In this lab, you will demonstrate your knowledge of process creation and manipulation. The

objective of the lab is for you to create your own "shell". When you log into Loki, you are

running the bash shell. It will recognize a number of different "verbs" and using new processes,

will run those commands and display their output on your screen.

Your solution for lab #$4$ will be to create a C program which will accept a number of fixed verb

names of your choosing. These will be replacements for variations of normal bash commands.

Your program is to recognize the verb requested and create a process to run the equivalent bash

command. To create the process you MUST use the fork$()$ function and have the parent process

wait$()$ for it to complete.

Within the child process you MUST use one of the exec??$()$ family of system calls to execute the

required bash verb with your selected parameters. You are not allowed to call aliases or functions

defined in your $.$bashrc $($ask me why!$).$

As an example, in my version I have created a verb called "jobtree". When I run my code and

type in "jobtree" at the prompt, it will produce the equivalent to the shell command:

ps $-$o user:$32,$pid,stime,tty$,$cmd $-$U jacques $--$forest

When I run jobtree in my shell, the C code will run the bash ps command followed by $5$

parameters: $"-$o$","$ user:$32,$pid,stime,tty$,$cmd$","-$U$",$ "jacques", $"--$forest".

I strongly suggest that you get your commands working$/$tested in bash before you port them into

your shell. It will help with debugging issues when things don't work later on$.$

Don't forget to add in an extra command to exit your shell and return to the bash shell. As an

example I would add an extra verb like "logout" which would "exit" my C code and return to the

original bash process. Of course, logout shouldn't trigger a fork$()$ as this would just create an

extra process and not just exit.

Ensure that you wait$()$ for every child created by fork$()$ or you will be creating zombie processes.

Which, I will have to clean up$.$ They waste system resources so$,$ be vigilant. Do a $"$ps $-$ef $|$ grep $-$e

$USER" before you log out and make sure you aren't leaving any zombies behind.

For this exercise, you are required to create your own verb equivalents. You are required to use

normal bash commands with parameters. Once you have selected $3$ or $4$ commands $($not counting

logout or help$),$ then use exec$()$ function call to execute the code.

Here are the ones I chose:

My shell's verb Bash equivalent

help None but prints out a help menu

diskuse du $-$sch

drives df $-$h $-$t ext$4$

chklog grep $-$e $USER $/$home$/$COIS$/3380/$secure

jobtree ps $-$o "user:$32,$pid,stime,tty$,$cmd$"-$U $USER $--$forest

logout Exits the shell

The help command will print out something like this $($change it to match your verbs$)$:

Valid verbs are:

help: displays this text

lastmod $[$number of days ago$]$ displays files modified on that day

diskuse shows the amount of disk being used in the current

directory.

drives Lists all of the ext$4$ mounted partitions as well as

their size and free space.

chklog $[$username$]$ scans the access log for a specific username.

If not present, it dumps the whole log file.

jobtree $[$username$]$ shows the processes running related to a specific

username.

logout Exits this shell and returns to your original shell.

NOTE: elements in square brackets are optional. Should you

omit them, the current username will be used.

$---$ EOT $---$

In my implementation, if the username is not specified on the appropriate command lines, I used

the getenv$()$ command to retrieve the name of the currently logged in user $($the environment

variable USER$).$ If the "number of days" was not specified, I assumed a $1.$

You are required to hand in to the Lab$4$ drop box on BlackBoard, a well document and modular

program plus sample output in a log file showing the functionality of your program. Remember

to use your script from lab$1$ to zip up your work for submission