question_8.cc

      //  $Id$
      
      /*  This is a solution to Question 8 on the Final Exam:
       *
       *    Write a complete C++ program that executes a command entered by
       *    the user on the command line.  If there are no command line
       *    arguments, the program is to do nothing, except to exit
       *    normally.  If the command that the user enters fails to execute,
       *    print an appropriate error message ant terminate with an exit
       *    code of 1.  If the command does execute, your program is to
       *    print the exit code from the command before exiting normally.
       *
       *    $Log$
       */
      #include <stdio.h>
      #include <stdlib.h>
      #include <unistd.h>
      #include <wait.h>
      
      //  main()
      //  ------------------------------------------------------------------
      /*
       *    If there are command line arguments, fork a child to execute the
       *    command specified there, print the command's exit code, and
       *    exit.
       *
       *    Arguments:  argc    Number of command line arguments.
       *                argv    Command line arguments.
       *                envp    Environment vector.
       */
      int
      main( int argc, char *argv[], char *envp[] )
      {
        //  Check if there is a command to execute.
        if ( argc < 2 )
          exit( 0 );
      
        //  Fork a process to execute the command.
        pid_t pid = fork();
        switch ( pid )
        {
          case -1:
            perror( "fork" );
            exit( 1 );
            break;
      
          case 0:
      
            //  Child: execute the command.
            execvp( argv[1], &argv[1] );
            perror( "execvp" );
            exit( 1 );
            break;
      
          default:
      
            //  Parent: print child's exit code, or its termination status
            //  if it did not exit.
            int status;
            wait( &status );
            if ( WIFEXITED(status) )
            {
              printf( "%s: exit code was %d\n", 
                                             argv[1], WEXITSTATUS(status) );
              exit( 0 );
            }
            printf( "%s: terminated with status %04X\n", argv[1], status );
            exit( 1 );
        }
      }