/* minish2.c - example of minimalistic shell
 *
 * script filename may be given as argument, else reads stdin
 * understands simple command language: 
 * (1) leading spaces are ignored
 * (2) leading '#' (also after spaces) means a comment line
 * (3) trailing '&' means background execution
 * (4) one internal command, 'exit' with optional argument for status code
 * (5) external commands, searches PATH (but doesn't allow changing it)
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <sys/wait.h>
#define MAXLINE 300

int main(int argc, char **argv)
{
  char line[MAXLINE+1]; /* command line being executed */
  char *args[MAXLINE];  /* pointers to the beginning of arguments */
  char *p, **arg;       /* pointers for above arrays */
  int bg;               /* indicates command is being backgrounded */
  int pid;              /* PID of child process */

  /* open file argument in place of stdin */
  if (argc>=2 && !freopen(argv[1], "r", stdin)) {
    perror(argv[1]);
    exit(errno);
  }

  /* read lines until EOF */
  while (fgets(line, MAXLINE, stdin)) {

    /* cleanup after possible terminated background processes */
    waitpid(-1, NULL, WNOHANG);

    /* verify we got \n */
    if ((p = strchr(line, '\n'))) {
      *p=0;     /* discard it */
    } else { 
      /* nope, line must've been too long */
      fprintf(stderr, "line too long\n"); 
      exit(1); 
    }

    /* remove possible trailing & and leave bg marking it was there */
    /* note: this does not allow spaces after the & */ 
    if ((bg = (*--p == '&'))) *p=0;

    /* skip leading spaces */
    p = line;
    while (*p == ' ') 
      ++p;

    /* if next char is # the line is comment, skip it */
    if (*p == '#') 
      continue;     

    /* p points now at first word in line, save it in args[0] */
    *(arg=args) = p;           
    /* search spaces */
    while ((p = strchr(p, ' '))) {
      /* clear all adjacent spaces */
      while (*p == ' ') 
	*p++ = 0;
      /* if a non-space follows it's an argument, save */
      /* pointer to it to next element of args */
      if (*p) 
	*++arg=p;
    }
    /* need NULL at end of args for execv */
    *++arg=NULL;

    /* if command is "exit", exit! */
    if (!strcmp(line,"exit")) {
      exit(args[1] ? atoi(args[1]) : 0);
    }

    /* start new process */
    switch ((pid=fork())) {
    case 0:       /* we're in child process now */
      /* try to execute the command */
      execvp(args[0], args); /* won't return if succesful */
      perror(args[0]);
      exit(errno);  
    case -1: 
      /* out of memory or something */
      perror("fork() failed");
      break;
    default: 
      /* wait for the child process to finish, unless */
      /* there was & meaning it should run in the background */
      if (!bg) waitpid(pid, NULL, 0);
    } 
  }

  exit(0);
}
/******************************The End******************************/