Run flex, bison, and compile by writing the following on the command line:

  make -f Makefile

or simply

  make

To run, write the following on the command line:

  ./stack_machine

Now you can type an expression ended by $ and <Ctrl>-D.
Example (leaving out the <Ctrl>-D):

  3*4-1/x$

You can also write your input on a FILE and run as

  ./stack_machine < FILE