EE 209: Programming Structures for EE

Assignment 5: A Unix Shell

(Acknowledgment: This assignment is borrowed and slightly modified from Princeton COS 217)


The purpose of this assignment is to help you learn about Unix processes, low-level input/output, and signals. It will also give you ample opportunity to define software modules; in that sense the assignment is a capstone for the course.


Signal handling (as described below) is the "on your own" part of this assignment. That part is worth 8% of this assignment.

You will get an extra 20% of the full score if you implement file redirection. See the extra credit section below.


A Unix shell is a program that makes the facilities of the operating system available to interactive users. There are several popular Unix shells: sh (the Bourne shell), csh (the C shell), and bash (the Bourne Again shell) are a few.

Your Task

Your task in this assignment is to create a program named ish. Your program should be a minimal but realistic interactive Unix shell. A Supplementary Information page lists detailed implementation requirements and recommendations.

You can work on this assignment with a partner in your class. A team should be no more than 2 people in the same class. Submit just one copy to KLMS if you work in a team.

If you choose to work alone (e.g., without a partner) on this assignment, you will receive extra credit as described below.

Building a Program

Use -D_BSD_SOURCE and -D_GNU_SOURCE options when building your program.

Initialization and Termination

When first started, your program should read and interpret lines from the file .ishrc in the user's HOME directory, provided that the file exists and is readable. Note that the file name is .ishrc (not ishrc), and that it resides in the user's HOME directory, not the current (alias working) directory.

To facilitate your debugging and our testing, your program should print each line that it reads from .ishrc immediately after reading it. Your program should print a percent sign and a space (% ) before each such line.

Your program should terminate when the user types Ctrl-d or issues the exit command. (See also the section below entitled "Signal Handling.")

Interactive Operation

After start-up processing, your program repeatedly should perform these actions:

Lexical Analysis

Informally, a token should be a word. More formally, a token should consist of a sequence of non-white-space characters that is separated from other tokens by white-space characters. There should be two exceptions:

Your program should assume that no line of the standard input stream contains more than 1023 characters; the terminating newline character is included in that count. In other words, your program should assume that a string composed from a line of input can fit in an array of characters of length 1024. If a line of the standard input stream is longer than 1023 characters, then your program need not handle it properly; but it should not corrupt memory.

You may use dfa.c as reference. This program uses the dynamic array and implements the example lexer from precept 11. Note, however, it does not have the full functionality required by ish.

Syntactic Analysis

A command should be a sequence of tokens, the first of which specifies the command name.

The '|' token should indicate that the immediate token after the '|' is another command. Your program should redirect the standard output of the command on the left to the standard input of the command on the right. If there's no following token after '|', your program should print out an appropriate error message. There can be multiple pipe operators in a single command.


Your program should interpret four shell built-in commands:

setenv var [value] If environment variable var does not exist, then your program should create it. Your program should set the value of var to value, or to the empty string if value is omitted. Note: Initially, your program inherits environment variables from its parent. Your program should be able to modify the value of an existing environment variable or create a new environment variable via the setenv command. Your program should be able to set the value of any environment variable; but the only environment variable that it explicitly uses is HOME.
unsetenv var Your program should destroy the environment variable var.
cd [dir] Your program should change its working directory to dir, or to the HOME directory if dir is omitted.
exit Your program should exit with exit status 0.

Note that those built-in commands should neither read from the standard input stream nor write to the standard output stream. Your program should print an error message if there is any piped commmand or file redirection with those built-in commands.

If the command is not a built-in command, then your program should consider the command name to be the name of a file that contains code to be executed. Your program should fork a child process and pass the file name, along with its arguments, to the execvp system call. If the attempt to execute the file fails, then your program should print an error message indicating the reason for the failure.

Process Handling

All child processes forked by your program should run in the foreground. Your program need not support background processes.

It is required to call wait for every child that has been created.

Signal Handling

[NOTE] Ctrl-d represents EOF, not a signal. Do NOT make a signal handler for Ctrl-d.

When the user types Ctrl-c, Linux sends a SIGINT signal to the parent process and its children. Upon receiving a SIGINT signal:

When the user types Ctrl-\, Linux sends a SIGQUIT signal to the parent process and its children. Upon receiving a SIGQUIT signal:

Error Handling

Your program should handle an erroneous line gracefully by rejecting the line and writing a descriptive error message to the standard error stream. An error message written by your program should begin with "programName: " where programName is argv[0], that is, the name of your program's executable binary file. Note that argv[0] typically will be ish, but need not be so.

The error messages written by your program need not be identical to those written by the given sampleish program. However, the error messages written by your program should be at least as descriptive as those written by sampleish.

Your program should handle all user errors. It should be impossible for the user's input to cause your program to crash.

Memory Management

Your program should contain no memory leaks. For every call of malloc or calloc, eventually there should be a corresponding call of free.

Extra Credit 1 (extra 20% of the full score of this assignment)

You are going to implement redirection of standard input and standard output as an extra credit.

Extra Credit 2 (extra 30% of your earned score including the extra credit1)

If you do this assignment on your own without a partner, you will receive extra credit which is worth 30% of (your basic score + extra credit1). Here is an example. If your score is 50 and you got extra credit 1, your earned score is 50 + 20 = 70. If you worked alone, you will receive 30% x 70 = 21 additional points as extra credit 2. So, your total score will be 91.


Test your program by creating multiple files containing lines which your program should interpret, repeatedly copying each to the .ishrc file in your HOME directory, and restarting your program. The file .ishrc contains a sequence of commands that can serve as a minimal test case. You should develop many more test files.

Of course you also should test your program manually by typing commands at its prompt.


Develop on lab machines. Use emacs to create source code. Use make to automate the build process. Use gdb to debug.

An executable version of the assignment solution is available in sampleish. Use it to resolve any issues concerning the desired functionality of your program. We also provide the interface and implementation of the DynArray ADT that we discussed in precepts. You are welcome to use that ADT in your program.

Your readme file should contain:


Use KAIST KLMS to submit your assignments. Your submission should be one gzipped tar file whose name is


Your submission need to include the following files:

Please do not submit emacs backup files, that is, files that end with '~'.


We will grade your work on quality from the user's point of view and from the programmer's point of view. From the user's point of view, your program has quality if it behaves as it should. The correct behavior of your program is defined by the previous sections of this assignment specification and by the given sampleish program. From the programmer's point of view, your program has quality if it is well styled and thereby simple to maintain. See the specifications of previous assignments for guidelines concerning style. Proper function-level and file-level modularity will be a prominent part of your grade. To encourage good coding practices, we will deduct points if gcc209 generates warning messages. Remember that the Supplementary Information page lists detailed implementation requirements and recommendations.