C - passing an unknown command into execvp()

Question

I'm writing a fake shell, where I create a child process and then call execvp(). In the normal shell, when I enter an unknown command such as 'hello' it returns 'hello: Command not found.' However, when I pass hello into execvp(), it doesn't return any error by default and just continues running the rest of my program like nothing happened. What's the easiest way to find out if nothing was actually run? here's my code:

if(fork() == 0)
{
     execvp(cmd, args);
}
else
{
     int status = 0;
     int corpse = wait(&status);
     printf(Child %d exited with a status of %d\n", corpse, status);
}

I know that if corpse < 0, then it's an unknown command, but there are other conditions in my code not listed where I don't want to wait (such as if & is entered at the end of a command). Any suggestions?

Answer 1

All of the exec methods can return -1 if there was an error (errno is set appropriately). You aren't checking the result of execvp so if it fails, the rest of your program will continue executing. You could have something like this to prevent the rest of your program from executing:

if (execvp(cmd, args) == -1)
    exit(EXIT_FAILURE);

You also want to check the result of fork() for <0.

Answer 2

You have two independent concerns.

1) is the return value of execvp. It shouldn't return. If it does there is a problem. Here's what I get execvp'ing a bad command. You don't want to wait if execvp fails. Always check the return values.

int res = execvp(argv[1], argv);
printf ("res is %i %s\n", res, strerror(errno));
// => res is -1 No such file or directory

2) The other concern is background processes and such. That's the job of a shell and you're going to need to figure out when your program should wait immediately and when you want to save the pid from fork and wait on it later.