Rshell Assignment

Introduction

This Rshell program processes commands entered by the user and executes them similarly to that of BASH in Linux, with the additional feature of being able to chain commands with the three operators ; , &&, and ||. The design pattern used in this project is a composite pattern. We choose to implement this partitioning pattern because it allows us significant extensibility when adding new features. Commands have the following format:

$ executable [argumentList] [connector] [cmd] ...

UML

Classes

In utilizing the composite design pattern, we built a Base class with two subclasses, a leaf node Command and a composite node Operator. The Operator class also had three subclasses representing the 3 types of connectors: Semicolon, Or, and And. In addition to the composite structure we also have two other classes Parser and ExpressionBuilder. The Parser class facilitates the translation if the user inputted string, into a vector<string> of commands and operators, to be handed over to the ExpressionBuilder class. The ExpressionBuilder class builds a tree based off of the vector<string> delivered by the Parser and dynamically creates this structure. An example of our process of breaking down the inputted string and building an tree of executions can be seen in the figure below.

Precedence Operators

Precedence operators (,) guide the affect Operators, controlling the logical flow of the command sequence. This precedece is implemented through both our, Parser and ExpressionBuilder classes. The Parser class recognizes parenthesis when it is parsing through the inputted string and creates its own string out of each parenthesis and adds it to its vector<string>. When the ExpressionBuilder class comes across this precedence operator, it adds everything between each parenthesis and recursively builds that subtree based off of its position in the command tree.

Known Bugs

1. Calling the exit command after a command fails, will not exit the program. The user has to type exit another time.
2. Calling a command with any sqaure brackets, parenthesis, semicolon, or, or and keyword will not work properly, such as echo ; will not print out a ; and will treat it like an empty echo command echo ""; .
3. Chaining pipes and redirects currently are not working