Spring 2010

Project 1 - Part 1: Writing a User Shell

Project 1: The Shell

Introduction

Using the Debugger

To start gdb type gdb program. For example, to debug your shell type:

csh> gdb shell

Then type

(gdb) break main

This will make the debugger stop your program before main is called. In general, to set a breakpoint in a given function type break <function-name>

To start running your program type:

(gdb)run

Your program will start running and then will stop at main.

Use "step"or "next" to execute the following line in your program. "step" will execute the following line and if it is a function, it will step into it. "next" will execute the following line and if it a function it will execute the function.

(gdb) next 

(gdb) step 

An empty line in gdb will rerun the previous gdb command.

Other useful commands are:

print var 

where 

quit 

For more complete tutorials on gdb see:

GDB Tutorial 1
GDB Tutorial 2
GDB Tutorial 3
GDB Tutorial 4

First part: Lex and Yacc

• Copy the tar file shell-src.tar.gz, that contains all the files in shell-src, to your home directory on spoof.cs.georgetown.edu from my home directory using the following command:

  cp ~clay/shell-src.tar.gz ~/

  and untar it using the following command:




gunzip shell-src.tar.gz

tar -xvf shell-src.tar

• Build the shell program by typing :

 make

To run it type:

 shell

Then type commands like

 ls -al

 ls -al aaa bbb > out

Check the output printed


• First, understand how the program works. Read the shell-src/Makefile to learn how the program is built. The file shell-src/command.h implements the data structure that represents a shell command. The struct SimpleCommand implements the list of arguments of a simple command. Usually a shell command can be represented by only one SimpleCommand. However, when pipes are used, a command will consist of more than one SimpleCommand. The struct Command represents a list of SimpleCommand structs. Other fields that the Command struct has are _outFile, _inputFile, and _errFile that represent input, output, and error redirection.


• Currently the shell program implements a very simple grammar:

 cmd [arg]* [> filename]

You will have to modify shell-src/shell.y to implement a more complex grammar

 cmd [arg]* [ | cmd [arg]* ]* 
[< filename] 
[ [> filename] [ >& filename] [>> filename] [>>& filename] ] 
[&]

• Insert the necessary actions in shell-src/shell.y to fill in the Command struct. Make sure that the Command struct is printed correctly.


• Run your program against the following commands:

 ls
 ls -al
 ls -al aaa bbb cc
 ls -al aaa bbb cc > outfile
 ls | cat | grep
 ls | cat | grep > out < inp
 ls aaaa | grep cccc | grep jjjj ssss dfdffdf
 ls aaaa | grep cccc | grep jjjj ssss dfdffdf >& out < in
 httpd &
 ls aaaa | grep cccc | grep jjjj ssss dfdffdf >>& out < in

The deadline of this part of the project is February 17th, before class. Follow these instructions to turnin your part one.

1. Login to spoof.cs.georgetown.edu.

2. cd to shell-src and type make clean

3. Type make to make sure that your shell is build correctly.

4. Type make clean again.

5. cd one directory above shell-src by typing "cd .."

6. Create a tar file named <user_name>.tar, where <user_name> is your spoof.cs.georgetown.edu login, by typing

tar -cf <user_name>.tar shell-src

7. Gzip the tar file by typing

gzip <user_name>.tar

8. Since this timestamp will be used to verify whether the work was completed on time or not, you should set the permissions on the file you submitted to make sure that the file timestamp is not changed. So this by typing:

chmod a-w <user_name>.tar.gz

9. Finally, mail the gzipped tar file to clay at cs dot georgetown dot edu as an attachment. We need to do this so I have a record in case the machine crashes. This will also allow you to make a backup to your own system in accordance with the class policies.

Resources