Start with program shellex.c from figures $8\mathrm{.}22-8\mathrm{.}24$ and create your own shell to

$1.$ Maintain variables using a syntax set abc$=$xyz

$2.$ Report all variables by using the command set with not arguments.

$3.$ Use the value of variable PATH as the path of one additional directory where executable

files may be found.

$4.$ Maintain information on all processes running in the background.

Your new version of shellex.c must maintain variables using a local array of strings abc$=$xyz$.$ It should not rely on the real shell to keep these strings. Which means that at the top of shellex.c there must be a declaration like char $*$vars$[10]$ to have an array that will contain up to $($for example$)10$ strings in the form abc$=$xyz$.$ When the user types a command like set path$=/$home$/$ck$47/$progs your shell should find an empty element in local array vars$[],$ call malloc$()$ and store the string there. When the user types a command to run a program from the disk, say $>$ test arg$1$ arg$2$ your shell should use execve$()$ to find the program in the current directory. If the program in not found, execve$()$ returns error and your shell should insert the PATH value before the filename $($by creating a new string and concatenating$)$ and call execve$()$ with that new string. So$,$ in the above example, either program test is executed or program $/$home$/$ck$47/$progs$/$test is executed.

$/*$ $begin shellmain $*/$

#include "csapp.h$"$

#define MAXARGS $128$

$/*$ function prototypes $*/$

void eval$($char$*$cmdline$)$;

int parseline$($char $*$buf$,$ char $**$argv$)$;

int builtin$\_$command$($char $**$argv$)$;

int main$()$

$\{$

char cmdline$[$MAXLINE$]$; $/*$ command line $*/$

while $(1)\{$

$/*$ read $*/$

printf$(">")$;

Fgets$($cmdline$,$ MAXLINE, stdin$)$;

if $($feof$($stdin$))$

exit$(0)$;

$/*$ evaluate $*/$

eval$($cmdline$)$;

$\}$

$\}$

$/*$ $end shellmain $*/$

$/*$ $begin eval $*/$

$/*$ eval $-$ evaluate a command line $*/$

void eval$($char $*$cmdline$)$

$\{$

char $*$argv$[$MAXARGS$]$; $/*$ argv for execve$()*/$

char buf$[$MAXLINE$]$; $/*$ holds modified command line $*/$

int bg; $/*$ should the job run in bg or fg$?*/$

pid$\_$t pid; $/*$ process id $*/$

strcpy$($buf$,$ cmdline$)$;

bg $=$ parseline$($buf$,$ argv$)$;

if $($argv$[0]==$ NULL$)$

return; $/*$ ignore empty lines $*/$

if $(!$builtin$\_$command$($argv$))\{$

if $(($pid $=$ Fork$())==0)\{/*$ child runs user job $*/$

if $($execve$($argv$[0],$ argv, environ$)<0)\{$

printf$("\%$s: Command not found.

$",$ argv$[0])$;

exit$(0)$;

$\}$

$\}$

$/*$ parent waits for foreground job to terminate $*/$

if $(!$bg$)\{$

int status;

if $($waitpid$($pid$,$ &status, $0)<0)$

unix$\_$error$("$waitfg: waitpid error"$)$;

$\}$

else

printf$("\%$d $\%$s$",$ pid, cmdline$)$;

$\}$

return;

$\}$

$/*$ if first arg is a builtin command, run it and return true $*/$

int builtin$\_$command$($char $**$argv$)$

$\{$

if $(!$strcmp$($argv$[0],$ "quit"$))/*$ quit command $*/$

exit$(0)$;

if $(!$strcmp$($argv$[0],"$&$"))/*$ ignore singleton & $*/$

return $1$;

return $0$; $/*$ not a builtin command $*/$

$\}$

$/*$ $end eval $*/$

$/*$ $begin parseline $*/$

$/*$ parseline $-$ parse the command line and build the argv array $*/$

int parseline$($char $*$buf$,$ char $**$argv$)$

$\{$

char $*$delim; $/*$ points to first space delimiter $*/$

int argc; $/*$ number of args $*/$

int bg; $/*$ background job? $*/$

buf$[$strlen$($buf$)-1]=\text{'}\text{'}$; $/*$ replace trailing $\text{'}$

$\text{'}$ with space $*/$

while $(*$buf && $(*$buf $==\text{'}\text{'}))/*$ ignore leading spaces $*/$

buf$++$;

$/*$ build the argv list $*/$

argc $=0$;

while $(($delim $=$ strchr$($buf$,\text{'}\text{'})))\{$

argv$[$argc$++]=$ buf;

$*$delim $=\text{'}\backslash 0\text{'}$;

buf $=$ delim $+1$;

while $(*$buf && $(*$buf $==\text{'}\text{'}))/*$ ignore spaces $*/$

buf$++$;

$\}$

argv$[$argc$]=$ NULL;

if $($argc $==0)/*$ ignore blank line $*/$

return $1$;

$/*$ should the job run in the background? $*/$

if $(($bg $=(*$argv$[$argc$-1]==\text{'}$&$\text{'}))!=0)$

argv$[--$argc$]=$ NULL;

return bg;

$\}$

$/*$ $end parseline $*/$