ostep-projects/processes-shell/README.md

Unix Shell

In this project, you'll build a simple Unix shell. The shell is the heart of the command-line interface, and thus is central to the Unix/C programming environment. Mastering use of the shell is necessary to become proficient in this world; knowing how the shell itself is built is the focus of this project.

There are three specific objectives to this assignment:

• To further familiarize yourself with the Linux programming environment.
• To learn how processes are created, destroyed, and managed.
• To gain exposure to the necessary functionality in shells.

Overview

In this assignment, you will implement a command line interpreter (CLI) or, as it is more commonly known, 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 shells you implement will be similar to, but simpler than, the one you run every day in Unix. If you don't know what shell you are running, it's probably bash. One thing you should do on your own time is learn more about your shell, by reading the man pages or other online materials.

Program Specifications

Basic Shell: wish

Your basic shell, called wish (short for Wisconsin Shell, naturally), is basically an interactive loop: it repeatedly prints a prompt wish> (note the space after the greater-than sign), parses the input, executes the command specified on that line of input, and waits for the command to finish. This is repeated until the user types exit. The name of your final executable should be wish:

prompt> ./wish

You should structure your shell such that it creates a new process for each new command (note that there are a few exceptions to this, which we discuss below). Your basic shell should be able to parse a command and run the program corresponding to the command. For example, if the user types ls -la /tmp, your shell should run the program /bin/ls with the given arguments -la and /tmp.

You might be wondering how the shell knows to run /bin/ls (which means the program binary ls is found in the directory /bin) when you type ls. The shells knows this thanks to a path variable that the user sets. The path variable contains the list of all directories to search, in order, when the user types a command. We'll learn more about how to deal with the path below.

Important: Note that the shell itself does not implement ls or really many other commands at all (it does implement a few, called built-ins, described further below). All it does is find those executables in one of the directories specified by path and create a new process to run them. More on this below.

Built-in Commands

Whenever your shell accepts a command, it should check whether the command is a built-in command or not. If it is, it should not be executed like other programs. Instead, your shell will invoke your implementation of the built-in command. For example, to implement the exit built-in command, you simply call exit(0); in your C program.

So far, you have added your own exit built-in command. Most Unix shells have many others such as cd, pwd, etc. In this project, you should implement exit, cd, pwd, and path.

The formats for exit, cd, and pwd are:

[optional-space]exit[optional-space]
[optional-space]pwd[optional-space]
[optional-space]cd[optional-space]
[optional-space]cd[oneOrMoreSpace]dir[optional-space]

When you run cd (without arguments), your shell should change the working directory to the path stored in the $HOME environment variable. Use the call getenv("HOME") in your wish source code to obtain this value.

You do not have to support tilde (~). Although in a typical Unix shell you could go to a user's directory by typing cd ~username, in this project you do not have to deal with tilde. You should treat it like a common character, i.e., you should just pass the whole word (e.g. "~username") to chdir(), and chdir will return an error.

Basically, when a user types pwd, you simply call getcwd(), and show the result. When a user changes the current working directory (e.g. "cd somepath"), you simply call chdir(). Hence, if you run your shell, and then run pwd, it should look like this:

% cd
% pwd
/afs/cs.wisc.edu/u/m/j/username
% echo $PWD
/u/m/j/username
% ./wish
wish> pwd
/afs/cs.wisc.edu/u/m/j/username

The format of the path built-in command is:

    [optionalSpace]path[oneOrMoreSpace]dir[optionalSpace] (and possibly more directories, space separated)

A typical usage would be like this:

wish> path /bin /usr/bin

By doing this, your shell will know to look in /bin and /usr/bin when a user types a command, to see if it can find the proper binary to execute. If the user sets path to be empty, then the shell should not be able to run any programs unless XXX (but built-in commands, such as path, should still work).

Redirection

Many times, a shell user prefers to send the output of his/her program to a file rather than to the screen. Usually, a shell provides this nice feature with the > character. Formally this is named as redirection of standard output. To make your shell users happy, your shell should also include this feature, but with a slight twist (explained below).

For example, if a user types ls -la /tmp > output, nothing should be printed on the screen. Instead, the standard output of the ls program should be rerouted to the output.out file. In addition, the standard error output of the file should be rerouted to the file output.err (the twist is that this is a little different than standard redirection).

If the output.out or output.err files already exists before you run your program, you should simple overwrite them (after truncating). If the output file is not specified (e.g., the user types ls > without a file), you should print an error message and not run the program ls.

Here are some redirections that should not work:

ls > out1 out2
ls > out1 out2 out3
ls > out1 > out2

Note: don't worry about redirection for built-in commands (e.g., we will not test what happens when you type path /bin > file).

Parallel Commands

Your shell will also allow the user to launch parallel commands.

Program Errors

The one and only error message. You should print this one and only error message whenever you encounter an error of any type:

    char error_message[30] = "An error has occurred\n";
    write(STDERR_FILENO, error_message, strlen(error_message)); 

The error message should be printed to stderr (standard error). Also, do not add whitespaces or tabs or extra error messages.

There is a difference between errors that your shell catches and those that the program catches. Your shell should catch all the syntax errors specified in this project page. If the syntax of the command looks perfect, you simply run the specified program. If there is any program-related errors (e.g., invalid arguments to ls when you run it, for example), let the program prints its specific error messages in any manner it desires (e.g., could be stdout or stderr).

White Spaces

The > operator will be separated by spaces. Valid input may include the following:

wish> ls
wish> ls > a
wish>    ls    > a

But not this (it is ok if this works, it just doesn't have to):

wish> ls>a

Defensive Programming and Error Messages

Defensive programming is good for you, so do it! It is also required. Your program should check all parameters, error-codes, etc. before it trusts them. In general, there should be no circumstances in which your C program will core dump, hang indefinitely, or prematurely terminate. Therefore, your program must respond to all input in a reasonable manner; by "reasonable", we mean print the error message (as specified in the next paragraph) and either continue processing or exit, depending upon the situation.

Since your code will be graded with automated testing, you should print this one and only error message whenever you encounter an error of any type:

    char error_message\[30\] = \"An error has occurred\\n\";
    write(STDERR_FILENO, error_message, strlen(error_message)); 

For this project, the error message should be printed to stderr. Also, do not attempt to add whitespaces or tabs or extra error messages.

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

• An incorrect number of command line arguments to your shell program.

For the following situation, you should print the error message to stderr and continue processing:

• A command does not exist or cannot be executed.
• A very long command line (over 128 bytes).

Your shell should also be able to handle the following scenarios below, which are not errors.

• An empty command line.
• Multiple white spaces on a command line.

Hints

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 will suggest a few library routines you may want to use to make your coding easier. 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.]

Basic Shell

Parsing: For reading lines of input, once again check out getline(). 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! You may find the strtok() routine useful for parsing the command line (i.e., for extracting the arguments within a command separated by whitespaces).

Executing Commands: Look into fork, exec, and wait/waitpid. See the man pages for these functions, and also read book chapter.

You will note that there are a variety of commands in the exec family; for this project, you must use execv. You should not use the system() library function call to run a command. Remember that if execv() 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 

Assuming that you find foo in directory /bin (or elsewhere in the defined path), then argv[0] = "/bin/foo", argv[1] = "205" and argv[2] = "535".

Important: the list of arguments must be terminated with a NULL pointer; in our example, this means argv[3] = NULL. We strongly recommend that you carefully check that you are constructing this array correctly!

Built-in Commands

For the exit built-in command, you should simply call exit() from within your source code. The corresponding shell process will exit, and the parent (i.e. your shell) will be notified.

For managing the current working directory, you should use getenv(), chdir(), and getcwd(). The getenv()call is useful when you want to go to your HOME directory. Thegetcwd()call is useful to know the current working directory, i.e., if a user typespwd, you simply call getcwd()and use those results. Finally,chdir` is useful for moving to different directories. For more information on these topics, read the man pages or the Advanced Unix Programming book (Chapters 4 and 7) or look around online.

Redirection

Redirection is relatively easy to implement. For example, to redirect standard output to a file, just use close() on stdout, and then open() on a file. More on this below.

With a file descriptor, you can perform read and write to a file. Maybe in your life so far, you have only used fopen(), fread(), and fwrite() for reading and writing to a file. Unfortunately, these functions work on FILE *, which is more of a C library support; the file descriptors are hidden.

To work on a file descriptor, you should use open(), read(), and write() system calls. These functions perform their work by using file descriptors. To understand more about file I/O and file descriptors you can read the Advanced Unix Programming book (Chapter 3) (specifically, 3.2 to 3.5, 3.7, 3.8, and 3.12), or just read the man pages. Before reading forward, at this point, you should become more familiar file descriptors.

The idea of redirection is to make the stdout descriptor point to your output file descriptor. First of all, let's understand the STDOUT_FILENO file descriptor. When a command ls -la /tmp runs, the ls program prints its output to the screen. But obviously, the ls program does not know what a screen is. All it knows is that the screen is basically pointed by the STDOUT_FILENO file descriptor. In other words, you could rewrite printf("hi"); in this way: write(STDOUT_FILENO, "hi", 2);.

To check if a particular file exists in a directory, use the stat() system call. For example, when the user types ls, and path is set to include both /bin and /usr/bin, try stat("/bin/ls"). If that fails, try stat("/usr/bin/ls"). If that fails too, print the only error message.

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 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. Next, add built-in commands. Finally, add redirection support.

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.

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 git. 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.