Make changes to the following program shellex.c to create your own shell that does the following: 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.

Submit your version of shellex.c with the code for the changes and screenshots of the program running and demonstrating all its features.

/* $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] = ' '; /* replace trailing ' ' with space */

while (*buf && (*buf == ' ')) /* ignore leading spaces */

buf++;

/* build the argv list */

argc = 0;

while ((delim = strchr(buf, ' '))) {

argv[argc++] = buf;

*delim = '\0';

buf = delim + 1;

while (*buf && (*buf == ' ')) /* 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] == '&')) != 0)

argv[--argc] = NULL;

return bg;

}

/* $end parseline */