Systems Programming
Assignment 3 - Improving a Shell
Due date COB - Thursday Week 13.
In this assignment you have to take an existing implementation of a shell program and
add some small improvements to it.
Preparation - Downloading, Compiling and Testing a simple Shell
Implementation
Download and uncompress the following file:
shell.tgz
The contents of the file are C sources that implement a simple shell. The contents are:
smsh.c - an implementation of a very simple shell program
smsh.h - a header file with function prototypes
execute.c - a set of helper functions for running processes within the program
splitline.c - some text processing utilities
To compile these sources run:
gcc -o smsh1 smsh1.c splitline.c execute.c
To run the shell type:
./smsh1
A prompt of the form. ">" will appear and then you can type commands like:
> ls
execute.c shell.tgz smsh.h smsh1 smsh1.c splitline.c
> wc execute.c
37 113 725 execute.c
The shell is terminated by typing the Control-D key (which signals end of input).
In this assignment you have to add functionality to this shell command. Each part builds
upon the last.
Part 1 - Adding the ability to pipe commands to your shell
(30 marks)
At the moment smsh1 doesn't support piping of commands. So, for example, if you type:
ls | wc
at the prompt you get:
ls: wc: No such file or directory
ls: |: No such file or directory
Write a new program called smsh2.c that is based on smsh1.c which performs all of the
shell operations of smsh1.c but also allows commands to be piped as above so that if
you type:
ls | wc
You get output like:
6 6 53
instead. You are free to add and modify files as required to accomplish this task. You
must add a Makefile to your submission so that you can compile all the files for part1 by
typing:
make part1
and the solution for part 1 can be run by typing:
./smsh2
Note, your program must still cater for all the behaviours that were correct in the original
version of smsh1. You may find the lectures on piping useful in completing this part.
Part 2 - redirecton of stdin and stdout (30 Marks)
Your version of smsh2 currently doesn't support redirection of output and input of
commands such as:
ls > tmp.txt
cat out.txt
Copy your smsh2.c program from part 1 to smsh3.c so that it can handle redirection of
standard input and standard output (don't worry about stderr) using the ">" and "<"
symbols. Again, you are free to add and modify files as required to accomplish this task
(without affecting the ability of your code to correctly execute the behaviour required for
part 1). You must add a Makefile to your submission so that you can compile all the files
for part 2 by typing:
make part2
and the solution for part 2 can be run by typing:
./smsh3
Note, your program must still cater for all the behaviours that were correct in the original
version of smsh2.
Part 3 - Adding Globbing - (20 marks)
Your version of smsh3 currently doesn't support wildcards in command lines such as:
ls *.c
cat *.h
Copy your smsh3.c program from part 2 to smsh4.c so that it can handle wilcard
symbols in the command line. This expansion of wildcards is called globbing.
Note, you can use the glob system call (type "man -s3 glob" to find out more) that helps
you to expand the wildcards to a list of actual filenames.
Again, you are free to add and modify files as required to accomplish this task (without
affecting the ability of your code to correctly execute the behaviour required for previous
parts). You must add a Makefile to your submission so that you can compile all the files
for part3 by typing:
make part3
and the solution for part 3 can be run by typing:
./smsh4
Note, your program must still cater for all the behaviours that were correct in the original
version of smsh3.
Handin Details to be posted in week 11.