Spring 2009 -- Junfeng Yang

1/27: Homework 1 is updated. All updates are marked with the label "UPDATE".

Homework 1 is due Thu 2/5 at 4:09pm EST

All written and programming problems in this assignment are to be done alone. We do not allow group collaboration for this assignment. You should submit your solutions electronically via CourseWorks. Refer to the homework submission page on the class web site for additional submission instructions.

The total points of this assignment is 110 (not a mistake).

Written Assignment (40 pts)

Exercise numbers refer to the course textbook, Operating System Concepts. Each problem is worth 5 points.
  1. (1.13) In a multiprogramming and time-sharing environment, several users share the system simultaneously. This situation can result in various security problems. a. What are two such problems? b. Can we ensure the same degree of security in a time-shared machine as in a dedicated machine? Explain your answer.
  2. (1.14) The issue of resource utilization shows up in different forms in different types of operating systems. List what resources must be managed carefully in the following settings: a. Mainframe or minicomputer systems. b. Workstations connected to servers. c. Handheld computers
  3. (1.22) What is the purpose of interrupts? What are the differences between a trap and an interrupt? Can traps be generated intentionally by a user program? If so, for what purpose?
  4. (1.24) Some computer systems do not provide a privileged mode of operation in hardware. Is it possible to construct a secure operating system for these computer systems? Give arguments both that it is and that it is not possible.
  5. What are the main requirements for an Interrupt Handler in Linux? Why?
  6. List the main differences between a system call and a regular function call.
  7. (2.13) Describe three general methods for passing parameters to the operating system.
  8. (2.23) What is the main advantage for an operating-system designer of using a virtual-machine architecture?What is the main advantage for a user?

Programming Assignment: Shell (70 pts)

For this programming assignment you will be required to submit source code, a README file documenting your files and code, and a test run of your programs. The README should explain any way in which your solution differs from what was assigned, and any assumptions you made. Refer to the homework submission page on the class web site for additional submission instructions.


A shell provides a command line interface for users. It interprets user commands and executes them. Some shells provide simple scripting terms such as if or while, and allow users to make a program which facilitates their computing life. There are two categories of shells: command line and graphical. Intuitively, command line shells provide command line user interface commonly used in Unix/Linux environments, while graphical shells provide graphic user interface(GUI) such as MS Windows. In this assignment, we will focus on command line shells.

Under the hood, a shell is just another user program. The files /bin/sh, /bin/bash, and /bin/tcsh are all executable files for shells. The only thing special about your login shell is that it is listed in your login record so that /bin/login (the program that prompts you for your password) knows what program to start when you log in. If you run ``cat /etc/passwd'', you'll see the login records of the machine and the login shell program listed as the last field.

A shell in Unix-like systems often supports two interesting features:

You can learn more about I/O redirection at this wiki page.

In this assignment, you will implement your own shell.

  1. (10 points) Write a simple shell. Basically your shell should read the line from standard input, parse the line with command and arguments, and execute the command with arguments. You'll need the fork() and exec*() system calls.
    • The shell should find the command in your current working directory first. If not found, it should search the directories in the shell's pathname list which will be explained in the part 2.
    • You are not allowed to use system(), as it just invokes the system's /bin/sh shell to do all the work. You are not allowed to use execlp() or execvp() in the standard library, because our shell has its own path variable, explained in part 2.
    • You may assume that arguments are separated by white spaces. You do not have to deal with special characters such as ', ", \, etc; however, you need to handle the redirection operators (<, >, and 2>) and the pipeline operator (|).
    • You can assume that the command line a user types is no longer than 4096 bytes. However, you should not assume that there is a restriction on the number of arguments to a given command.
    • The executable file for your shell should be named myshell, for the TAs' grading pleasure.

  2. (20 points) Implement the following built-in commands.
    • exit: users can exit from the shell with the exit command.
    • cd: cd is a command to change directories. You will need to invoke the chdir system call.
    • path: path is a command not only to show the current pathname list, but also to append or remove several pathnames. In your shell implementation, you may keep an variable or data structure to deal with pathname list (referred to as "path" variable below). This list helps searching for executables when users enter specific commands.
      • path (without arguments) displays the pathnames currently set. It should show pathnames separated by colons. ex) "/bin:/usr/bin"
      • path + /foo/bar appends the pathname to the "path" variable. Only one pathname will be given to each invocation of the path command. It's okay to add a pathname to the "path" variable even if it already contains the same pathname, i.e., duplicates in the "path" variable are fine.
      • path - /foo/bar removes the pathname from the "path" variable. It should remove all duplicates of the given pathname. Only one pathname will be given to each invocation of the path command.

  3. (20 points) Extend your shell with I/O redirection (<, >, and 2>), as described above.
    • You'll need to understand Unix file descriptors. stdin maps to file descriptor 0, stdout to 1, and stderr to 2.
    • You'll need the open(), close(), and dup2() system calls.
    • You can assume that there is no space inside the stderr redirection operator "2>"
    • You should handle cases when all three streams are redirected, such as cmd < in.txt > out.txt 2> err.txt (not necessarily in this order).
    • You don't need to handle other forms of redirections. For example, you don't need to handle the following redirections : cmd 1> filename (another way to redirect standard output)), cmd 2>&1 (redirect standard error to standard output). (Most other shells do handle the above cases.)
    • UPDATE: Your shell do not need to handle I/O redirection for built-in commands (cd, exit, path).

  4. (20 points) Extend your shell with pipeline (|), as described above.
    • You'll need the pipe system call.
    • There is no restriction on the depth of a pipeline. That is, your solution should handle arbitrary number of commands chained together with operator |
    • You should handle combinations of redirection and pipeline when they can be combined. An example is: sort < file.txt | uniq. However, if there is a conflict between redirection and pipeline, i.e. one file descriptor gets associated with two things, you should report a syntax error. An example is: ls > 1.txt | grep FOO; we cannot redirect the stdout of ls to both 1.txt and grep at the same time.
    • UPDATE: Your shell do not need to handle built-in commands (cd, exit, path) in pipeline.
    • UPDATE: Your shell should not accept new user commands while the pipeline is still running.

Additional Requirements