To Make Pineapple Filling: Mix 1 cup crushed pineapple and juice (8 1/4-ounce can), 1 cup sugar, 2 1/2 Tablespoons cornstarch, 1 Tablespoon butter, and 1 cup water in saucepan. Cook until thickened, cool.
Preheat oven to 400 degrees F (200 degrees C).
Cream together shortening and sugar thoroughly. Stir in flour, baking soda and salt. Mix in rolled oats. Press and flatten half of the mixture over bottom of a 13 x 9 1/2 x 2 inch greased pan. Spread with cooled filling.
Top with remaining crumb mixture, patting lightly. Bake 25 to 30 minutes, or until lightly browned. While warm cut into bars and remove from pan.