shell

Shell is a term that may refer to several concepts across science, technology, and industry. It commonly denotes a computing command-line interface, a model of atomic and nuclear structure, a hard protective covering produced by living organisms, and the multinational energy company Shell plc.

In computing, a shell is a user interface that enables interaction with an operating system. It interprets commands, provides scripting capabilities, and connects programs through pipes and redirection. Popular examples include Bash and Zsh on Unix-like systems and PowerShell on Windows, with many other variants for different environments.

In chemistry and physics, electron shells are principal energy levels around an atomic nucleus in which electrons reside. Shells are filled in a defined order and are central to models of atomic structure; in the nuclear shell model, protons and neutrons occupy energy levels within the nucleus.

In biology, shells are hard external coverings produced by many mollusks and some turtles and crustaceans. They vary in composition, shape, and structure, from calcium carbonate to conchiolin, and range from spiraled to bivalved forms. They provide protection, support, and, in some cases, buoyancy.

Shell plc, commonly referred to as Shell, is a British multinational energy company and one of the largest oil and gas firms. Formed by the 1907 merger of Royal Dutch Petroleum and the Shell Transport and Trading Company, it operates in exploration, refining, distribution, and energy transition initiatives.