Table Of Contents

Previous topic

Learn Shell Scripting

Next topic

Script / Program Organization

This Page

Linux Lives in a Shell

What a Shell Does All Day

Every interactive operating system has a program which does one of two things: it receives user input events and processes them, and/or it interprets and executes commands. A Linux/Unix shell is a system program which interprets and executes commands which may be input at a console, read from a file, or passed through a data stream.

Command shells are not necessarily interpreted programming languages: Microsoft’s COMMAND.COM for MS_DOS certainly is not. But it so happens that Unix terminal shells are examples of structured REPL programming languages (see The History of REPL), and learning shell scripting is a good way to learn computer programming.

How to Collect Shells

Computer operations are easily automated through shell scripts, as this guide will demonstrate for your Linux computer. (What – You don’t have a Linux system? What are you waiting for?) Lacking a Linux computer to practice on, there are on-line alternatives –,, and others – where one can practice programming skills with just an Internet connection. These guides will show how this is done.

Caring for Shells

Unix-like operating systems have experienced a public domain rebirth in Linux, where Linux servers now account for a large majority of all active web servers. Also, Apple’s OS-X operating system for the Macintosh is a Unix, and these computers are quite popular in the programming community. As a result, anyone doing collaborative web development is almost certain to be working with shell programming either on the desktop, or on servers, or both. So, developers care for shells because their livelihood depends on them.

The History of REPL

Once there was a company, AT&T, whose Bell Labs division conducted extensive research into computer operating systems and programming languages. This produced stuff like UNIX, ALGOL, C, PLAN9, and various other computer softwares which are mostly no longer around in their original form, if at all. The AT&T phone service monopoly was divested in 1982, and intellectual property from Bell Labs became redundant over time.

In the stew of Bell Labs projects was a command shell for UNIX, an interpreter to execute commands which might be obtained through user input, or read from a file or a data stream.

The idea of an command interpreter may have come from the LISP programming language. While ALGOL and C are compiled languages, LISP can be interpreted. A LISP interpreter processes each statement in four steps: Read, Eval, Print, and Loop; whence comes the acronym REPL. But, LISP was not a Bell Labs project, so shell programming syntax was ripped from ALGOL, C, and UNIX commands instead. History is replete with mistakes, of which this was one. This one example is enough to justify the open source programming paradigm.