A command interpreter is an operating system program that understands and executes commands entered by a program or a user. If the command interpreter is used interactively, a response is displayed to the user after each command is executed.

Usage of the command interpreter is often indicated by a prompt symbol and a blinking cursor. When commands are initiated, the command interpreter first checks the syntax to make sure it is valid. If it cannot be validated within an internal operating-system library, an error is displayed. If the command is found in the operating-system library, the command interpreter either executes the command or loads another executable program that can execute the command.

Command interpreters are specific to an operating system. Some are utilities or processes within the operating system, and others are interfaces to the operating system. The command interpreter is also known as the shell in some programming languages.