Assignment 2 - Make your own shell! [View Files Here]

Considering that I started it the day the assignment was released, the specs changed tirelessly.
An endless tirade of constant bug squelching and tweaking, but it’s good to see an original assignment.
EDIT: no it’s not.

Command Execution

Marks: 1

The shell was to be able to execute programs entered into the prompt, locating any programs that were in one of the PATH directories.
Nothing too fancy.

Command History

Marks: 2

The shell was to be able to store (and recall) the most recent 20 valid commands, using the h or history command.
Command recalling was accessible with !(cmdNo), or !! which recalled the most recent command.

I did have a few disagreements with the assignment spec’s requested implementation of the command history.

cmdNo

From my programming experience, I would much prefer longer code that is modular and easily separable than short code intertwined in a mess of different functionalities. _ironic_

The history ‘module’ exposed the current command number (cmdNo) to the main program. This left the history functionality as an important dependency to the success execution of the program (even its compilation!). A few of my friends were concerned that their initial program code was not working because they had not yet completed the history component.

Of course, history.c isn’t a library, and has a use case very specific to this assignment - but I decided to use cmdNo internally within the history component.

The history command

Something that I wanted to change (but didn’t because it’s an assignment and I gotta follow the specs) was the entries that the h and history commands displayed.

Consider a case where twenty arbitrary valid commands were executed.
Upon typing history we would see

   1  aaa
   2  bbb
   3  ccc
   4  ddd
  ..  ...
  18  rrr
  19  sss
  20  ttt

Cool, so we can use !1 to !20 right!?

Wrong.

!2 would work,
!3 would work,
!20 would work,
But !1 won’t work.

history also counts as a valid command, and unfortunately, the command history is displayed prior to this command being added into the history itself.

Our command history, after typing in history now actually becomes something like this

   2  bbb
   3  ccc
   4  ddd
  ..  ...
  18  rrr
  19  sss
  20  ttt
  21  history

This means that after we perform history (cmdNo 21), we have the recall range from !2 to !21.

It’s counter-intuitive to show the cmdNo 1 if we can’t use it, and it’s easily fixable by adding the h and history command to the history prior to its execution, however, we got to follow the assignment spec!

Builtins

Marks: 1

The builtin commands to be implemented were h, history, pwd, cd and exit.
Simple!

(really, it was)

Wildcard Expansion

Marks: 2

glob is a pretty cool feature, huh!
It allows you to expand wildcards and aliases like ~ and *.c.

As this functionality would result in more tokens, to implement in C, I basically had to replicate the token list, exploding the wildcards and aliases. In terms of memory management, reminder to free the old tokens!

I/O Redirection

Marks: 3

Input and output redirection was pretty straightforward, just utilising the dup2 function to change the sinks for stdin and stdout.
I originally also implemented the stdout append redirection, but took that feature out incase it broke the autotests

Challenge: Piping

Marks: 0

I did it!
Implementing piping involved having to reimplement the most of my pre-execution procedures.
I had to retokenise the input line by the pipe character, and then tokenise each of those pipe-delimited tokens.
And I had to store them somewhere
and I had to create an array of pipe pairs
and I had to create an array of path-resolved executables
and I had to figure out how to efficiently redirect the inputs and outputs
and I had to get the memory management working

But we did it.

Happy ^_^

Remarks

Overall a quite interesting assignment for students to understand what exactly a shell does.
I’ll stick to bash though :)

(Flex) Time taken to complete: 4 hours 37 minutes. Woo!
Though the challenge part took an additional 3 hours…