Linux Shell: hfsh26

Operating Systems

Due February 16 by 23:59

In this project, you will build a simple Unix shell. The shell is the heart of the command-line interface, and thus is central to the Linux/UNIX/C/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.

Logistics

• This project is worth 100 points.
• This is an individual assignment. I will be checking your source code using MOSS against other programs on GitHub, CoPilot, ChatGPT, etc. as well as other students. Click here for the MOSS site.
• Your hfsh26 shell program must be written in C/C++. The use of the standard template library is permitted.
• A Makefile is supplied. Include additional targets to clean your source code subdirectory and another target to test your program.
• Your source code file is hfsh26.cpp.
• The name of your executable must be hfsh26.
• Your program must be well documented.
• Your program must be well structured. Programs that are all in main() or have excessively long functions will result in loss of points.
• You are not permitted to use the system() library function.
• You are permitted to use the functions in csapp.h.
• Your program will be tested on babbage.cs.tcu.edu (Ubuntu Server 24.04 LTS).
• Keep your program up-to-date using git.
• The test cases included with this project are the minimum test cases for submission. Note: I may test your program with other test cases.

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 Linux/macOS/Windows. If you don't know what shell you are running, it is probably bash on Linux machines (or bash or zsh on macOS). 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: hfsh26

Your basic shell, called hfsh26 (short for Horned Frog Shell v2), is basically an interactive loop: it repeatedly prints a prompt hfsh26> (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 must be hfsh26.

The shell can be invoked with either no arguments or a single argument; anything else is an error. Here is the no-argument example:

prompt> ./hfsh26
hfsh26> 

At this point, hfsh26 is running, and ready to accept commands. Type away!

The mode above is called interactive mode, and allows the user to type commands directly. The shell also supports a batch mode, which instead reads input from a batch file and executes commands from therein. Here is how you run the shell with a batch file named batch.txt:

prompt> ./hfsh26 batch.txt

One difference between batch and interactive modes: in interactive mode, a prompt is printed (hfsh26> ). In batch mode, no prompt should be printed.

You should structure your shell such that it creates a process for each new command (the exception are built-in commands, discussed 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 (how does the shell know to run /bin/ls? It's something called the shell path; more on this below).

Structure

Basic Shell

The shell is very simple (conceptually): it runs in a while loop, repeatedly asking for input to tell it what command to execute. It then executes that command. The loop continues indefinitely, until the user types the built-in command exit, at which point it exits. That's it!

The reading and parsing of lines of text has been completed for you! See the file hfsh26.cpp.

In either mode (interactive or batch), when your shell hits the end-of-file marker (EOF), your shell will exit(0).

To execute commands, look into fork(), exec(), and wait()/waitpid(). See the man pages or notes from last semester for these functions. You are not permitted to use the system() library function call to run a command. Remember that if exec() 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.

Paths

In our example above, the user typed ls but the shell knew to execute the program /bin/ls. How does your shell know this?

It turns out that the user must specify a path variable to describe the set of directories to search for executables; the set of directories that comprise the path are sometimes called the search path of the shell. The path variable contains the list of all directories to search, in order, when the user types a command.

Important: Note that the shell itself does not implement ls or other commands (except built-ins). All it does is find those executables in one of the directories specified by path and create a new process to run them.

To check if a particular file exists in a directory and is executable, consider the access() system call. For example, when the user types ls, and path is set to include both /bin and /usr/bin, try access("/bin/ls", X_OK). If that fails, try "/usr/bin/ls". If that fails too, it is an error.

Your initial shell path should contain one directory: `/bin'

Note: Most shells allow you to specify a binary specifically without using a search path, using either absolute paths or relative paths. For example, a user could type the absolute path /bin/ls and execute the ls binary without a search path being needed. A user could also specify a relative path which starts with the current working directory and specifies the executable directly, e.g., ./main. In this project, you do not have to worry about these features.

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 hfsh source code, which then will exit the shell.

In this project, you should implement exit, cd, and path as built-in commands.

• exit: When the user types exit, your shell should simply call the exit system call with 0 as a parameter. It is an error to pass any arguments to exit.

• cd: cd always take one argument (0 or >1 args should be signaled as an error). To change directories, use the chdir() system call with the argument supplied by the user; if chdir fails, that is also an error.

• path: The path command takes 0 or more arguments, with each argument separated by whitespace from the others. A typical usage would be like this: hfsh> path /bin /usr/bin, which would add /bin and /usr/bin to the search path of the shell. If the user sets path to be empty, then the shell should not be able to run any programs (except built-in commands). The path command always overwrites the old path with the newly specified path.

Redirection

Many times, a shell user prefers to send the output of a 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 file output. In addition, the standard error output of the program should be rerouted to the file output (the twist is that this is a little different than standard redirection).

If the output file exists before you run your program, you should simple overwrite it (after truncating it).

The exact format of redirection is a command (and possibly some arguments) followed by the redirection symbol followed by a filename. Multiple redirection operators or multiple files to the right of the redirection sign are errors.

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

Parallel Commands

Your shell will also allow the user to launch parallel commands. This is accomplished with the ampersand operator as follows:

hfsh> cmd1 & cmd2 args1 args2 & cmd3 args1

In this case, instead of running cmd1 and then waiting for it to finish, your shell should run cmd1, cmd2, and cmd3 (each with whatever arguments the user has passed to it) in parallel, before waiting for any of them to complete.

Then, after starting all such processes, you must make sure to use wait() (or waitpid) to wait for them to complete. After all processes are done, return control to the user as usual (or, if in batch mode, move on to the next line).

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), as shown above.

After any most errors, your shell simply continue processing after printing the one and only error message. However, if the shell is invoked with more than one file, or if the shell is passed a bad batch file, it should exit by calling exit(1).

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 are any program-related errors (e.g., invalid arguments to ls when you run it, for example), the shell does not have to worry about that (rather, the program will print its own error messages and exit).

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).

Next, add built-in commands. Then, try working on redirection. Finally, think about parallel commands. Each of these requires a little more effort on parsing, but each should not be too hard to implement.

At some point, you should make sure your code is robust to white space of various kinds, including spaces ( ) and tabs (\t). In general, the user should be able to put variable amounts of white space before and after commands, arguments, and various operators; however, the operators (redirection and parallel commands) do not require whitespace.

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. It's also 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 different inputs at it and make sure the shell behaves well. Good code comes through testing; you must run many different tests to make sure things work as desired. Don't be gentle -- other users certainly won't be.

Finally, 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.

Points

• Documentation (10 points)
• Well structured (10 points)
• Built-in commands (cd, exit) (Example test cases: 1, 2, 5, 15, 21) (10 points)
• Basic shell (fork, exec) (Example test cases: 3, 4) (10 points)
• Paths (Example test cases: 6, 7) (15 points)
• Redirection (Example test cases: 8, 9, 10, 11, 12) (15 points)
• Batch (Example test cases: 13, 14) (10 points)
• Parallel commands (execute in background) (Example test cases: 16, 17, 18, 19) (15 points)
• Other/combo (Example test cases: 20, 22) (5 points)