CIS 3110: Operating Systems
Assignment 1: Writing a Shell
Due Date: Feb. 1, 2013
The requirement
In this assignment you are to implement a Unix shell program. A shell is simply a program that
conveniently allows you to run other programs. Read up on your favorite shell to see what it does.
The requirements for completing this assignment successfully are described under specification.
You are provided with files called lex.c and myshell.c which contains some code that uses
getline(), a function provided by lex.c to get and parse a line of input. getline() returns an array
of pointers to character strings. Each string is either a word containing the letters, numbers, ., and
/, or a single character string containing one of the special characters: ( ) < > | & ;.
To compile lex.c, you have to use flex: "flex lex.c". This will produce a file called lex.yy.c.
lex.yy.c and myshell.c must then be compiled and linked in order to get a running program. In
the link step you also have to use "-lfl" to get everything to work properly. Use gcc for the
compilation and linking.
Specification
(1) Your shell must support the following:
1. The internal shell command "exit" which terminates the shell.
Concepts: shell commands, exiting the shell
System calls: exit()
2. A command with no arguments
Example: ls
Details: Your shell must block until the command completes and, if the return code is
abnormal, print out a message to that effect.
Concepts: Forking a child process, waiting for it to complete, synchronous execution
System calls: fork(), execvp(), exit(), wait()
(2) Your shell can also support the following:
1. A command with arguments
Example: ls -l
Details: Argument 0 is the name of the command
Concepts: Command-line parameters
2. A command, with or without arguments, executed in the background using &.
For simplicity, assume that if present the & is always the last thing on the line.Example: xemacs &
Details: In this case, your shell must execute the command and return immediately, not
blocking until the command finishes.
Concepts: Background execution, signals, signal handlers, processes, asynchronous
execution
System calls: sigset()
3. A command, with or without arguments, whose output is redirected to a file
Example: ls -l > foo
Details: This takes the output of the command and put it in the named file
Concepts: File operations, output redirection
System calls: freopen()
4. A command, with or without arguments, whose input is redirected from a file
Example: sort < testfile
Details: This takes the named file as input to the command
Concepts: Input redirection, more file operations
System calls: freopen()
5. A command, with or without arguments, whose output is piped to the input of
another command.
Example: ls -l | more
Details: This takes the output of the first command and makes it the input to the second
command
Concepts: Pipes, synchronous operation
System calls: pipe()
(3) Your shell should implement the following new commands:
1. A command "add" that adds all numbers in command line (decimal or
hexadecimal).
Example: add 100 210 0xa
Details: add all numbers and output the sum
Output: 100 + 210 + 0xa = 320
2. A command "arg" that counts and lists command line arguments.
Example: args 1 2 3 4 5 "six, seven"
Details: count and list arguments
Output: argc = 6, args = 1, 2, 3, 4, 5, "six, seven"
3. A command "???" that is named and designed by yourself.
Example: you make an example
Details: you specify the function
Output: you give a sample output
Note: You must check and correctly handle all return values. This means that you need to read
the man pages for each function to figure out what the possible return values are, what errors
they indicate, and what you must do when you get that error. Submission
 If you have any problems in the development of your programs, contact the teaching
assistant (TA) assigned to this course.
 You are encouraged to discuss this project with fellow students. However, you are not
allowed to share code with any student.
 Each student should submit a brief report (2-3 paragraphs) that explains your algorithm,
any assumptions you made, and the way to run your program.
 If your TA is unable to run/test your program, you should present a demo arranged by
your TA’s request.