create a xv$6$ small shell in c$,$ has to follow this template, only putting code where the comments say. it needs to execute simple commands including cd$,$ implelemt input output redirects, pipes, multi element pipelines and the ; operator which allows commands to run one after another in one line. the hardest part i find is implement cd and the ; operator. #include "kernel$/$types$.$h$"$

#include "user$/$user$.$h$"$

#include "kernel$/$fcntl$.$h$"$

$/*$ Read a line of characters from stdin. $*/$

int getcmd$($char $*$buf$,$ int nbuf$)\{$

$//$ ##### Place your code here

return $0$;

$\}$

$/*$

A recursive function which parses the command

at $*$buf and executes it$.$

$*/$

$\_\_$attribute$\_\_(($noreturn$))$

void run$\_$command$($char $*$buf$,$ int nbuf, int $*$pcp$)\{$

$/*$ Useful data structures and flags. $*/$

char $*$arguments$[10]$;

int numargs $=0$;

$/*$ Word start$/$end $*/$

int ws $=1$;

int we $=0$;

int redirection$\_$left $=0$;

int redirection$\_$right $=0$;

char $*$file$\_$name$\_$l $=0$;

char $*$file$\_$name$\_$r $=0$;

int p$[2]$;

int pipe$\_$cmd $=0$;

int sequence$\_$cmd $=0$;

int i $=0$;

$/*$ Parse the command character by character. $*/$

for $($; i $<$ nbuf; i$++)\{$

$/*$ Parse the current character and set$-$up various flags:

sequence$\_$cmd$,$ redirection, pipe$\_$cmd and similar. $*/$

$/*$ ##### Place your code here. $*/$

if $(!($redirection$\_$left $||$ redirection$\_$right$))\{$

$/*$ No redirection, continue parsing command. $*/$

$//$ Place your code here.

$\}$ else $\{$

$/*$ Redirection command. Capture the file names. $*/$

$//$ ##### Place your code here.

$\}$

$\}$

$/*$

Sequence command. Continue this command in a new process.

Wait for it to complete and execute the command following $\text{'}$;$\text{'}.$

$*/$

if $($sequence$\_$cmd$)\{$

sequence$\_$cmd $=0$;

if $($fork$()!=0)\{$

wait$(0)$;

$//$ ##### Place your code here.

$\}$

$\}$

$/*$

If this is a redirection command,

tie the specified files to std in$/$out$.$

$*/$

if $($redirection$\_$left$)\{$

$//$ ##### Place your code here.

$\}$

if $($redirection$\_$right$)\{$

$//$ ##### Place your code here.

$\}$

$/*$ Parsing done. Execute the command. $*/$

$/*$

If this command is a CD command, write the arguments to the pcp pipe

and exit with $\text{'}2\text{'}$ to tell the parent process about this.

$*/$

if $($strcmp$($arguments$[0],"$cd$")==0)\{$

$//$ ##### Place your code here.

$\}$ else $\{$

$/*$

Pipe command: fork twice. Execute the left hand side directly.

Call run$\_$command recursion for the right side of the pipe.

$*/$

if $($pipe$\_$cmd$)\{$

$//$ ##### Place your code here.

$\}$ else $\{$

$//$ ##### Place your code here.

$\}$

$\}$

exit$(0)$;

$\}$

int main$($void$)\{$

static char buf$[100]$;

int pcp$[2]$;

pipe$($pcp$)$;

$/*$ Read and run input commands. $*/$

while$($getcmd$($buf$,$ sizeof$($buf$))>=0)\{$

if$($fork$()==0)$

run$\_$command$($buf$,100,$ pcp$)$;

$/*$

Check if run$\_$command found this is

a CD command and run it if required.

$*/$

int child$\_$status;

$//$ ##### Place your code here

$\}$

exit$(0)$;

$\}$