A simple Linux shell: Your shell should repeatedly display a prompt and allow a user to enter a command to run. Your shell is supposed to read the input from system standard input, parse the line with command and arguments, and execute. You must only use fork$()$ and execv$()$ system calls to support the shell command execution.

Your shell should find the internal commands first $($described below$).$ If not found, it should search the directories in the shell's pathname environment.

You are not allowed to use system$(),$ as it invokes the system's $/$bin$/$sh shell. You cannot use execlp$()$ or execvp$(),$ because your shell has its own pathname variable.

By convention, the command arguments are separated by white spaces. Your shell does not need to handle special characters, such as $<,>,|,$ &$,$ and so on$.$

Internal Commands: Implement the following $3$ internal commands $($running in the main process rather than the child process$).$

cd: is a command, obviously, to change directories. You are required to use chdir$()$ system call.

path: is not only a command to show the current pathname variable content $($if no argument is provided$),$ but also a utility to modify $($either add or remove$)$ the pathname variable content.

path $+/$abc$/$def should append the pathname $"/$abc$/$def$"$ to the pathname variable, which is a string. E$.$g$.,"/$bin:$/$sbin:$/$abc$/$def$"($where $\text{'}$:$\text{'}$ is a delimiter$).$

path $-/$abc$/$def should remove the pathname $"/$abc$/$def$"$ from the existing pathname variable.

quit: is a command to leave the shell.

Give the above path command, you are NOT ALLOWED to use following functions to access a file name or obtain the pathname information: access$(),$ getenv$(),$ setenv$(),$ etc.

A string parser: A $($string$)$ parser is a function that breaks an input string into parts, such as commands and parameters, which can be managed by other programming components. In this project, you are highly suggested to implement a string parser. The functionality and the structure of your string parser should be organized in the following steps.

Keyboard input: you are required to use fgets$($buf$,256,$ stdin$)$ library function to take the keyboard input, where buf is a char pointer $($to be declared$),256$ is the command length limit$,$ and stdin the system standard input.

Parsing: given a command string stored in buf, suppose the content can be represented as $"$cmd arg$1$ arg$2$ arg$3...".$

your parser first breaks the string into a string array, for example token$[].$ The size of token$[]$ should always be the number of words plus one. For example, given a command with $3$ argument, your token$[]$ size should be $1+3+1=5,$ where token$[4]$ stores NULL.

your parser then checks whether $"$cmd$"($stored in token$[0])$ is an internal command $($as described above$).$ If it is$,$ then you perform this internal command in the parent process.

If it is not an internal command, then you must use fork$()/$execv$()$ pair to arrange $"$cmd$"$ to be executed in a child process. If $"$cmd$"$ cannot be found $($e$.$g$.,$ due to a typo$),$ you need to output corresponding error message to the screen.