#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;
char *infile;
char *outfile;
char *errfile;
char *outcmd;
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;

  outcmd = infile = outfile = errfile = NULL;
  for (n = 0; n < avsize; n++)
    av[n] = NULL;

  a = strtok (line, " \t\r\n");
  for (n = 0; a; n++) {
    if (a[0] == '<')
      infile = a[1] ? a + 1 : strtok (NULL, " \t\r\n");
    else if (a[0] == '>')
      outfile = a[1] ? a + 1 : strtok (NULL, " \t\r\n");
    else if (a[0] == '|') {
      if (!a[1])
	outcmd = strtok (NULL, "");
      else {
	outcmd = a + 1;
	a = strtok (NULL, "");
	while (a > outcmd && !a[-1])
	  *--a = ' ';
      }
    }
    else if (a[0] == '2' && a[1] == '>')
      errfile = a[2] ? a + 2 : strtok (NULL, " \t\r\n");
    else {
      avreserve (n);
      av[n] = a;
    }
    a = strtok (NULL, " \t\r\n");
  }
}

void
doexec (void)
{
  int fd;

  while (outcmd) {
    int pipefds[2];

    if (outfile) {
      fprintf (stderr, "syntax error: > in pipe writer\n");
      exit (1);
    }

    if (pipe (pipefds) < 0) {
      perror ("pipe");
      exit (0);
    }

    switch (fork ()) {
    case -1:
      perror ("fork");
      exit (1);
    case 0:
      if (pipefds[1] != 1) {
	dup2 (pipefds[1], 1);
	close (pipefds[1]);
      }
      close (pipefds[0]);
      outcmd = NULL;
      break;
    default:
      if (pipefds[0] != 0) {
	dup2 (pipefds[0], 0);
	close (pipefds[0]);
      }
      close (pipefds[1]);
      parseline (outcmd);
      if (infile) {
	fprintf (stderr, "syntax error: < in pipe reader\n");
	exit (1);
      }
      break;
    }
  }

  if (infile) {
    if ((fd = open (infile, O_RDONLY)) < 0) {
      perror (infile);
      exit (1);
    }
    if (fd != 0) {
      dup2 (fd, 0);
      close (fd);
    }
  }

  if (outfile) {
    if ((fd = open (outfile, O_WRONLY|O_CREAT|O_TRUNC, 0666)) < 0) {
      perror (outfile);
      exit (1);
    }
    if (fd != 1) {
      dup2 (fd, 1);
      close (fd);
    }
  }

  if (errfile) {
    if ((fd = open (errfile, O_WRONLY|O_CREAT|O_TRUNC, 0666)) < 0) {
      perror (errfile);
      exit (1);
    }
    if (fd != 2) {
      dup2 (fd, 2);
      close (fd);
    }
  }

  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;
    }
  }
}