1.3. History of UNIX Shells

The independence of the shell from the UNIX operating system per se has led to the development of dozens of shells throughout UNIX history—although only a few have achieved widespread use.

The first major shell was the Bourne shell (named after its inventor, Steven Bourne); it was included in the first popular version of UNIX, Version 7, starting in 1979. The Bourne shell is known on the system as sh. Although UNIX has gone through many, many changes, the Bourne shell is still popular and essentially unchanged. Several UNIX utilities and administration features depend on it.

The first widely used alternative shell was the C shell, or csh. This was written by Bill Joy at the University of California at Berkeley as part of the Berkeley Software Distribution (BSD) version of UNIX that came out a couple of years after Version 7.

The C shell gets its name from the resemblance of its commands to statements in the C Programming Language, which makes the shell easier for programmers on UNIX systems to learn. It supports a number of operating system features (e.g., job control; see Chapter 8) that were unique to BSD UNIX but by now have migrated to most other modern versions. It also has a few important features (e.g., aliases; see Chapter 3) that make it easier to use in general.

In recent years a number of other shells have become popular. The most notable of these is the Korn shell. This shell is a commercial product that incorporates the best features of the Bourne and C shells, plus many features of its own.^[1] The Korn shell is similar to bash in most respects; both have an abundance of features that make them easy to work with. The advantage of bash is that it is free. For further information on the Korn shell see Appendix A.

^[1] The Korn shell can be downloaded for free but it comes with a license that will require payment if the shell is used in certain situations.

1.3.1. The Bourne Again Shell

The Bourne Again shell (named in punning tribute to Steve Bourne's shell) was created for use in the GNU project.^[2] The GNU project was started by Richard Stallman of the Free Software Foundation (FSF) for the purpose of creating a UNIX-compatible operating system and replacing all of the commercial UNIX utilities with freely distributable ones. GNU embodies not only new software utilities, but a new distribution concept: the copyleft. Copylefted software may be freely distributed so long as no restrictions are placed on further distribution (for example, the source code must be made freely available).

^[2] GNU is a recursive acronym, standing for "GNU's Not UNIX."

bash, intended to be the standard shell for the GNU system, was officially "born" on Sunday, January 10, 1988. Brian Fox wrote the original versions of bash and readline and continued to improve the shell up until 1993. Early in 1989 he was joined by Chet Ramey, who was responsible for numerous bug fixes and the inclusion of many useful features. Chet Ramey is now the official maintainer of bash and continues to make further enhancements.

In keeping with the GNU principles, all versions of bash since 0.99 have been freely available from the FSF. bash has found its way onto every major version of UNIX and is rapidly becoming the most popular Bourne shell derivative. It is the standard shell included with Linux, a widely used free UNIX operating system, and Apple's Mac OS X.

In 1995 Chet Ramey began working on a major new release, 2.0, which was released to the public for the first time on December 23, 1996. bash 2.0 added a range of new features to the old release (the one before being 1.14.7) and brought the shell into better compliance with various standards. bash 3.0 improves on the previous version and rounds out the feature list and standards compliance.

This book describes bash 3.0. It is applicable to all previous releases of bash. Any features of the current release that are different in, or missing from, previous releases will be noted in the text.

1.3.2. Features of bash

Although the Bourne shell is still known as the "standard" shell, bash is becoming increasingly popular. In addition to its Bourne shell compatibility, it includes the best features of the C and Korn shells as well as several advantages of its own.

bash's command-line editing modes are the features that tend to attract people to it first. With command-line editing, it's much easier to go back and fix mistakes or modify previous commands than it is with the C shell's history mechanism—and the Bourne shell doesn't let you do this at all.

The other major bash feature that is intended mostly for interactive users is job control. As Chapter 8 explains, job control gives you the ability to stop, start, and pause any number of commands at the same time. This feature was borrowed almost verbatim from the C shell.

The rest of bash's important advantages are meant mainly for shell customizers and programmers. It has many new options and variables for customization, and its programming features have been significantly expanded to include function definition, more control structures, integer arithmetic, advanced I/O control, and more.