Project 2a: The Unix Shell
There are three objectives to this assignment:
In this assignment, you will implement a command line interpreter (aka a 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 shell you implement will be similar to, but simpler than, the
one you run every day in Unix. You can find out which shell you are
running by typing
Your shell is basically an interactive loop: it repeatedly prints a
prompt> ./mysh mysh> pwd /home/tyler/cs537/p1 mysh> echo hello > out mysh> echo world >> out mysh> ls out ... mysh> cat out hello world mysh> cat out | wc 2 2 12 mysh> exit prompt>
You should structure your shell such that it creates a new process for each new command (there are a few exceptions to this, which we will discuss below). Running commands in a new process protects the main shell process from any errors that occur in the new command.
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). Some have found strchr() useful as well.
Your shell must be able to run any program in your path. The
Running ProgramsMost commands will instruct the shell to run a program with some specified arguments. For example:
mysh> prog arg1 arg2 ...
In order to execute the programs given as commands to your shell, look into fork, execvp, and wait/waitpid system calls. See the UNIX man pages for these functions, and also read the Advance Programming in the UNIX Environment, Chapter 8 (specifically, 8.1, 8.2, 8.3, 8.6, 8.10). Before starting this project, you should definitely play around with these functions.
You will note that there are a variety of commands in the
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 = "foo", argv = "205" and argv = "535".
Important: the list of arguments must be terminated with a NULL pointer; that is, argv = NULL. We strongly recommend that you carefully check that you are constructing this array correctly!
There are three special cases where your shell should execute a command directly itself instead of running a separate process.
First, if the user enters "exit" as a command, the shell should
terminate (either by returning from main, or with a call
Second, if the user enters "cd dir", you should change the
current directory to "dir" by using the
Third, if the user enters "pwd", print the current working
directory. This can be obtained with
Your shell should have three special features: overwrite redirection (">"), append redirection (">>"), and pipes ("|").
Instead of writing a program's output to the terminal, a user may want write the output to a file (redirection) or use the output as the input to another program (piping).
Overwrite redirection: if the user types "program args > outfile", save the output from running the program to outfile, overwriting any file that already exists with that name.
Append redirection: if the user types "program args >> outfile" append the output of the program to the end of the outfile, creating it if it does not already exist.
Pipes: if the user types "program1 args1 | program2 args2", use the output from program1 as the input to program2.These features are relatively easy to implement. After fork (but before exec), the STDIN and STDOUT file descriptors are already set up to refer to user-typed input and output to the terminal respectively. The
Submit a Makefile so that we can simply run "make" to compile
We will release some (80%) of the test cases we will use for grading
before the project due date. If your programs passes these tests, you will get
at least 80% of the grade unless you are not following specifications (for
example, you will receive a very low score if you use the
The remaining 20% of your grade will be based on tests not released. These will test whether you have implemented all the details in the specifications - so if you miss some corner case, you will likely lose points here.
Writing your shell in a simple manner is a matter of finding the
relevant library routines and calling them properly. To simplify
things for you in this assignment, we have suggested 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 (the
one exception is that you must use the
Remember to get the basic functionality of your program working before worrying about all of the error conditions and corner cases. For example, for your shell, first get a single command running (probably first a command with no arguments, such as "ls"). Then try adding more arguments. Next, try working on multiple commands. Make sure that you are correctly handling all of the cases where there is miscellaneous white space around commands or missing commands. Finally, support for built-in commands, redirection, and pipes.
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. And, it's just good programming sense.
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.