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

char **av;
int avsize;

void
avreserve (int n)
{
  int oldavsize = avsize;

  if (avsize > n + 1)
    return;

  avsize = 2 * (oldavsize + 1);
  if (avsize <= n)
    avsize = n + 1;
  av = realloc (av, avsize * sizeof (*av));
  while (oldavsize < avsize)
    av[oldavsize++] = NULL;
}

void
parseline (char *line)
{
  char *a;
  int n;

  for (n = 0; n < avsize; n++)
    av[n] = NULL;

  a = strtok (line, " \t\r\n");
  for (n = 0; a; n++) {
    avreserve (n);
    av[n] = a;
    a = strtok (NULL, " \t\r\n");
  }
}

void
doexec (void)
{
  execvp (av[0], av);
  perror (av[0]);
  exit (1);
}

int
main (void)
{
  char buf[512];
  char *line;
  int pid;

  avreserve (10);

  for (;;) {
    write (2, "$ ", 2);
    if (!(line = fgets (buf, sizeof (buf), stdin))) {
      write (2, "EOF\n", 4);
      exit (0);
    }
    parseline (line);
    if (!av[0])
      continue;

    switch (pid = fork ()) {
    case -1:
      perror ("fork");
      break;
    case 0:
      doexec ();
      break;
    default:
      waitpid (pid, NULL, 0);
      break;
    }
  }
}