Stir together the garam masala, coriander, 3/4 teaspoon salt, 1/2 teaspoon sugar and the cumin. Add the shrimp and toss to coat.
Heat the oil in a large skillet over medium-low heat. Cook the shrimp, working in batches to prevent crowding, until just cooked through, about 2 minutes per side. Transfer to a platter.
Meanwhile, combine the pineapple juice and the remaining 1/2 teaspoon sugar in a medium skillet or saucepan and bring to a boil. Cook until syrupy and reduced to 1/3 cup, about 7 minutes. Stir in the remaining 1/4 teaspoon salt and pour the glaze over the shrimp and serve.