exec command arg1 arg2 ...[< input]
      The exec command treats its command argument as the
      name of a program to execute.  It searches the
      directories in the PATH environment variable to find an
      executable file by the name command, then executes the
      file, passing it an argument list consisting of command
      plus all of the args.  If an argument < appears
      anywhere among the arguments to exec, then neither it
      or the following argument is passed to command.
      Instead, the following argument (input) consists of
      input to the command;  exec will create a pipe and use
      it to pass input to command as standard input.  Exec
      also creates a pipe to receive command's output (both
      standard output and standard error).  The information
      received over this pipe is returned as the result of
      the exec command.  The exec command also looks at the
      return status returned by command.  Normally this
      should be zero; if it is then exec returns normally.
      If command returns a non-zero status, then exec will
      return that code;  it should be one of the ones defined
      in the section ``COMMAND RESULTS'' above.  If an out-of
      range code is returned by the command, it will cause
      command unwinding just as if TCL_ERROR had been
      returned; at the outermost level of command
      interpretation, the Tcl interpreter will turn the code
      into TCL_ERROR, with an appropriate error message.