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

char prompt[] = "$ ";

static int
Fork()
{
  pid_t pid;

  if ((pid = fork()) < 0)
    error(EXIT_FAILURE, errno, "fork error");
    return(pid);
}

int
main(void)
{
  long MAX = sysconf(_SC_LINE_MAX);
  char buf[MAX];
  pid_t pid;
  int status, n;

  do {
    write(STDOUT_FILENO, prompt, strlen(prompt));
    fflush(NULL);
    memset(buf, 0, MAX);
    if((n = read(STDIN_FILENO, buf, MAX)) == 0) {
      printf("use exit to exit shelln");
      continue;
    }
    buf[strlen(buf) - 1] = ''; // chomp 'n'

    if (strncmp(buf, "exit", MAX) == 0) { // match
      break;
    }
    pid = Fork();
    if (pid == 0) {  // child
      execlp(buf, buf, (char *)NULL);
      error(EXIT_FAILURE, errno, "exec failure");
    }
    // parent
    if ((pid = waitpid(pid, &status, 0)) < 0)
      error(EXIT_FAILURE, errno, "waitpid error");
  } while(1);
  exit(EXIT_SUCCESS);
}