Programming Assignment 1: The Unix Shell

Due: Thursday, September 20 at 9 pm
You are to do this project BY YOURSELF
This project must be implemented in C (and not C++ or anything else)


8/25: Any updates to the project specification will go here. You should read through the archives before sending a question -- perhaps it is already answered!


There are five objectives to this assignment:
  • To familiarize yourself with the Linux programming environment.
  • To develop your defensive programming skills in C.
  • To gain exposure to the necessary functionality in shells.
  • To learn how processes are handled (i.e., starting and waiting for their termination).
  • To turn in a picture of yourself.


In this assignment, you will implement a command line interpreter or shell. The shell should operate in this basic way: when you type in a command (in response to its prompt), the shell creates a child process that executes the command you entered and then prompts for more user input when it has finished.

The shells you implement will be similar to, but much simpler than, the one you run every day in Unix. You can find out which shell you are running by typing echo $SHELL at a prompt. You may then wish to look at the man pages for sh or the shell you are running (more likely tcsh or bash) to learn more about all of the functionality that can be present. For this project, you do not need to implement much functionality; but you will need to be able to handle running multiple commands simultaneously.

Your shell can be run in two ways: interactive and batch. In interactive mode, you will display a prompt (any string of your choosing) and the user of the shell will type in a command at the prompt. In batch mode, your shell is started by specifying a batch file on its command line; the batch file contains the list of commands that should be executed. In batch mode, you should not display a prompt. In batch mode you should echo each line you read from the batch file back to the user before executing it; this will help you when you debug your shells (and us when we test your programs). In both interactive and batch mode, your shell stops accepting new commands when it sees the quit command on a line or reaches the end of the input stream (i.e., the end of the batch file or the user types 'Ctrl-D'). The shell should then exit after all running processes have terminated.

Each line (of the batch file or typed at the prompt) may contain multiple commands separated with the ; character. Each of the commands separated by a ; should be run simultaneously, or concurrently. (Note that this is different behavior than standard Linux shells which run these commands one at a time, in order.) The shell should not print the next prompt or take more input until all of these commands have finished executing (the wait() and/or waitpid() system calls may be useful here). For example, the following lines are all valid and have reasonable commands specified:

prompt> ls
prompt> /bin/ls
prompt> ls -l
prompt> ls -l ; cat file
prompt> ls -l ; cat file ; grep foo file2

For example, on the last line, the commands ls -l , cat file and grep foo file2 should all be running at the same time; as a result, you may see that their output is intermixed.

To exit the shell, the user can type quit. This should just exit the shell and be done with it (the exit() system call will be useful here). Note that quit is a built-in shell command; it is not to be executed like other programs the user types in. If the "quit" command is on the same line with other commands, you should ensure that the other commands execute (and finish) before you exit your shell.

These are all valid examples for quitting the shell.
prompt> quit
prompt> quit ; cat file
prompt> cat file ; quit
This project is not as hard as it may seem at first reading (or perhaps it doesn't seem that hard at all, which is good!); in fact, the code you write will be much smaller than this specification. Writing your shell in a simple manner is a matter of finding the relevant library routines and calling them properly. Your finished programs will probably be under 200 lines, including comments. If you find that you are writing a lot of code, it probably means that you are doing something wrong and should take a break from hacking and instead think about what you are trying to do.

Program Specifications

Your C program must be invoked exactly as follows:

shell [batchFile]

The command line arguments to your shell are to be interpreted as follows.
batchFile: an optional argument (often indicated by square brackets as above). If present, your shell will read each line of the batchFile for commands to be executed. If not present, your shell will run in interactive mode by printing a prompt to the user at stdout and reading the command from stdin.
For example, if you run your program as

shell /u/j/v/batchfile

then your program will read commands from /u/j/v/batchfile until it sees the quit command.

Defensive programming is an important concept in operating systems: an OS can't simply fail when it encounters an error; it must check all parameters before it trusts them. In general, there should be no circumstances in which your C program will core dump, hang indefinately, or prematurely terminate. Therefore, your program must respond to all input in a reasonable manner; by "reasonable", we mean print an understandable error message and either continue processing or exit, depending upon the situation.

You should consider the following situations as errors; in each case, your shell should print a message (to stderr) and exit gracefully:

  • An incorrect number of command line arguments to your shell program.
  • The batch file does not exist or cannot be opened.

For the following situation, you should print a message to the user (stderr) and continue processing:

  • A command does not exist or cannot be executed.

Optionally, to make coding your shell easier, you may print an error message and continue processing in the following situation:

  • A very long command line (for this project, over 512 characters including the '\n').

Your shell should also be able to handle the following scenarios, which are not errors (i.e., your shell should not print an error message):

  • An empty command line.
  • Extra white spaces within a command line.
  • Batch file ends without quit command or user types 'Ctrl-D' as command in interactive mode.
In no case, should any input or any command line format cause your shell program to crash or to exit prematurely. You should think carefully about how you want to handle oddly formatted command lines (e.g., lines with no commands between a ;). In these cases, you may choose to print a warning message and/or execute some subset of the commands. However, in all cases, your shell should continue to execute!

prompt> ; cat file ; grep foo file2
prompt> cat file ; ; grep foo file2
prompt> cat file ; ls -l ;
prompt> cat file ;;;; ls -l
prompt> ;; ls -l
prompt> ;


Your shell is basically a loop: it repeatedly prints a prompt (if in interactive mode), parses the input, executes the command specified on that line of input, and waits for the command to finish, if it is in the foreground. This is repeated until the user types "quit" or ends their input.

You should structure your shell such that it creates a new process for each new command. There are two advantages of creating a new process. First, it protects the main shell process from any errors that occur in the new command. Second, it allows easy concurrency; that is, multiple commands can be started and allowed to execute simultaneously (i.e., in parallel style).

To simplify things for you in this first assignment, we will suggest a few library routines you may want to use to make your coding easier. (Do not expect this detailed of advice for future assignments!) You are free to use these routines if you want or to disregard our suggestions.

To find information on these library routines, look at the manual pages (using the Unix command man ). You will also find man pages useful for seeing which header files you should include.


For reading lines of input, you may want to look at fgets(). To open a file and get a handle with type FILE * , look into fopen(). Be sure to check the return code of these routines for errors! (If you see an error, the routine perror() is useful for displaying the problem.) You may find the strtok() routine useful for parsing the command line (i.e., for extracting the arguments within a command separated by whitespace or a tab or ...).

Executing Commands

Look into fork(), execvp(), and wait/waitpid().

The fork() system call creates a new process. After this point, two processes will be executing within your code. You will be able to differentiate the child from the parent by looking at the return value of fork; the child sees a 0, the parent sees the pid of the child.

You will note that there are a variety of commands in the exec family; for this project, you must use execvp(). Remember that if execvp() is successful, it will not return; if it does return, there was an error (e.g., the command does not exist). The most challenging part is getting the arguments correctly specified. The first argument specifies the program that should be executed, with the full path specified; this is straight-forward. The second argument, char *argv[] matches those that the program sees in its function prototype:

int main(int argc, char *argv[]);

Note that this argument is an array of strings, or an array of pointers to characters. For example, if you invoke a program with:

foo 205 535

then argv[0] = "foo", argv[1] = "205" and argv[2] = "535". Important: the list of arguments must be terminated with a NULL pointer; that is, argv[3] = NULL. We strongly recommend that you carefully check that you are constructing this array correctly!

The wait()/waitpid() system calls allow the parent process to wait for its children. Read the man pages for more details.

Miscellaneous Hints

Remember to get the basic functionality of your shell working before worrying about all of the error conditions and end cases. For example, first focus on interactive mode, and get a single command running (probably first a command with no arguments, such as "ls"). Then, add in the functionality to work in batch mode (most of our test cases will use batch mode, so make sure this works!). Next, try working on multiple jobs separated with the ; character. Finally, make sure that you are correctly handling all of the cases where there is miscellaneous white space around commands or missing commands.

We strongly recommend that you check the return codes of all system calls from the very beginning of your work. This will often catch errors in how you are invoking these new system calls.

Beat up your own code! You are the best (and in this case, the only) tester of this code. Throw lots of junk at it and make sure the shell behaves well. Good code comes through testing -- you must run all sorts of different tests to make sure things work as desired. Don't be gentle -- other users certainly won't be. Break it now so we don't have to break it later.

Keep versions of your code. More advanced programmers will use a source control system such as CVS. Minimally, when you get a piece of functionality working, make a copy of your .c file (perhaps a subdirectory with a version number, such as v1, v2, etc.). By keeping older, working versions around, you can comfortably work on adding new functionality, safe in the knowledge you can always go back to an older, working version if need be.
For testing your code, you will probably want to run commands that take awhile to complete. Try compiling and running this very simple C program; when multiple copies are run in parallel you should see the output from each process interleaved. See the code for more details.


For this project, you need to hand in four distinct items:
  • Your source code (no object files or executables, please!)
  • A Makefile for compiling your source code
  • A picture of yourself
  • A README file with some basic documentation about your code

Hand in your source code. We have created a directory ~cs537-SECTION/handin/NAME, where SECTION is either 1 or 2 and NAME is your login name. For example, if you are in section 2 and your login is jimmyv, your handin directory is ~cs537-2/handin/jimmyv. Your handin directory has subdirectories: p1, p2, p3, etc. For this assignment, use directory p1.

Copy all of your .c source files into the appropriate subdirectory. Do not submit any .o files. After the deadline for this project, you will be prevented from making any changes in these directory. Remember: No late projects will be accepted!

To ensure that we compile your C correctly for the demo, you will need to create a simple makefile; this way our scripts can just run make to compile your code with the right libraries and flags. If you don't know how to write a makefile, you might want to look at the man pages for make, or better yet, read this little tutorial. Otherwise, check out this very simple sample makefile.

You will also need to turn in a digital picture of yourself. Put this in your handin directory (a jpg or a gif is fine), in the form Firstname.Lastname.gif (or whatever). We want to get to know you better, and what better way than to be able to associate a name with a face? If you don't turn in a picture, your project will not be graded!

Finally, we would like to see a file called README describing your code. This file should have contain the following four sections:

  • Your name and login information
  • Design overview: A few paragraphs describing the overall structure of your code and any important structures.
  • Complete specification: Describe how you handled any ambiguities in the specification. For example, for this project, explain how your shell will handle lines that have no commands between semi-colons.
  • Known bugs or problems: A list of any features that you did not implement or that you know are not working correctly
Due to the simplificity of this project, the documentation for this project is fairly minimal. It may be more extensive for future projects. The majority of your grade for this assignment will depend upon how well your implementation works and a smaller portion will be given for documentation and style. We will run your program on a suite of about 20 test cases, some of which will exercise your programs ability to correctly execute commands and some of which will test your programs ability to catch error conditions. Be sure that you thoroughly exercise your program's capabilities on a wide range of test suites, so that you will not be unpleasantly surprised when we run our tests.

Even though you will not be heavily graded on style for this assignment, you should still follow all the principles of software engineering you learned in CS 302, CS 367, and elsewhere, such as top-down design, good indentation, meaningful variable names, modularity, and helpful comments. Don't be sloppy! You should be following these principles for your own benefit.

Finally, while you can develop your code on any system that you want, make sure that your code runs correctly on a machine that runs the Linux operating system. Specifically, since libraries and environments sometimes vary in small and large ways across systems, you should verify your code on the linux machines in the 13XX labs (e.g., the royal or emperor clusters). These machines are where your projects will be tested.

Other Shell Fun

The shell you are building is very simplistic; it doesn't have a PATH variable, it doesn't support changing directories, there is no shell history of previous commands you have run, the user can't customize the prompt, etc. Feel free to play around with adding such functionality; it will give you some more insight into how things really work. However, please don't let your fun ruin your code -- please turn in a copy of the shell that implements only the functionality described in this specification for the project.