Before your shell forks a new process to call execvp(), it should parse the input string and separate it into a collection of substrings representing the executable file and any command-line arguments. If the user entered an empty line, report an error and fetch a new line of input. Your code must handle at least four command-line arguments (in addition to the name of the executable file itself).

#define LSH_RL_BUFSIZE 1024

char *lsh_read_line(void)

{

int bufsize = LSH_RL_BUFSIZE;

int position = 0;

char buffer = malloc(sizeof(char) bufsize);

int c;

if (!buffer) {

   fprintf(stderr, "lsh: allocation error\n");

   exit(EXIT_FAILURE);

}

while (1) {

   // Read a character

   c = getchar();

   // If we hit EOF, replace it with a null character and return.

   if (c == EOF || c == '\n') {

     buffer[position] = '\0';

     return buffer;

   } else {

     buffer[position] = c;

   }

   position++;

   // If we have exceeded the buffer, reallocate.

   if (position >= bufsize) {

     bufsize += LSH_RL_BUFSIZE;

     buffer = realloc(buffer, bufsize);

     if (!buffer) {

       fprintf(stderr, "lsh: allocation error\n");

       exit(EXIT_FAILURE);

     }

   }

}

}

Explanation:

answer; defense;