Shells

Shells are hard, protective outer coverings produced by many animals, especially mollusks such as snails, clams, and nautiluses. They are mainly calcium carbonate in calcite or aragonite, reinforced by an organic matrix. Shells grow as the animal adds material at the opening, guided by the mantle. Structural layers include the periostracum, the prismatic layer, and the nacreous layer. Shells defend against predators and desiccation, provide support for soft bodies, and often show species-specific shapes and colors. They leave a rich fossil record for studying ancient life.

In military usage, a shell is a munition fired from a gun or mortar, containing an explosive or payload. A shell consists of a metal casing, propellant charge, and a fuze to trigger detonation. Variants include high-explosive, armor-piercing, fragmentation, incendiary, and chemical designs. Fuzes may be timed, contact, or proximity types. Modern shells may include specialized payloads, programmable fuzes, or guidance components. The term typically refers to artillery munitions, though some small arms ammunition is also called shells in informal use.

In computing, a shell is a user interface that interprets commands and mediates access to the operating system. Unix-like systems commonly use shells such as Bash, Zsh, or Fish; Windows uses PowerShell and the Command Prompt. Shells provide features including command history, scripting, environment variables, piping, and redirection. Shell scripting enables automation of tasks, configuration, software builds, and maintenance. Although the shell is distinct from the terminal, the terminal provides access to the shell environment and input.

Shells have also long been used by humans for ornament, tools, and currency in various cultures. They appear in art, archaeology, and natural history as fossils that illuminate past ecosystems.