For our final project, we extended the capabilities of the basic shell user/sh.c provided to us. Specifically, our goals were to implement output append redirection, piping, linking, and directory navigation commands (mkdir, cd, pwd).

ssh me@node.zoo.cs.yale.edu
git clone https://github.com/rafi-khan/pios.git
cd pios
make qemu

This should run the tests and start the shell

The PIOS files system keeps a reference to the current directory via files->cwd. However this is simply a inode number, and doesn’t give the full path as we'd like through a typical pwd. For that to be useful though, we need a way to make directories. We've done that by doing a mkdir.c, which implements a lot of the UNIX mkdir.

mkdir os

But that's slow so let’s just go ahead and make lots of them

mkdir -pv os/is/awesome/yay

Verify:

ls -l

We've written a cd that takes relative or absolute pathnames

cd os
ls 
cd is
cwd # => Courtesy of Prof. Ford, should return 'is'
cd /os/is/awesome 
cwd # => 'awesome'

But who’s to know where awesome is? We’d like the full path, which can be gotten from by tracing dinos on files->cwd. The string manipulation, however, is non-trivial and can be tricky. In the end, we have pwd:

pwd # => '/os/is/awesome'

That’s pretty good. Now let’s get to appending. Normally, we can echo out to files, but we've added the standard >> append as well:

echo hi > out
cat out # => 'hi'
echo hi >> out
cat out # => 'hihi'

We would have added things like standard input, output and error redirection, but most of it was repetitive drudgery--plus we did all that in CS 323 anyway. Most of our efforts were devoted to doing command history, linking.

After make qemu, please make sure to not click on any windows (the bash console or the x11 console), as this makes the arrow key codes mess up. I've only gotten this error running on Mac OS, so if you see question marks or white blocks in the bash/x11 console, this may be why. Please recompile with make qemu and type the commands without clicking anywhere.

Our command history is a step above bash because bash keeps all history (not just distinct, non-consecutive commands) and doesn’t consider what’s already been typed when scrolling through command history. For example, if you type ls and press up, you’d expect only commands that begin with ls to show up. That’s exactly what our history does.

type e, then press up 

Pressing up with e typed will skip the previous cats and go on to the echos (you should see echo hi >> out, and if you press up again should see echo hi > outt)

You may notice that there are two t’s at the end of out in the bash console -- that’s a byproduct of bash not properly handling the \b character. If you look at the x11 console (cmd-tab over to x11 on Mac), however, you can see the history worked properly.

Switch back to bash, and press enter: this does cat hi > out, which we can verify with

cat out # => should show just “hi”

For demonstration purposes, we have programmed the down key to show the entire history -- in practice they would obviously change this to scroll the other way through the previous commands. This is simply a matter of changing ++ to a -- in readline.c. If you enter the same command twice, and press down (press enter to return to the shell), you'll see that the history only includes the command once. This only works with consecutive commands.

We implemented symbolic links in pios as a special type of inode. (Still in user space).

We have included a link program that creates link. You can do

link linkfile targetfile

And have a symbolic link to targetfile. We implemented symbolic links by storing the path in the inode data, and checking the type of inode mode in filedesc_open. To test:

echo linked > out
link linked_out out
cat linked_out # => "linked"
ls -l # => note 'l' by linked_out