CSCIGA 2250 Shell Lab
Lab: Shell 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 Usable SHell, or nyush for short.
Please review the first lecture of MIT’s The Missing Semester of Your CS Education if you are not familiar with the shell.
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The shell is essentially a command-line interpreter. It works as follows:
1. It prompts you to enter a command.
2. 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.
3. 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
Your shell should follow these specifications carefully. These specifications may be slight different from the default Linux shell (bash) for simplicity.
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 $.
• Another whitespace.
For example, if you are in /home/abc123/2250/lab2, then the prompt should be:
[nyush lab2]$ █
If you are in the root directory (/), then the prompt should be: [nyush /]$ █
Note that the final █ character in these examples 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). A command may contain multiple programs separated by the pipe (|) symbol. A valid command must satisfy the following requirements:
• If there are multiple programs in a command, only
the first program may redirect its input (using <), and only the last program may redirect its output (using > or >>). If there is only one program in a command, it may redirect both input and output.
• In each command, there are no more than one input redirection and one output redirection.
• Built-in commands (e.g., cd) cannot be I/O redirected or piped. For simplicity, our test cases have the following assumptions:
• Each command has no more than 1000 characters.
• There is always a single space separating filenames,
arguments, and the pipe and redirection symbols (|, <, >, >>).
• There are no spaces within a filename or an argument.
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>file.txt|cat
• cat|cat
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]$ █
(Again, the final █ character represents your cursor.) Locating programs
You can specify a program by either an absolute path, a relative path, or base name only.
1. An absolute path begins with a slash (/). If the user specifies an absolute path, then your shell must run the program at that location.
2. 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.
3. 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
[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. To do so, you can connect to the running Docker container from another terminal window using:
docker exec -it 2250 bash
...and then kill your shell using:
2250]# killall nyush
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 filename. 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 filename. For example:
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.
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.
[nyush lab2]$ ls -l > output.txt
[nyush lab2]$ ls -l >> output.txt
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.
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:
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 STDERR and prompt for the next command.
Error: invalid directory
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]$ cd /usr/local
[nyush local]$ cd bin
[nyush bin]$ █
[nyush lab2]$ jobs
[1] ./hello
[2] /usr/bin/top -c
[3] cat > output.txt
[nyush lab2]$ █
(If there are no currently suspended jobs, this command should print nothing.)
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
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:
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] /usr/bin/top -c
[3] cat > output.txt
[nyush lab2]$ fg 2
[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
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.
Compilation
We will grade your submission in an x86_64 8 container on Gradescope. We will compile your program using gcc 12.1.1 with the C17 standard and GNU extensions.
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 or execute / bin/sh. Otherwise, what is the whole point of this lab?
Submission
You must submit a .zip archive containing all files needed to compile nyush in the root of the archive. You can create the archive file with the following command in the Docker container:
$ zip nyush.zip Makefile *.h *.c
Note that other file formats (e.g., rar) will not be accepted.
You need to upload the .zip archive to Gradescope. If you need to acknowledge any influences per our academic integrity policy, write them as comments in your source code.
The total of this lab is 100 points, mapped to 15% of your final grade of this course.
• Compile successfully and can print the correct prompt. (40 points)
• Process creation and termination. (20 points)
• Simple built-in commands (cd and exit) and error handling. (10
• Input and output redirection. (10 points)
• Pipes. (10 points)
• Handling suspended jobs (jobs and fg). (10 points)
You will get 0 points for this lab if you call the system() function or execute /bin/sh.
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.
How to get started?
This lab requires you to write a complete system from scratch, so it may be daunting at first. Remember to get the basic functionality working first, and build up your shell step-by-step.
Here is how I would tackle this lab:
Milestone 1. Write a simple program that prints the prompt and flushes STDOUT. You may use the getcwd() system call to get the current working directory.
Milestone 2. Write a loop that repeatedly prints the prompt and gets the user input. You may use the getline() library function to obtain user input.
Milestone 3. Extend Milestone 2 to be able to run a simple program, such as ls. At this point, what you’ve written is already a shell! Conceptually, it might look like:
while (true) {
// print prompt
// read command
pid = fork();
if (pid < 0) {
// fork failed (this shouldn't happen)
} else if (pid == 0) {
// child (new process)
execv(...); // or another variant
// exec failed
waitpid(-1, ...);
Milestone 4. Extend Milestone 3 to run a program with arguments, such as ls -l.
Milestone 5. Handle simple built-in commands (cd and exit). You may use the chdir() system call to change the working directory.
Milestone 6. Handle output redirection, such as cat > output.txt. Milestone 7. Handle input redirection, such as cat < input.txt.
Milestone 8. Run two programs with one pipe, such as cat | cat. The example code in man 2 pipe is very helpful.
Milestone 9. Handle multiple pipes, such as cat | cat | cat.
Milestone 10. Handle suspended jobs and related built-in commands (jobs and fg). Read man waitpid to see how to wait for a stopped child.
Some students may find handling suspended jobs easier than implementing pipes, so feel free to rearrange these milestones as you see fit.
Keep versions of your code! Use git or similar tools, but don’t make your repository public.
Useful system calls
Your shell will make many system calls. Here are a few that you may find useful.
• Process management: access(), fork(), execv() (or other variants), waitpid().
• I/O redirection and
pipes: 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 (and possibly, errno) of all system calls and library function calls from the very beginning of your work! This will often catch errors early, and it is a good programming practice.
Reading 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 kill shows the kill command in Section 1 by default. If you need to look up the kill() system call, you need to invoke man 2 kill.
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?
In gdb, how to debug the child process after fork()?
By default, when a program forks, gdb will continue to debug the parent process, and the child process will run unimpeded.
If you want to follow the child process instead of the parent process, use the command set follow-fork-mode child.
If you want to debug both the parent and child processes, use the command set detach-on-fork off. Then, you can use info inferiors to show all processes and use inferior to switch to another process.
I’m still confused about pipes. Any more hints?
If you can already support I/O redirection for a single process, adding support for pipes shouldn’t be too difficult since a pipe is just two I/O redirections. Start by reading the example code in man 3 pipe and writing a simple program to see how pipe works.
Here are some additional hints:
Make sure a single pipe works before trying multiple pipes.
To support a single pipe, the parent (shell) process should
create the pipe and fork two children. The first child redirects
its output to the write end of the pipe, and the second child
redirects its input from the read end of the pipe. In other
words, instead of opening a file for redirection, use the file
descriptors returned by
Both children should run concurrently. The parent (shell)
process should wait for both children. In other words, a child
shouldn’t wait for another child.
Make sure to close all unused file descriptors, otherwise your
child process may not terminate. In the above case, the
parent holds two ends of the pipe, and after
, each child
holds two ends of the pipe. So altogether you need to close
six file descriptors. The parent should close them before
, and the child should close them before
calling calling
Supporting multiple pipes is similar. For N processes, your
shell should create N-1 pipes and fork N children. You can
either write a loop or use recursion to do that.
execv(). •
I’m still confused about suspended jobs. Any more hints?
To support suspended jobs, your shell should call
the WUNTRACED WIFSTOPPED()
waitpid() with
option and
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