Question

The original code using the gets() function is written below. You need to do (a) change the provided code so that you now use fgets() function to obtain input from the user instead of gets(), (b) make any other necessary changes in the code because of using fgets() function, and (c) fill in the code for the execute() function so that the whole program works as expected (a simple shell program).

Note: part c is already done, and the execute function is already filled, just need to do part a and b.

CODE BELOW:

/* The program asks for UNIX commands to be typed and inputted to a string*/
/* The string is then "parsed" by locating blanks etc. */
/* Each command and corresponding arguments are put in a args array */
/* execvp is called to execute these commands in child process */

#include
#include
#include
#include
#include

void parse(char *buf, char **args);
void execute(char **args);
char *gets(char *s);

int main(int argc, char *argv[])
{
    char buf[1024];
    char *args[64];

    for (;;) {
        /*
         * Prompt for and read a command.
         */
        printf("Command: ");

        if (gets(buf) == NULL) {
            printf("\n");
            exit(0);
        }

        /*
         * Split the string into arguments.
         */
        parse(buf, args);

        /*
         * Execute the command.
         */
        execute(args);
    }
}

/*
* parse--split the command in buf into
*         individual arguments.
*/
void parse(char *buf, char **args)
{
    while (*buf != '\0') {
        /*
         * Strip whitespace. Use nulls, so
         * that the previous argument is terminated
         * automatically.
         */
        while ((*buf == ' ') || (*buf == '\t'))
            *buf++ = '\0';

        /*
         * Save the argument.
         */
        *args++ = buf;

        /*
         * Skip over the argument.
         */
        while ((*buf != '\0') && (*buf != ' ') && (*buf != '\t'))
            buf++;
    }

    *args = NULL;
}

/*
* execute--spawn a child process and execute
*           the program.
*/
void execute(char **args)

{
//fill in your code here
   pid_t pid;
   switch (pid = fork()) {
       case -1:
           perror("error");

       case 0:
           execvp(args[0], args);
           perror(*args);
           exit(1);
   }

Answer 1

In the code you have posted, the #include lines are clipped off. So I could not get the different files that are included. I did include them looking at code. So the order may be differnt from what you have in your original code. Just make sure you have <string.h> also for the new code to work. If you are not supposed to use string.h , let me know.

Modified code-
------------

/* The program asks for UNIX commands to be typed and inputted to a string*/
/* The string is then "parsed" by locating blanks etc. */
/* Each command and corresponding arguments are put in a args array */
/* execvp is called to execute these commands in child process */

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <wait.h>

void parse(char *buf, char **args);
void execute(char **args);

int main(int argc, char *argv[])
{
char buf[1024];
char *args[64];
int n = 0;
for (;;) {
/*
* Prompt for and read a command.
*/
printf("Command: ");
  
if (fgets(buf, 1024, stdin) == NULL) {
printf("\n");
exit(0);
}
  
   while(buf[n] != '\n')
       n++;
     
   buf[n] = '\0'; //remove the newline character at end
  
/*
* Split the string into arguments.
*/
parse(buf, args);

/*
* Execute the command.
*/
execute(args);
}
}

/*
* parse--split the command in buf into
* individual arguments.
*/
void parse(char *buf, char **args)
{
while (*buf != '\0') {
/*
* Strip whitespace. Use nulls, so
* that the previous argument is terminated
* automatically.
*/
while ((*buf == ' ') || (*buf == '\t'))
*buf++ = '\0';

/*
* Save the argument.
*/
*args++ = buf;

/*
* Skip over the argument.
*/
while ((*buf != '\0') && (*buf != ' ') && (*buf != '\t'))
buf++;
}

*args = NULL;
}

/*
* execute--spawn a child process and execute
* the program.
*/
void execute(char **args)

{
//fill in your code here
pid_t pid;
switch (pid = fork()) {
case -1:
perror("error");

case 0:
execvp(args[0], args);
perror(*args);
exit(1);
}
}