In ancient times, people would eat “Melilot”-stalks of wheat-which had to be opened to expose the edible kernel inside. It is a problem to eat such things on Shabbat, since separating the chaff from the kernel constitutes the Melacha of “Mefarek” which is a Tolada of “Dash” (threshing). However, Shulhan Aruch (319:6) permits eating such stalks if the separating is done B’shinui-in an unusual manner. For example, he suggests using the tips of the fingers to break it open. Otherwise it would be a problem.
The Rema applies this prohibition to types of nuts that have a soft green shell or peas and chickpeas that grow in a pod. The Mishna Berura also applies it to sesame seeds. All of these would be forbidden to open unless done with a Shinui.
However, it seems clear that the problem is only with the green shell; there is no problem cracking open the hard shell. This is the consensus of many Poskim, including Hacham Ovadia, who permit cracking nuts on Shabbat, since that is the way it is eaten. Rabbi Moshe Feinstein (Russia – New York, 1895-1986) in Iggerot Moshe (OC 1:125), is also lenient, as long as the nuts are eaten right away. The Ben Ish Hai, as well, in Parashat Beshalach (Par. 8) permits peeling pistachios.
However, it should be pointed out that the Menuhat Ahaba (Rabbi Moshe Halevi, Israel, 1961-2001) was Mahmir (strict) and ruled that peeling nuts today is analogous to taking peas out of the pod. Since nowadays most nuts are sold without a shell, removing the shell is not considered an integral part of the act of eating and is a violation of Mefarek, unless done with a Shinui. This is the opinion of the Shevet Halevi, as well. Most authorities disagree with this strict approach, including the Sis Eliezer, Shemirat Shabbat K’hilchata and Hacham Bension.
Thus, there is a difference between taking a pea out of a pod and cracking nuts on Shabbat. It is even permitted to use a nutcracker. When cracking a walnut containing a soft green shell, it may also be removed if done in the same action as cracking the outer shell.
Removing a pea from its pod and a nut from its soft green shell may only be done with a Shinui, but it is permitted to crack the shells of nuts in a regular fashion.