HW 1: Shell Solution

In this homework, you’ll be building a shell, similar to the Bash shell you use on your CS 162 Virtual Machine. The purpose of a shell is to allow users to run and manage programs. The operating system kernel provides well-documented interfaces for building shells. By building your own shell, you’ll become more familiar with these interfaces and you’ll probably learn more about other shells as well. This assignment will be due February 12, 2019 at 11:59 pm PST.

    • Getting started

Log in to your Vagrant Virtual Machine and run:

    • cd ~/code/personal/

    • git pull staff master

    • cd hw1

We have added starter code for your shell and a simple Make le in the hw1 directory. It includes a string tokenizer, which splits a string into words. In order to run the shell:

$ make

$ ./shell

In order to terminate the shell after it starts, either type exit or press CTRL-D.

    • Add support for cd and pwd

The skeleton code for your shell has a dispatcher for \built-in" commands. Every shell needs to support a number of built-in commands, which are functions in the shell itself, not external programs. For example, the exit command needs to be implemented as a built-in command, because it exits the shell itself. So far, the only two built-ins supported are ?, which brings up the help menu, and exit, which exits the shell.

Add a new built-in pwd that prints the current working directory to standard output. Then, add a new built-in cd that takes one argument, a directory path, and changes the current working directory to that directory.

Once you’re done, push your code to the autograder. In your VM:

$ git add shell.c

    • git commit -m "Finished adding basic functionality into the shell."

    • git push personal master

You should commit your code periodically and often so you can go back to a previous version of your code if you want to.

    • Program execution

If you try to type something into your shell that isn’t a built-in command, you’ll get a message that the shell doesn’t know how to execute programs. Modify your shell so that it can execute programs when they are entered into the shell. The rst word of the command is the name of the program. The rest of the words are the command-line arguments to the program.

For this step, you can assume that the rst word of the command will be the full path to the program. So instead of running wc, you would have to run /usr/bin/wc. In the next section, you will implement support for simple program names like wc. But you can pass some autograder tests by only supporting full paths.


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You should use the functions de ned in tokenizer.c for separating the input text into words. You do not need to support any parsing features that are not supported by tokenizer.c. Once you implement this step, you should be able to execute programs like this:

$ ./shell

0: /usr/bin/wc shell.c

        77 2621843 shell.c

    1: exit

When your shell needs to execute a program, it should fork a child process, which calls one of the

exec functions to run the new program. The parent process should wait until the child process completes and then continue listening for more commands.

    • Path resolution

You probably found that it was a pain to test your shell in the previous part because you had to type the full path of every program. Luckily, every program (including your shell program) has access to a set of \environment variables", which is structured as a hashtable of string keys to string values. One of these environment variables is the PATH variable. You can print the PATH variable of your current environment on your Vagrant VM: (use bash for this, not your homemade shell!)

    • echo $PATH /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:...


When bash or any other shell executes a program like wc, it looks for a program called \wc" in each directory on the PATH environment variable and runs the rst one that it nds. The directories on the path are separated with a colon.

Modify your shell so that it uses the PATH variable from the environment to resolve program names. Typing in the full pathname of the executable should still be supported. Do not use \execvp". The autograder looks for \execvp", and you won’t receive a grade if that word is found. Use execv instead and implement your own PATH resolution.

    • Input/Output Redirection

When running programs, it is sometimes useful to provide input from a le or to direct output to a le. The syntax \[process] > [file]" tells your shell to redirect the process’s standard output to a le. Similarly, the syntax "[process] < [file]" tells your shell to feed the contents of a le to the process’s standard input.

Mod y your shell so that it supports redirecting stdin and stdout to les. You do not need to support redirection for shell built-in commands. You do not need to support stderr redirection or appending to les (e.g. \[process] >> [file]"). You can assume that there will always be spaces around special characters < and >. Be aware that the \< [file]" or \> [file]" are NOT passed as arguments to the program.

    • Signal Handling and Terminal Control

Most shells let you stop or pause processes with special key strokes. These special keystrokes, such as Ctrl-C or Ctrl-Z, work by sending signals to the shell’s subprocesses. For example, pressing CTRL-C sends the SIGINT signal which usually stops the current program, and pressing CTRL-Z sends the SIGTSTP signal which usually sends the current program to the background. If you try these keystrokes in your


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shell, the signals are sent directly to the shell process itself. This is not what we want since, for example, attempting to CTRL-Z a subprocess of your shell will also stop the shell itself. We want to have the signals a ect only the subprocesses that our shell creates.

Before we explain how you can achieve this e ect, let’s discuss some more operating system concepts.

6.1    Process groups

We have already established that every process has a unique process ID (pid). Every process also has a (possibly non-unique) process group ID (pgid) which, by default, is the same as the pgid of its parent process. Processes can get and set their process group ID with getpgid(), setpgid(), getpgrp(), or setpgrp().

Keep in mind that, when your shell starts a new program, that program might require multiple processes to function correctly. All of these processes will inherit the same process group ID of the original process. So, it may be a good idea to put each shell subprocess in its own process group, to simplify your bookkeeping. When you move each subprocess into its own process group, the pgid should be equal to the pid.

6.2    Foreground terminal

Every terminal has an associated \foreground" process group ID. When you type CTRL-C, your terminal sends a signal to every process inside the foreground process group. You can change which process group is in the foreground of a terminal with \tcsetpgrp(int fd, pid_t pgrp)". The fd should be 0 for \standard input".

6.3    Overview of signals

Signals are asynchronous messages that are delivered to processes. They are identi ed by their signal number, but they also have somewhat human-friendly names that all start with SIG. Some common ones include:

SIGINT - Delivered when you type CTRL-C. By default, this stops the program.

SIGQUIT - Delivered when you type CTRL-\. By default, this also stops the program, but programs treat this signal more seriously than SIGINT. This signal also attempts to produce a core dump of the program before exiting.

SIGKILL - There is no keyboard shortcut for this. This signal stops the program forcibly and cannot be overridden by the program. (Most other signals can be ignored by the program.)

SIGTERM - There is no keyboard shortcut for this either. It behaves the same way as SIGQUIT. SIGTSTP - Delivered when you type CTRL-Z. By default, this pauses the program. In bash, if you type CTRL-Z, the current program will be paused and bash (which can detect that you paused
the current program) will start accepting more commands.

SIGCONT - Delivered when you run fg or fg %NUMBER in bash. This signal resumes a paused program.

SIGTTIN - Delivered to a background process that is trying to read input from the keyboard. By default, this pauses the program, since background processes cannot read input from the keyboard. When you resume the background process with SIGCONT and put it in the foreground, it can try to read input from the keyboard again.




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SIGTTOU - Delivered to a background process that is trying to write output to the terminal console, but there is another foreground process that is using the terminal. Behaves the same as SIGTTIN by default.

In your shell, you can use kill -XXX PID, where XXX is the human-friendly su x of the desired signal, to send any signal to the process with process id PID. For example, kill -TERM PID sends a SIGTERM to the process with process id PID.

In C, you can use the signal function to change how signals are handled by the current process. The shell should basically ignore most of these signals, whereas the shell’s subprocesses should respond with the default action. For example, the shell should ignore SIGTTOU, but the subprocesses should not. Beware: forked processes will inherit the signal handlers of the original process. Reading man 2 signal and man 7 signal will provide more information. Be sure to check out the SIG_DFL and SIG_IGN constants. For more information about how signals work, please work through the tutorial here.

Your task is to ensure that each program you start is in its own process group. When you start a process, its process group should be placed in the foreground. Stopping signals should only a ect the foregrounded program, not the backgrounded shell.

    • Background processing

So far, your shell waits for each program to nish before starting the next one. Many shells allow you run a command in the background by putting an \&" at the end of the command line. After the background program is started, the shell allows you to start more processes without waiting for background process to nish.

Modify your shell so that it runs commands that end in an \&" in the background. You only need to support background processing for programs, not built-in commands. Once you’ve implemented this feature, you should be able to run programs in the background with a command such as \/bin/ls &".

You should also add a new built-in command wait, which waits until all background jobs have terminated before returning to the prompt.

You can assume that there will always be spaces around the & character. You can assume that, if there is a & character, it will be the last token on that line.

    • Optional: Foreground/Background Switching

Most shells allow for running processes to be toggled between running in the foreground versus in background. You can optionally add two built-in commands to support this:

\fg [pid]" { Move the process with id pid to the foreground. The process should resume if it was paused. If pid is not speci ed, then move the most recently launched process to the foreground.

\bg [pid]" { Resume a paused background process. If pid is not speci ed, then resume the most recently launched process.

You should keep a list of all programs you’ve started, whether they are in the foreground or back-ground. Inside this list, you should also keep a \struct termios" to store the terminal settings of each program.

    • Autograder and Submission

To submit and push to autograder,    rst commit your changes, then do:

$ git push personal master


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CS 162      HW 1: Shell



Within 30 minutes you should receive an email from the autograder. (If you haven’t received an email within half an hour, please notify the instructors via a private post on Piazza.)

Please ensure that your solution does not print any extraneous output when stdin is not a terminal. That is, any time a built-in or a process in run with your shell, only the output of the built-in or process should be printed. Please do not print anything extra for debugging, as this can mess up your autograder results.
























































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