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/*********************************************************************************
 * myshell.c, primitive core shall wrapper for Unix
 * Copyright (C) 2009 Konstantin Ignatiev, ignatiev@users.sourceforge.net
 *
 * This program is distributed under the terms of GNU General Public License,
 * version 2, as published by the Free Software Foundation.
 * See <URL:http://www.fsf.org/licenses/gpl.txt>
 *
 * ********** Informal subversion log *******************************************
 * 138 2009-10-22 Minor improvements
 * 137 2009-10-22 Initial version
 * *******************************************************************************
 */

/* On Debian, make sure you have package libreadline5-dev installed and do this:
 * gcc -g --pedantic -Wall -DRL -o myshell myshell.c -lreadline
 */

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

#ifdef RL
#include <readline/readline.h>
#include <readline/history.h>
#endif /* defined(RL) */

#define PATH_SEPARATOR ':'  

static char ** split_by_words ( char cmd[] );
static char * get_executable ( char path[], char cmd[] ) ;
static int my_gets (const char prompt[], char line[], int n_alloc);
static char * find_in_path ( char path[], char name[] ) ;

int main ( int argc, char * argv[] ) 
{
   int			ii, n_read, status;
   pid_t		childpid;
   char			cmd[512], ** args, *executable;

   printf ( "MyShell interface, compiled %s%s\n",
	    __DATE__,
#ifdef RL
	    ", with GNU readline"
#else
	    ""
#endif
      );

   if (getuid() == geteuid())
      printf ("UID = %d (both \"real\" and \"effective\", so setuid mode)\n",
	      getuid() );
   else
      printf ("setuid mode active: \"real\" UID = %d, \"effective\" UID = %d\n",
	      getuid(), geteuid());
	 
   printf ("Use Ctrl-D or command \"quit\" to quit\n" );
   
   while ( 1 ) 
   {
      n_read = my_gets ("> ", cmd, sizeof cmd - 1);

      if (n_read < 0 || strcmp(cmd, "quit") == 0) 
      {
	 printf ( "%sBye!\n", (n_read < 0)?"^D\n":"" );
	 return 0;
      }
      args = split_by_words ( cmd );
      if (NULL == (executable = get_executable (getenv("PATH"), args[0])))
	 continue;
      printf ( "%s(", executable );
      for (ii = 1; args[ii] != NULL; ii ++ )
	 printf ( "%s%s", args[ii], (args[ii + 1] == NULL)?"":"," );
      printf ( ")\n" );
      if (0 == (childpid = fork())) 
      {
	 /* CHILD process */
	 status = execv(executable, args); /* !!! args[0] passed as EXE name */
	 fprintf ( stderr, "execv() failed with code %d\n", status );
	 exit(-1);
      }
      else if (childpid > 0) 
      {
	 /* PARENT, OK */
	 waitpid (childpid, &status, 0);

	 printf ("Child process %d finished with status %d\n", childpid, status);
      }
      else 
      {
	 fprintf ( stderr, "Cannot fork()\n" );
      }
	 
      for (ii = 0; args[ii] != NULL; ii ++ )
	 free (args[ii]);
   }
}

static char * get_executable ( char path[], char cmd[] ) 
{
   struct stat		st;
   char			* exe;
   if (strchr(cmd, '/') == NULL) 
   {
      exe = find_in_path (path, cmd);
      if (!exe) 
      {
	 fprintf ( stderr, "\"%s\" not found in $PATH\n", cmd );
	 return NULL;
      }
   }
   else
      exe = cmd;

   if (stat(exe, &st) != 0) 
   {
      fprintf ( stderr, "No such file \"%s\"\n", exe );
      return NULL;
   }

   if ((st.st_mode & S_IFMT) != S_IFREG)
   {
      fprintf ( stderr, "Not a plain file \"%s\"\n", exe );
      return NULL;
   }

   if ((st.st_mode & S_IXUSR) == 0 &&
       (st.st_mode & S_IXGRP) == 0 &&
       (st.st_mode & S_IXOTH) == 0    )
   {
      fprintf ( stderr, "No execution permission \"%s\"\n", exe );
      return NULL;
   }
      
   return exe;
}

static char * find_in_path ( char path[], char name[] ) 
{
   int		ii, idx = 0;
   struct stat  st;
   char		test_name[512];
   
   while (path[idx] != '\0') 
   {
      for (; path[idx] == PATH_SEPARATOR; idx ++ );
      if (path[idx] != '\0') 
      {
	 for (ii = idx + 1; path[ii] != PATH_SEPARATOR && path[ii] != '\0'; ii ++);

	 strncpy (test_name, path + idx, ii - idx);
	 test_name[ii - idx] = '/';
	 strcpy ( test_name + ii - idx + 1, name );

	 if (0)
	    printf ( "Trying '%s'\n", test_name );
	 
	 if (stat(test_name,&st) == 0)
	    return strdup(test_name);
      }
      idx = ii;
   }
   return NULL;
}

static char ** split_by_words ( char cmd[] ) 
{
   int		idx, n, ii;
   static char	* words[512];
   
   idx = n = 0;

   while ( cmd[idx] != '\0' ) 
   {
      for (; isspace(cmd[idx]); idx ++ );
      
      if (cmd[idx] == '"') 
      {
	 for (ii = idx + 1; cmd[ii] != '\0' && cmd[ii] != '"'; ii ++ );
	 if (cmd[ii] != '"') 
	 {
	    fprintf ( stderr, "Cannot find matching quote to one in position %d\n", idx + 1 );
	    exit(-1);
	 }
	 cmd[ii] = '\0';
	 words[n ++] = strdup(cmd + idx + 1);
	 idx = ii + 1;
      }
      else if (cmd[idx] != '\0') 
      {
	 for(ii = idx + 1; !isspace(cmd[ii]) && cmd[ii] != '\0'; ii ++ );
	 cmd[ii] = '\0';
	 words[n ++] = strdup(cmd + idx);
	 idx = ii + 1;
      }
   }
   
   words[n] = NULL;

   return words;
}

static int my_gets (const char prompt[], char line[], int n_alloc)
{
   int			n_in;
#ifdef RL
   char			* rl_in;
   rl_in = readline (prompt);   
   if (!rl_in)
      return -1;
   n_in = strlen(rl_in);
   strncpy ( line, rl_in, n_alloc);
   if (rl_in && rl_in[0])
      add_history (rl_in);
   free (rl_in);
#else
   printf ( "%s", prompt );
   if (NULL == fgets (line, n_alloc, stdin))
      return -1;
   n_in = strlen(line) - 1;
   line[n_in] = '\0';
#endif
   return n_in;
}