/*
 * set uid and gid, then copy stdin to "output.txt"
 *
 * invoked as:
 *
 *   ./save [--setuidgid uid gid] [--chdir dir]
 *
 * for example:
 *
 *   ./save --setuidgid 3012 3012 --chdir /home/frodo/a3
 *
 * R. Perry, Feb. 2005
 */

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

#define SAVE_FILE "output.txt"

static char *progname; /* my name */

int main( int argc, char *argv[])
{
  uid_t uid;
  gid_t gid;
  gid_t grouplist[1];

  FILE *f;
  char line[BUFSIZ];
  
/* get my name */

  progname = strrchr( argv[0], '/');
  progname = progname ? progname + 1 : argv[0];

/*** Must be the first command-line option: --setuidgid uid gid ***/

    if( argc >= 4 && strcmp( argv[1], "--setuidgid") == 0)
    {
	uid = atoi( argv[2]);
	gid = atoi( argv[3]);

	grouplist[0] = gid;

	if( setgroups( 1, grouplist))
	{
	    fprintf( stderr, "%s: setgroups() failed: %s\n", progname, strerror(errno));
	    exit(1);
	}

	if( setgid( gid))
	{
	    fprintf( stderr, "%s: setgid() failed: %s\n", progname, strerror(errno));
	    exit(1);
	}

	if( setuid( uid))
	{
	    fprintf( stderr, "%s: setuid() failed: %s\n", progname, strerror(errno));
	    exit(1);
	}

	argc -= 3;
	argv += 3;
    }

/*** Must be the first remaining command-line option: --chdir dir ***/

    if( argc >= 3 && strcmp( argv[1], "--chdir") == 0)
    {
	if( chdir( argv[2]))
	{
	    fprintf( stderr, "%s: chdir() failed: %s\n", progname, strerror(errno));
	    exit(1);
	}

	argc -= 2;
	argv += 2;
    }

/* check usage */

  if( argc > 1)
  {
    fprintf( stderr, "Usage: %s [--setuidgid uid gid] [--chdir dir]\n", progname);
    return 1;
  }

/* open output file */

  if( (f = fopen( SAVE_FILE, "w")) == NULL)
  {
    fprintf( stderr, "%s: can't open %s for writing: %s\n", progname, SAVE_FILE, strerror(errno));
    return 1;
  }

/* copy */

  while( fgets( line, BUFSIZ, stdin))
    fputs( line, f);

  fputc( '\n', f); /* ensure final newline */

  fclose( f);

  return 0;
}