Description: An external command is a type of instruction in Unix-like operating systems that is not directly integrated into the shell but is available as an executable file on the system. Unlike internal commands, which are part of the shell itself and run in its context, external commands are independent programs residing in the file system. These commands can be invoked from the shell and typically perform specific tasks, such as file manipulation, process management, or system administration. The ability to use external commands allows users to extend the shell’s functionality and leverage a wide range of tools and utilities available on the system. External commands are fundamental to the flexibility and power of Unix-like environments, as they enable the integration of third-party software and customization of the user’s working environment.
History: The concept of external commands dates back to the early Unix operating systems developed in the 1970s. Unix was designed to be a modular system where tools could be developed and maintained independently. This allowed external commands to become an integral part of the Unix ecosystem, facilitating the creation of scripts and automation of tasks. As Unix-like systems evolved, so did external commands, with the introduction of new utilities and tools that expanded the system’s capabilities.
Uses: External commands are used to perform a variety of tasks in Unix-like systems, such as file manipulation (e.g., ‘cp’ for copying files), process management (like ‘ps’ for listing running processes), and system administration (e.g., ‘df’ for displaying disk usage). They are also essential for script creation, where multiple external commands can be chained together to automate complex tasks.
Examples: Examples of external commands include ‘ls’ for listing files in a directory, ‘grep’ for searching text within files, and ‘tar’ for archiving and compressing files. These commands are invoked from the shell and can be used in combination with other commands to perform more complex tasks.
