Working with the CLI

The Command-Line Interface (CLI) is perhaps the most widely available method for accessing a network device. It is a term imported from computers, which was a replacement for teletypewriter (TTY) machines. A CLI is normally implemented by using a program that runs inside the device to interpret the keys being typed. Early implementations of the CLI program monitored the device’s serial port, where a terminal with a keyboard was connected to communicate.

In UNIX, the CLI program was called a shell, and the first shell, called the V6 shell, was created in 1971 by Ken Thompson at Bell Labs. The Bourne shell was introduced in 1977 as a replacement for the V6 shell. Although the UNIX shell is used as an interactive command interpreter, it was also intended to be a scripting language and contains most of the features that are commonly considered to produce structured programs.

Network devices use a simplified version of a shell for their CLIs. Let’...