Question

I have an assignment in which I need to create a unix shell using fork(). I’ve got this working correctly. Now I need to check user input to see if it is a valid unix command. If it is not valid (ie its “1035813”) I need to tell the user to enter a valid command.

Is there a way I can get a list of every possible unix command so I can compare the user input with every string in this list? Or is there an easier way to do this?

Answer 1

The appropriate way to do this is:

1. check if it’s a built-in command in your shell. For example, cd should probably be a built in command.
2. fork and try to exec it. (execvp is probably what you really want, actually). If that fails, check errno to determine why.

Example:

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

int main(int argc, char* argv[])
{
  if (argc != 2) {
    printf("usage: %s <program-to-run>\n", argv[0]);
    return -1;
  }

  char* program      = argv[1];
  /* in this case we aren't passing any arguments to the program */
  char* const args[] = { program, NULL };

  printf("trying to run %s...\n", program);

  pid_t pid = fork();

  if (pid == -1) {
    perror("failed to fork");
    return -1;
  }

  if (pid == 0) {
    /* child */
    if (execvp(program, args) == -1) {
      /* here errno is set.  You can retrieve a message with either
       * perror() or strerror()
       */
      perror(program);
      return -1;
    }
  } else {
    /* parent */
    int status;
    waitpid(pid, &status, 0);
    printf("%s exited with status %d\n", program, WEXITSTATUS(status));
  }

}