at 21:46

Go for the cmd module :

The Cmd class provides a simple framework for writing line-oriented command interpreters. These are often useful for test harnesses, administrative tools, and prototypes that will later be wrapped in a more sophisticated interface.

Developing an interactive shell

4 Answers4