Can someone help me with the $//$ TO$-$DO P$5\mathrm{.}3$ section of my program. Instructions: $3.$ Modify this project so that $\text{'}!$NN$\text{'}$ re$-$executes the n$\text{'}$th command entered. You can assume that NN will only be tested with values $1$ through $9,$ no more than $9$ values will be entered. If it goes over $9,$ loop back to $1($use a circular queue structure$)$ Shell$(...)1>$ ls Shell$(...)2>!1$ # re$-$executes ls Shell$(...)3>!2$ # re$-$executes ls Shell$(...)4>!4$ # prints "Not valid" to stderr Shell$(...)4>!5$ # prints "Not valid" to stderr Test: $./$shell $-$test rerun $/*$ SMP$1$: Simple Shell $*//*$ LIBRARY SECTION $*/$ #include $/*$ Character types $*/$ #include $/*$ Standard buffered input$/$output $*/$ #include $/*$ Standard library functions $*/$ #include $/*$ String operations $*/$ #include $/*$ Data types $*/$ #include $/*$ Declarations for waiting $*/$ #include $/*$ Standard symbolic constants and types $*/$ #include $"$smp$1\_$tests.h$"/*$ Built$-$in test system $*//*$ DEFINE SECTION $*/$ #define SHELL$\_$BUFFER$\_$SIZE $256/*$ Size of the Shell input buffer $*/$ #define SHELL$\_$MAX$\_$ARGS $8/*$ Maximum number of arguments parsed $*//*$ VARIABLE SECTION $*/$ enum STATE$\_$SPACE, STATE$\_$NON$\_$SPACE ; $/*$ Parser states $*/$ int counter $=1$; int imthechild$($const char $*$path$\_$to$\_$exec, char $*$const args$[])//$ TO$-$DO P$5\mathrm{.}1//$ Split the command into its name and path components char $*$command$\_$name $=$ strrchr$($path$\_$to$\_$exec, $\text{'}/\text{'})+1$; char $*$path $=$ strtok$($getenv$("$PATH$"),"$:$")$; int status $=-1$; $//$ Loop through each directory in the path while $($path $!=$ NULL$)//$ Construct the full path to the executable file char file$\_$path$[256]$; snprintf$($file$\_$path, sizeof$($file$\_$path$),"\%$s$/\%$s$",$ path, command$\_$name$)$; $//$ Try to execute the file execv$($file$\_$path, args$)$; $//$ If execv returns, it means the file could not be executed $//$ using this path, so try the next one path $=$ strtok$($NULL$,"$:$")$; $//$ If we get here, the command could not be executed using any path return $-1$; void imtheparent$($pid$\_$t child$\_$pid, int run$\_$in$\_$background$)$ int child$\_$return$\_$val, child$\_$error$\_$code; $/*$ fork returned a positive pid so we are the parent $*/$ fprintf$($stderr$,"$ Parent says 'child process has been forked with pid$=\%$d$\text{'}",$ child$\_$pid$)$; if $($run$\_$in$\_$background$)$ fprintf$($stderr$,"$ Parent says 'run$\_$in$\_$background$=1...$ so we're not waiting for the child'"$)$; return; $//$ TO$-$DO P$5\mathrm{.}4$ waitpid$($child$\_$pid, child$\_$return$\_$val,$0)$; $/*$ Use the WEXITSTATUS to extract the status code from the return value $*/$ child$\_$error$\_$code $=$ WEXITSTATUS$($child$\_$return$\_$val$)$; fprintf$($stderr$,"$ Parent says 'wait$()$ returned so the child with pid$=\%$d is finished.$\text{'}",$ child$\_$pid$)$; if $($child$\_$error$\_$code $!=0)/*$ Error: Child process failed. Most likely a failed exec $*/$ fprintf$($stderr$,"$ Parent says 'Child process $\%$d failed with code $\%$d$\text{'}",$ child$\_$pid, child$\_$error$\_$code$)$; $/*$ MAIN PROCEDURE SECTION $*/$ int main$($int argc, char $**$argv$)$ pid$\_$t shell$\_$pid, pid$\_$from$\_$fork; int n$\_$read, i$,$ exec$\_$argc, parser$\_$state, run$\_$in$\_$background; $/*$ buffer: The Shell's input buffer. $*/$ char buffer$[$SHELL$\_$BUFFER$\_$SIZE$]$; $/*$ exec$\_$argv: Arguments passed to exec call including NULL terminator. $*/$ char $*$exec$\_$argv$[$SHELL$\_$MAX$\_$ARGS $+1]$; $//$ TO$-$DO new variables for P$5\mathrm{.}2,$ P$5\mathrm{.}3,$ P$5\mathrm{.}6/*$ Entrypoint for the testrunner program $*/$ if $($argc $>1!$strcmp$($argv$[1],"-$test"$))$ return run$\_$smp$1\_$tests$($argc $-1,$ argv $+1)$; $/*$ Allow the Shell prompt to display the pid of this process $*/$ shell$\_$pid $=$ getpid$()$; while $(1)//...$ if $($exec$\_$argc $>0)$ counter$++$; $//$ Print shell prompt fprintf$($stdout$,$ "Shell$($pid$=\%$d$)\%$d$>",$ shell$\_$pid, counter$)$; fflush$($stdout$)$; $//.../*$ Read a line of input. $*/$ if $($fgets$($buffer$,$ SHELL$\_$BUFFER$\_$SIZE, stdin$)==$ NULL$)$ return EXIT$\_$SUCCESS; n$\_$read $=$ strlen$($buffer$)$; run$\_$in$\_$background $=$ n$\_$read $>2$ buffer$[$n$\_$read $-2]==\text{'}\text{'}$; buffer$[$n$\_$read $-$ run$\_$in$\_$background $-1]=\text{'}\text{'}$; $//$ TO$-$DO P$5\mathrm{.}3/*$ Parse the arguments: the first argument is the file or command $**$ we want to run. $*/$ parser$\_$state $=$ STATE$\_$SPACE; for $($exec$\_$argc $=0,$ i $=0$; $($buffer$[$i$]!=\text{'}\text{'})($exec$\_$argc $<$ SHELL$\_$MAX$\_$ARGS$)$; i$++)$ if $(!$isspace$($buffer$[$i$]))$ if $($parser$\_$state $==$ STATE$\_$SPACE$)$ exec$\_$argv$[$exec$\_$argc$++]=$ buffer$[$i$]$; parser$\_$state $=$ STATE$\_$NON$\_$SPACE; else buffer$[$i$]=\text{'}\text{'}$; parser$\_$state $=$ STATE$\_$SPACE; $/*$ run$\_$in$\_$background is $1$ if the input line's last character $**$ is an ampersand $($indicating background execution$).*/$ buffer$[$i$]=\text{'}\text{'}$; $/*$ Terminate input, overwriting the $\text{'}\text{'}$ if it exists $*//*$ If no command was given $($empty line$)$ the Shell just prints the prompt again $*/$ if $(!$exec$\_$argc$)$ continue; $/*$ Terminate the list of exec parameters with NULL $*/$ exec$\_$argv$[$exec$\_$argc$]=$ NULL; $/*$ If Shell runs 'exit' it exits the program. $*/$ if $(!$strcmp$($exec$\_$argv$[0],$ "exit"$))$ printf$("$Exiting process $\%$d$",$ shell$\_$pid$)$; return EXIT$\_$SUCCESS; $/*$ End Shell program $*/$ else if $(!$strcmp$($exec$\_$argv$[0],"$cd$")$ exec$\_$argc $>1)/*$ Running $\text{'}$cd$\text{'}$ changes the Shell's working directory. $*//*$ Alternative: try chdir inside a forked child: if$($fork$()==0)*/$ if $($chdir$($exec$\_$argv$[1]))/*$ Error: change directory failed $*/$ fprintf$($stderr$,"$cd: failed to chdir $\%$s$",$ exec$\_$argv$[1])$; $/*$ End alternative: exit$($EXIT$\_$SUCCESS$)$; $*/$ else $/*$ Execute Commands $*//*$ Try replacing 'fork$()\text{'}$ with $\text{'}0\text{'}.$ What happens? $*/$ pid$\_$from$\_$fork $=$ fork$()$; if $($pid$\_$from$\_$fork $<0)/*$ Error: fork$()$ failed. Unlikely, but possible $($e$.$g$.$ OS $**$ kernel runs out of memory or process descriptors$).*/$ fprintf$($stderr$,$ "fork failed"$)$; continue; if $($pid$\_$from$\_$fork $==0)//$ TO$-$DO P$5\mathrm{.}6$ return imthechild$($exec$\_$argv$[0],$ exec$\_$argv$[0])$; $/*$ Exit from main. $*/$ else imtheparent$($pid$\_$from$\_$fork, run$\_$in$\_$background$)$; $/*$ Parent will continue around the loop. $*//*$ end if $*//*$ end while loop $*/$ return EXIT$\_$SUCCESS; $/*$ end main$()*/$