Writing Your Own Shell

The object of this assignment is to gain experience with some advanced programming techniques like process creation and contol, file descriptors, signals and possibly pipes. To do this, you will be writing your own command shell - much like csh, bsh or the DOS command shell.

Wait! Have you written your parser already? Do that first.

Basic Psuedocode for a Shell

From experience using a command shell, you should be able to write basic pseudocode for a shell (Note: The pseducode below uses the UNIX style fork/exec not the Windows style CreateProcess/WaitForSingleObject.):

     main (int argc, char **argv)
	while (1){
		int childPid;
		char * cmdLine;


	        cmdLine= readCommandLine(); //or GNU readline("");
		cmd = parseCommand(cmdLine);

		record command in history list (GNU readline history ?)
		if ( isBuiltInCommand(cmd)){
		} else {		
		     childPid = fork();
		     if (childPid == 0){
			executeCommand(cmd); //calls execvp  
		     } else {
			if (isBackgroundJob(cmd)){
			        record in list of background jobs
			} else {
				waitpid (childPid);


Required Features

Between this simple pseudocode and full featured shells, there are many optional features. Here are the features you should support in your shell:

  1. The prompt you print should indicate the current working directory. For example:

    The directory: /usr/foo/bar%

    It may also indicate other things like machine name or username or any other information you would like.

    Try getcwd(char * buf, size_t size) .

  2. You should allow the user to specify commands either by relative or absolute pathnames. To read in the command line, you may want to consider the readline function from the GNU readline library as it supports user editing of the command line. I do not know if there is an equivalent for Windows so you may end up needing to write your own version of readline for Windows. In that case, it does not need to support editing of the command line etc.

    Linux TIP! Example of a relative command df, same command with absolute path: /bin/df. To find the absolute path of a command use which, which ls gives /bin/ls

  3. Try execvp it will search the path automatically for you. First argument should be pointer to command string and the second arguments should be a pointer to an array which contains the command string as arg[0] and the other arguments as arg[1] through arg[n].

  4. You do not need to support setting of environment variables. However, you may find it useful to know about these variables especially PATH which is the list of directories in which to look for a specified executable. You may use execvp to have the system search the PATH inherited by your own shell from its parent.

    Linux TIP! Use printenv $PATH to see what's in your executable path

    Windows TIP! Right Click on 'My Computer', go to Properties and find the Environment Variables

  5. You should be able to redirect STDIN and STDOUT for the new processes by using < and >. For example, foo < infile > outfile would create a new process to run foo and assign STDIN for the new process to infile and STDOUT for the new process to outfile. In many real shells it gets much more complicated than this (e.g. >> to append, > to overwrite, >& redirect STDERR and STDOUT, etc.)! (WARNING: I am told that I/O redirection may be quite tricky on Windows. We may substitute a different feature here for Windows) You also do not have to support I/O redirection for built-in commands (it shouldn't be too hard but you don't have to do it.)

    Note: one redirect in each direction is fine, not ls > foo < foo2

  6. First open the file (use open or creat, open read only for infiles and creat writable for outfiles ) and then use dup2. 0 is the filedescriptor for STDIN and 1 is the file descriptor for STDOUT.
    dup2 (fdFileYouOpened, fileno(stdin))
    dup2 (fdFileYouOpened, fileno(stdout))

  7. You should be able to place commands in the background with an & at the end of the command line. You do not need to support moving processes between the foreground and the background (ex. bg and fg). You also do not need to support putting built-in commands in the background.
  8. Try waitpid(pid, status, options).

  9. You should maintain a history of commands previously issued. The number of previous commands recorded can be a compile time constant of at least 10. This is a FIFO list, you should start numbering them with 1 and then when you exhaust your buffer you should discard the lowest number *BUT* keep incrementing the number of the next item. For example, if storing 10 commands, when the 11th is issued, you would be recording commands 2 through 11.

    Linux TIP! The history command can show you the remembered commands by the linux shell, available with the up and down arrows

    Windows TIP! Use the up and down arrows to access the history from the cmd app.

  10. A user should be able to repeat a previously issued command by typing !number where number indicates which command to repeat. !-1 would mean to repeat the last command. !1 would mean repeat the command numbered 1 in the list of command returned by history.

    Note: You can probably think of better syntax for this, but I thought it was good to stay as close as possible to syntax used by real shells

  11. A built-in command is one for which no new process is created but instead the functionality is build directly into the shell itself. You should support the following built-in commands: jobs, cd, history, exit and kill. I would also *highly* recommend the built-in function help that lists the available built-in commands and their syntax. (If you don't follow the syntax expected, then a help function would let the graders proceed anyway.)
  12. Try kill (pid, SIGKILL) .

  13. If the user chooses to exit while there are background processes, notify the user that these background processes exist, do not exit and return to the command prompt. The user must kill the background processes before exiting.
  14. You may assume that each item in the command string is seperated on either side by at least on space (e.g. prog > outfile rather than prog>outfile).

How to start


Here is a text file that I wrote (not computer output so beware) to show an example of the shell in operation.

Here is a tar file that contains a Makefile that will get you started compiling on a UNIX/Linux platform. Copy the file and then execute the command tar -xf exampleShell.tar to unzip the contents. Then simply cd exampleShell into the exampleShell directory and type make. This will compile shell.c into the executable shell. You can execute shell by typing ./shell at the command prompt. You can remove the executable by typing make clean.

Here is a tar file that contains a Makefile and two programs that illustrate the use of the dup system call to support I/O redirection. Note that the file f1 is present in the directory and is needed as the input file for the forkDupExec program. That program when run will produce f2 an identical copy of f1 by forking off a new process, reassigning its stdout and stdin and execing cat.

Optional Features

If you are enjoying this project and would like to add more advanced features to your shell, here are some sugguestions:

  1. You could support optional parameters to some of the built-in commands. For example, history -s num could set the size of the history buffer and history num could return the last num commands. You could also support additional built-in commands like which, pushd/popd or alias. If you make modifcations of this type, I would recommend help command to return more detailed information on a single command.
  2. You could support | , a pipe, between two processes. For example, foo | bar would send the STDOUT of foo to the STDIN of bar using a pipe. You may want to start by supporting pipes only between two processes before considering longer chains. Longer chains will probably require something like handle process n and then recursively handle the other n-1.
  3. You could implement more advanced I/O redirection as described above (>&, >!, etc.).
  4. You could implement the built-in shell functions, fg and bg, to move processes between the background and the foreground.
  5. You could support the editing of shell variables with built-in shell functions like printenv and setenv.
  6. I wouldn't recommend it :-), but you could even write support for shell programming (e.g. if/then, while, for constructs).
  7. Tab completion and command prompt editing. The GNU readline library makes this easy.
  8. Up and down errors to scroll through the history list. The GNU history library makes easy.
  9. Adding the builtin function ls on Windows.
  10. You could relax the parsing constraints on the command line itself (e.g. correctly recognize the space free command prog>outfile).
  11. Terminal support (messy!)(You may notice that some programs like more or pine are aware of the screen real estate they need - that requires terminal emulation support.
  12. You could also try porting it to yet another OS. (PalmOS?)

Any advanced shell feature is likely to earn you some extra credit, but you should do it only if you've finished the required functions, are having fun and would like to learn more. In particular, we will *not* say how much extra credit each feature or sub feature may be worth.

Submitting Your Assignment

Instructions will be given on how to submit your assignment

Helpful Resources

Here are a list of resources you may find helpful. Feel free to send mail suggesting others.


GNU C Library

GNU history library (for the required functionality might be easier without it?)

The following functions are likely to be helpful (consult the man pages for details):
fork, exec, execvp, wait, waitpid, kill, dup, pipe, strncmp, strlen, malloc, free, getcwd, chdir , open, close, readline, gets, fgets, getchar, signal (*not* system!)

Debugging with GDB



Sample I/O redirection

Last modified: