Lab 2: Shell

This is the repository for Lab 2 of CS202 Operating Systems at NYU. Under src/, the fall22/ folder contains the code when I was a student of this course, and the spring23/ folder contains the code when I was a tutor of this course. By running make either under src/fall22/main/ or under src/spring23, you would be able to compile an executable named nyush, which is the shell. As follows are the lab instructions when I was a student of this course.

Introduction

The shell is the main command-line interface between a user and the operating system, and it is an essential part of the daily lives of computer scientists, software engineers, system administrators, and such. It makes heavy use of many OS features. In this lab, you will build a simplified version of the Unix shell called the New Yet Usable SHell, or nyush for short.

Objectives

Through this lab, you will:

Familiarize yourself with the Linux programming environment and the shell, of course.
Learn how to write an interactive command-line program.
Learn how processes are created, destroyed, and managed.
Learn how to handle signals and I/O redirection.
Get a better understanding of the OS and system calls.
Be a better C programmer and be better prepared for your future technical job interviews. In particular, the string parsing skill that you will practice in this lab is desired in many interview questions.

Overview

The shell is essentially a command-line interpreter. It works as follows:

It prompts you to enter a command.
It interprets the command you entered.
- If you entered a built-in command (e.g., cd), then the shell runs that command.
- If you entered an external program (e.g., /bin/ls), or multiple programs connected through pipes (e.g., ls -l | less), then the shell creates child processes, executes these programs, and waits for all these processes to either terminate or be suspended.
- If you entered something wrong, then the shell prints an error message.
Rinse and repeat until you press Ctrl-D to close STDIN or enter the built-in command exit, at which point the shell exits.

Specifications

The prompt

The prompt is what the shell prints before waiting for you to enter a command. In this lab, your prompt must have exactly the following format:

An opening bracket [.
The word nyush.
A whitespace.
The basename of the current working directory.
A closing bracket ].
A dollar sign $.
Another whitespace.

For example, if you are in /home/abc123/cs202/lab2, then the prompt should be:

[nyush lab2]$ █

If you are in the root directory (/), then the prompt should be:

[nyush /]$ █

Note that the final █ character in this example represents your cursor; you should not print that character in your shell prompt.

Keep in mind that STDOUT is line-buffered by default. Therefore, don't forget to flush STDOUT immediately after you print the prompt. Otherwise, your program may not work correctly with the autograder.

The command

In each iteration, the user inputs a command terminated by the "enter" key (i.e., newline). For simplicity, we have the following assumptions:

Each command has no more than 1000 characters.
Program arguments, if any, are separated by a single space.
There is no space within the program name or any command-line argument.
A command may contain multiple programs separated by the pipe (|) symbol.
In each command, the first program may redirect its input using <, and the last program may redirect its output using > or >>. If there is only one program in the command, it may redirect both input and output.
In each command, there may be at most one input redirection and one output redirection.
Built-in commands (e.g., cd) cannot be I/O redirected or piped.

For your reference, here is the grammar for valid commands (don't worry if you can't understand it; just look at the examples below):

[command] := ""; or
          := [cd] [arg]; or
          := [exit]; or
          := [fg] [arg]; or
          := [jobs]; or
          := [cmd] '<' [filename] [recursive]; or
          := [cmd] '<' [filename] [terminate]; or
          := [cmd] [recursive]; or
          := [cmd] [terminate] < [filename]; or
          := [cmd] [terminate].

[recursive] := '|' [cmd] [recursive]; or
            := '|' [cmd] [terminate].

[terminate] := ""; or
            := '>' [filename]; or
            := '>>' [filename].

[cmd] := [cmdname] [arg]*

[cmdname] := A string without any space, tab, > (ASCII 62), < (ASCII 60), | (ASCII 124), * (ASCII 42), ! (ASCII 33), ` (ASCII 96), ' (ASCII 39), nor " (ASCII 34) characters. Besides, the cmdname is not cd, exit, fg, jobs.

[arg] := A string without any space, tab, > (ASCII 62), < (ASCII 60), | (ASCII 124), * (ASCII 42), ! (ASCII 33), ` (ASCII 96), ' (ASCII 39), nor " (ASCII 34) characters.

[filename] := A string without any space, tab, > (ASCII 62), < (ASCII 60), | (ASCII 124), * (ASCII 42), ! (ASCII 33), ` (ASCII 96), ' (ASCII 39), nor " (ASCII 34) characters.

Here are some examples of valid commands:

A blank line.
/usr/bin/ls -a -l
cat shell.c | grep main | less
cat < input.txt
cat > output.txt
cat >> output.txt
cat < input.txt > output.txt
cat < input.txt >> output.txt
cat > output.txt < input.txt
cat >> output.txt < input.txt
cat < input.txt | cat > output.txt
cat < input.txt | cat | cat >> output.txt

Here are some examples of invalid commands:

cat <
cat >
cat |
| cat
cat << file.txt
cat < file.txt < file2.txt
cat < file.txt file2.txt
cat > file.txt > file2.txt
cat > file.txt >> file2.txt
cat > file.txt file2.txt
cat > file.txt | cat
cat | cat < file.txt
cd / > file.txt

In our test cases, you can assume that there is always a single space separating filenames, arguments, and the pipe and redirection symbols (|, <, >, >>). You can also assume all filenames and arguments contain only valid characters (i.e., no space or special characters), but the filenames might not necessarily correspond to existing files on the disk.

If there is any error in parsing the command, then your shell should print the following error message to STDERR and prompt for the next command.

Error: invalid command

Note that there should be a newline at the end of the error message. For example:

[nyush lab2]$ cat <
Error: invalid command
[nyush lab2]$ █

Locating programs

You can specify a program by either an absolute path, a relative path, or base name only.

An absolute path begins with a slash (/). If the user specifies an absolute path, then your shell must run the program at that location.
A relative path contains, but not begins with, a slash (/). If the user specifies a relative path, then your shell should locate the program by following the path from the current working directory. For example, dir1/dir2/program is equivalent to ./dir1/dir2/program.
Otherwise, if the user specifies only the base name without any slash (/), then your shell must search for the program under /usr/bin. For example, when the user types ls, then your shell should try /usr/bin/ls. If that fails, it is an error. In this case, your shell should not search the current working directory. For example, suppose there is a program named hello in the current working directory. Entering hello should result in an error, whereas ./hello runs the program.

In any case, if the program cannot be located, your shell should print the following error message to STDERR and prompt for the next command.

Error: invalid program

Process termination and suspension

After creating the processes, your shell must wait until all the processes have stopped running — either terminated or suspended. Then, your shell should prompt the user for the next command.

Your shell must not leave any zombies in the system when it is ready to read the next command from the user.

Signal handling

If a user presses Ctrl-C or Ctrl-Z, they don't expect to terminate or suspend the shell. Therefore, your shell should ignore the following signals: SIGINT, SIGQUIT, and SIGTSTP. All other signals not listed here should keep the default signal handlers.

Note that only the shell itself, not the child processes created by the shell, should ignore these signals. For example,

[nyush lab2]$ cat
^C
[nyush lab2]$ █

Here, the signal SIGINT generated by Ctrl-C terminates only the process cat, not the shell itself.

As a side note, if your shell ever hangs and you would like to kill the shell, you can still send it the SIGTERM or SIGKILL signal.

I/O redirection

Sometimes, a user would read the input to a program from a file rather than the keyboard, or send the output of a program to a file rather than the screen. Your shell should be able to redirect the standard input (STDIN) and the standard output (STDOUT). For simplicity, you are not required to redirect the standard error (STDERR).

Input redirection

Input redirection is achieved by a < symbol followed by a file name. For example:

[nyush lab2]$ cat < input.txt

If the file does not exist, your shell should print the following error message to STDERR and prompt for the next command.

Error: invalid file

Output redirection

Output redirection is achieved by > or >> followed by a file name. For example:

[nyush lab2]$ ls -l > output.txt
[nyush lab2]$ ls -l >> output.txt

If the file does not exist, a new file should be created. If the file already exists, redirecting with > should overwrite the file (after truncating it), whereas redirecting with >> should append to the existing file.

Pipe

A pipe (|) connects the standard output of the first program to the standard input of the second program. For example:

[nyush lab2]$ cat shell.c | wc -l

The user may invoke $n$ programs chained through $(n-1)$ pipes. Each pipe connects the output of the program immediately before the pipe to the input of the program immediately after the pipe. For example:

[nyush lab2]$ cat shell.c | grep main | less

Here, the output of cat shell.c is the input of grep main, and the output of grep main is the input of less.

Built-in commands

Every shell has a few built-in commands. When the user issues a command, the shell should first check if it is a built-in command. If so, it should not be executed like other programs.

In this lab, you will implement four built-in commands: cd, jobs, fg, and exit.

`cd <dir>`

This command changes the current working directory of the shell. It takes exactly one argument: the directory, which may be an absolute or relative path. For example:

[nyush lab2]$ cd /usr/local
[nyush local]$ cd bin
[nyush bin]$ █

If cd is called with 0 or 2+ arguments, your shell should print the following error message to STDERR and prompt for the next command.

Error: invalid command

If the directory does not exist, your shell should print the following error message to STRERR and prompt for the next command.

Error: invalid directory

`jobs`

This command prints a list of currently suspended jobs to STDOUT, one job per line. Each line has the following format: [index] command. For example:

[nyush lab2]$ jobs
[1] ./hello
[2] /usr/bin/top -c
[3] cat > output.txt
[nyush lab2]$ █

A job is the whole command, including any arguments and I/O redirections. A job may be suspended by Ctrl-Z, the SIGTSTP signal, or the SIGSTOP signal. This list is sorted by the time each job is suspended (oldest first), and the index starts from 1.

For simplicity, we have the following assumptions:

There are no more than 100 suspended jobs at one time.
There are no pipes in any suspended jobs.
The only way to resume a suspended job is by using the fg command (see below). We will not try to resume or terminate a suspended job by other means. We will not try to press Ctrl-C or Ctrl-D while there are suspended jobs.
You don't need to worry about "process groups." (If you don't know what process groups are, don't worry.)

The jobs command takes no arguments. If it is called with any arguments, your shell should print the following error message to STDERR and prompt for the next command.

Error: invalid command

`fg <index>`

This command resumes a job in the foreground. It takes exactly one argument: the job index, which is the number inside the bracket printed by the jobs command. For example:

[nyush lab2]$ jobs
[1] ./hello
[2] /usr/bin/top -c
[3] cat > output.txt
[nyush lab2]$ fg 2

The last command would resume /usr/bin/top -c in the foreground. Note that the job index of cat > output.txt would become 2 as a result. Should the job /usr/bin/top -c be suspended again, it would be inserted to the end of the job list:

[nyush lab2]$ jobs
[1] ./hello
[2] cat > output.txt
[3] /usr/bin/top -c
[nyush lab2]$ █

If fg is called with 0 or 2+ arguments, your shell should print the following error message to STDERR and prompt for the next command.

Error: invalid command

If the job index does not exist in the list of currently suspended jobs, your shell should print the following error message to STDERR and prompt for the next command.

Error: invalid job

`exit`

This command terminates your shell. However, if there are currently suspended jobs, your shell should not terminate. Instead, it should print the following error message to STDERR and prompt for the next command.

Error: there are suspended jobs

The exit command takes no arguments. If it is called with any arguments, your shell should print the following error message to STDERR and prompt for the next command.

Error: invalid command

Note that if the STDIN of your shell is closed (e.g., by pressing Ctrl-D at the prompt), your shell should terminate regardless of whether there are suspended jobs.

Compiling

We will grade your submission on linserv1.cims.nyu.edu, which runs CentOS Linux release 7.9.2009. We will compile your program using gcc 9.2.0. You need to run the following command to load it:

$ module load gcc-9.2

You must provide a Makefile, and by running make, it should generate an executable file named nyush in the current working directory.

Your program must not call the system() function. Otherwise, what is the whole point of this lab?

Testing

Beat up your own code extensively. Better yet, eat your own dog food. I would happily use nyush as my main shell (at least for the duration of this lab), so why wouldn't you?

We are providing a sample autograder with a few test cases. Please extract them on a CIMS compute server and follow the instructions in the README file.

Note that these test cases are not exhaustive. The test cases for final grading will be different from the ones provided and will not be shared. Do not try to hack or exploit the autograder or the CIMS computer servers.

Submission

You must submit a .tar.xz archive containing all files needed to compile nyush. You can create the archive file with the following command: (replace YourNetID with your NetID)

$ tar cvJf nyush-YourNetID.tar.xz Makefile *.h *.c

Note that other file formats (e.g., zip or rar) will not be accepted.

Rubric

The total of this lab is 100 points, mapped to 15% of your final grade of this course.

Compile successfully and can prompt the user for input. (40 points)
Process creation and termination. (20 points)
I/O redirection and pipe. (20 points)
Handling suspended jobs (jobs and fg). (10 points)
Other built-in commands (cd and exit) and error handling. (10 points)

You will get 0 points for this lab if you call the system() function.

Please make sure that your shell prompt and all error messages are exactly as specified in this document. Any discrepancy may lead to point deductions.

Tips

Don't procrastinate!

This lab requires significant programming effort. Therefore, start as early as possible! Don't wait until the last week.

Step by step

Remember to get the basic functionality working first, and build up your shell step-by-step.

Here is how I would tackle this lab:

Write a simple command parser.
Be able to run a simple program, such as ls.
Run a program with arguments, such as ls -l.
Handle simple built-in commands (cd and exit).
Handle output redirection, such as cat > output.txt.
Handle input redirection, such as cat < input.txt.
Run two programs with one pipe, such as cat | cat.
Handle multiple pipes, such as cat | cat | cat.
Handle suspended jobs.
Handle more built-in commands (jobs and fg).

Feel free to rearrange as you see fit.

Keep versions of your code! Use git or similar tools, but don't make your repository public.

Read man pages

Man pages are of vital importance for programmers working on Linux and such. It’s a treasure trove of information.

Man pages are divided into sections. Please see man man for the description of each section. In particular, Section 2 contains system calls. You will need to look them up a lot in this lab.

Sometimes, you need to specify the section number explicitly. For example, man pipe shows the page in Section 8 by default. If you need to look up the pipe() system call, you need to invoke man 2 pipe.

Useful system calls

Your shell will make many system calls. Here are a few that you may find useful.

Process management: fork(), exec*(), wait(), waitpid().
I/O redirection and pipe: dup2(), creat(), open(), close(), pipe().
Signal handling: signal().
Built-in commands: chdir(), getcwd(), kill().

You might not need to use all of them, and you are free to use other system calls not mentioned above.

Check the return values of all system calls from the very beginning of your work. This will often catch errors early, and it is a good programming practice.

Parsing the command line

You might find writing the command parser troublesome. Don’t be frustrated. You are not alone. However, it is an essential skill for any programmer, and it often appears in software engineer interviews. Once you get through it, you will never be afraid of it again.

I personally find the strtok_r() function extremely helpful. You don’t have to use it, but why not give it a try?

This lab has borrowed some ideas from Prof. Arpaci-Dusseau and Dr. T. Y. Wong.