While working on your shell, if you accidentally forkbomb, you will probably have to close your VM and restart it. (The bug is, you forgot to exit after calling execvp.)

In this project, you’ll be making a simple Unix shell. A shell is what you interact with when you log into your VM - it’s a command-line interface for running programs.

Unlike the previous programming projects/labs, you are being left to figure many things out for yourself. By now you should be familiar enough with “how things work” in C that you can be mostly self-sufficient, and figure out how to do things based on examples and documentation.

To that end, here are some useful places for documentation:

Additionally, I have linked relevant code examples in the instructions below.

Grading Rubric

Note: Error handling is not explicitly mentioned in any of the following categories, but you should be checking for errors in everything. You’ll lose points if not.

Isn’t a shell a special kind of program?

Nope! A shell is just a user-mode process that lets you interact with the operating system. The basic operation of a shell can be summed up as:

  1. Read a command from the user
  2. Parse that command
  3. If the command is valid, run it
  4. Go back to step 1

The shell you interact with when you log into your VM is called bash. You can even run bash inside itself:

[cs0449vm ~]: bash
[cs0449vm ~]: pstree student
sshd───bash───bash───pstree

systemd───(sd-pam)
[cs0449vm ~]: exit
[cs0449vm ~]: _

pstree shows a bash process nested inside another bash process!

When you write your shell, you can test it like any other program you’ve written.

[cs0449vm ~]: ./myshell
myshell> ls
myshell    myshell.c
myshell> exit
[cs0449vm ~]: _

1. Input tokenization

Use fgets() with a generously-sized input buffer, like 300 characters.

Once you have the input, you can tokenize it (split it into “words”) with the strtok_r() function. It behaves oddly, so be sure to read up on it.

Put #define _GNU_SOURCE at the top of your program like in the following example.

Here is a sample program that demonstrates strtok_r.. Feel free to use it as the basis for your command parsing, but remember… You cannot return the resulting token array from a function. So you have to allocate that array in the function that needs it.

For strtok_r()’s “delim” parameter, you can give it this string:

" \t\n"

Get the string tokenization working first. Test it out well, and try edge cases - typing nothing, typing many things, typing several spaces in a row, using tab characters…

Commands

Many of the commands you’re used to running in the shell are actually builtins - commands that the shell understands and executes instead of having another program execute them. This makes the shell somewhat faster, because it doesn’t have to start a new process for each command.

Anything that isn’t a builtin should be interpreted as a command to run a program.

Following is a list of commands you need to support.

2. exit and exit number

Functions needed: exit()

The simplest command is exit, as it just… exits the shell.

NOTE: In all these examples, myshell> indicates your shell’s prompt, and $ indicates bash’s prompt.

$ ./myshell
myshell> exit
$ _

You also need to support giving an argument to exit. It should be a number, and it will be returned to bash. You can check it like so:

Exit codes can only go up to 255. If you type a bigger number, it will wrap around. That’s not a bug, so don’t worry!

myshell> exit 45
$ echo $?
45
$ _

The echo $? command in bash will show the exit code from the last program.

If no argument is given to exit, it should return 0:

myshell> exit
$ echo $?
0
$ _

Hint: there are a few functions in the C standard library you can use to parse integers from strings. You’ve used at least one before…

3. cd dirname

Functions needed: chdir()

You know how cd works! You don’t have to do anything special for the stuff that comes after the cd. chdir() handles it all for you.

Really, chdir() handles it all for you. You don’t have to parse the path, or look for ‘..’, or make sure paths are relative/absolute etc. chdir() is like cd in function form.

You do not need to support cd without an argument. Just regular old cd.

You do not need to support cd ~. This is actually a bash feature, but it’s kind of complicated, so don’t worry about it.

You can see if it works properly using the pwd program, once your shell can run regular programs.

myshell> cd test
myshell> pwd
/home/student/private/test
myshell> cd ..
myshell> pwd
/home/student/private
myshell> _

4. Regular programs

Functions needed: fork(), execvp(), exit(), waitpid(), signal()

If something doesn’t look like any built-in command, run it as a regular program. You should support commands with or without arguments.

There are two examples that will form the basis of this part, which you can kinda smash together:

Your shell should support ANY number of arguments to programs, not just zero or one.

For example, and these are just examples: ANY program should be able to be run like this:

myshell> ls
myshell.c    myshell    Makefile
myshell> pwd
/home/student/private
myshell> echo "hello"
"hello"
myshell> echo 1 2 3 4 5
1 2 3 4 5
myshell> touch one two three
myshell> ls -lh .
total 9K
-rw-r--r-- 1 abc123 UNKNOWN1 2.8K Dec  3 22:04 myshell.c
-rwxr-xr-x 1 abc123 UNKNOWN1 4.4K Dec  3 22:04 myshell
-rw-r--r-- 1 abc123 UNKNOWN1  319 Dec  3 18:51 Makefile
-rw-r--r-- 1 abc123 UNKNOWN1    0 Dec  3 22:05 one
-rw-r--r-- 1 abc123 UNKNOWN1    0 Dec  3 22:05 two
-rw-r--r-- 1 abc123 UNKNOWN1    0 Dec  3 22:05 three
myshell> _

The Parent Process

After using fork(), the parent process should wait for its child to complete. Things to make sure to implement:

If you get errors about “implicit declaration of function ‘strsignal’” then add #define _GNU_SOURCE to the very top of your code, before any #include lines.

The Child Process

After using fork(), the child process is responsible for running the program. Things to make sure to implement:

AND THEN…. exit() after you print the error. DON’T FORGET TO EXIT HERE. This is how you forkbomb.

Notes on using execvp:

Catching Ctrl+C

Ctrl+C is a useful way to stop a running process. However by default, if you Ctrl+C while a child process is running, the parent will terminate too. So if you try to use it while running a program in your shell…

$ ./myshell
myshell> cat
typing stuff here...
typing stuff here...
cat just copies everything I type.
cat just copies everything I type.
<ctrl+C>
$ _

I tried to exit cat by using Ctrl+C but it exited my shell too!

Making this work right is pretty easy.

Once that’s done, you can use Ctrl+C with abandon:

Ctrl+C will make a ^C print and kinda mess up the display. That’s okay.

$ ./myshell
myshell> cat
blah
blah
blahhhhh
blahhhhh
<ctrl+C>
myshell> exit
$ _

5. Input and Output redirection

Functions needed: freopen()

Any regular program should also support having its stdin, stdout, or both redirected with the < and > symbols.

The redirections can come in either order, like cat < input > output or cat > output < input. Do not hardcode your shell to assume one will come before the other.

Your shell should support using input and output redirection on any non-builtin command with any number of parameters.

This means you should look for the redirections by looking starting at the last tokens. Then you can replace each redirection token (< and >) with NULL to ensure the right arguments get passed to the program.

There should be at most one > and one <. If the user uses < or > more than once, give an error and don’t run anything. (Since you are in the child process when doing this, you can just exit(0) when you encounter a redirection error.)

bash lets you write ls>out without spaces, but you don’t have to support that. ls > out is fine for your shell.

myshell> ls > output
myshell> cat output
myshell.c
myshell
Makefile
output
myshell> less < Makefile

<then less runs and shows the makefile>

myshell> cat < Makefile > copy
myshell> ls
myshell.c    myshell    Makefile    output    copy
myshell> less copy

<then less runs and shows that 'copy' is identical to the original makefile>

myshell> ls -lh . > output
myshell> cat output
total 31K
-rw-r--r-- 1 abc123 UNKNOWN1 2.8K Dec  3 23:18 myshell.c
-rwxr-xr-x 1 abc123 UNKNOWN1 4.4K Dec  3 23:18 myshell
-rw-r--r-- 1 abc123 UNKNOWN1  319 Dec  3 18:51 Makefile
-rw-r--r-- 1 abc123 UNKNOWN1   39 Dec  3 23:20 output
-rw-r--r-- 1 abc123 UNKNOWN1  319 Dec  3 23:21 copy
myshell> ls > output > output
Error: Too many redirections
myshell> _

Input and output redirection should detect and report the following errors:

Again, you can just exit the child process when encountering any of these errors.

Opening the redirection files

You should open the redirection files in the child process after using fork, but before using execvp().

In order to redirect stdin and stdout, you have to open new files to take their place. freopen() is the right choice for this.

Submission

It’ll be on gradescooooope