Bourne Shell Scripting/Comparing Shells

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Almost all books like this one have a section on (or very similar to) "why you should use the shell/program flavor/language/etc. discussed in this book and not any of the others that perform the same tasks in a slightly different way". It seems to be pretty well mandatory.

However, this book will not do that. We'll talk a bit about "why Bourne Shell" of course. But you'll soon see that doesn't preclude other shells at all. And there's no good reason not to use another shell either, as we will explain a little further down.

Bourne shell and other Unix command shells[edit]

There are many Unix command shells available today. Bourne Shell is just one drop in a very large ocean. How do all these shells relate? Do they do the same things? Is one better than the other? Let's take a look at what makes a shell and what that means for all the different shells out there.

How it all got started...[edit]

The Unix operating system has had a unique outlook on the world ever since it was created back in the 1970s. It stands apart from most other operating systems in that its focus has always been towards power users: people who want to squeeze every drop of performance out of their system and have the technical knowledge to do so. Unix was designed to be programmed and modified to the desires of the user. At its core, Unix does not have a user interface; instead it is comprised of a stable OS kernel and a versatile C library. If you're not trying to do actual hard-core programming but rather are trying to do day-to-day tasks (or even just want to put a little program together quickly), pure Unix is a tremendous pain in the backside.

In other words, it was clear from the start that a tool would be needed that would allow a user to make use of the functions offered him by the coding library and kernel without actually doing serious programming work. A tool in other words that could pass small commands on to the lower-level system quickly and easily. Without the need for a compiler or other fancy editor, but with the ability to tap into the enormous power of the underlying system. Stephen Bourne set himself to the task and came up with what he called a shell: a small, on-the-fly compiler that could take one command at a time, translate it into the sequence of bits understood by the machine and have that command carried out. We now call this type of program an interpreter, but at the time, the term "shell" was much more common (since it was a shell over the underlying system for the user). Stephen's shell was slim, fast, and though a bit unwieldy at times, its power is still the envy of many current operating system command-line interfaces today. Since it was designed by Stephen Bourne, this shell is called the Bourne Shell. The executable is simply called sh and use of this shell in scripting is still so ubiquitous, there isn't a Unix-based system on this earth that doesn't offer a shell whose executable can be reached under the name sh.

...And how it ended up[edit]

Of course, everyone's a critic. The Bourne Shell saw tremendous use (indeed, it still does) and as a result, it became the de facto standard among Unix shells. But all sorts of people almost immediately (as well as with use) wanted new features in the shell, or a more familiar way of expressing commands, or something else. Many people built new shells that they felt continued where Bourne Shell ended. Some were completely compatible with Bourne Shell, others were less so. Some became famous, others flopped. But pretty much all of them look fondly upon Bourne Shell, the shell they call "Dad..."

A number of these shells can be run in sh-like mode, to more closely emulate that very first sh, though most people tend just to run their shells in the default mode, which provides more power than the minimum sh.

It's Bourne Shell, but not as we know it....[edit]

So there are a lot of shells around but you can find Bourne Shell everywhere, right? Good old sh, just sitting there faithfully until the end of time....

Well, no, not really. Most of the sh exectuables out there nowadays aren't really the Bourne Shell anymore. Through a bit of Unix magic called a link (which allows one file to masquerade as another) the sh executable you find on any Unix system is likely actually to be one of the shells that is based on the Bourne shell. One of the most frequently used shells nowadays (with the ascent of free and open-source operating systems like Linux and FreeBSD) is a heavily extended form of the Bourne Shell produced by the Free Software Foundation, called Bash. Bash hasn't forgotten its roots, though: it stands for the Bourne Again SHell.

Another example of a descendant shell standing in for its ancestor is the Korn Shell (ksh). Also an extension shell, it is completely compatible with sh -- it simply adds some features. Much the same is true for zsh.

Finally, a slightly different category is formed by the C Shell (csh) and its descendant tcsh. These shells do break compatibility to some extent, using different syntax for many commands. Systems that use these shells as standard shells often provide a real Bourne Shell executable to run generic Bourne Shell scripts.

Having read the above, you will understand why this book doesn't have to convince you to use Bourne Shell instead of any other shell: in most cases, there's no noticeable difference. Bourne Shell and its legacy have become so ingrained in the heart and soul of the Unix environment that you are using Bourne Shell when you are using practically any shell available to you.

Why Bourne Shell[edit]

So only one real question remains: now that you find yourself on your own, cozy slice of a Unix system, with your own shell and all its capabilities, is there any real reason to use Bourne Shell rather than using the whole range of your shell's capabilities?

Well, it depends. Probably, there isn't. For the most part of course, you are using Bourne Shell by using the whole potential of your shell -- your shell is probably that similar to the Bourne Shell. But there is one thing you might want to keep in mind: someday, you might want to write a script that you might want to pass around to other people. Of course you can write your script using the full range of options that your shell offers you; but then it might not work on another machine with another shell. This is where the role of Bourne Shell as the Latin of Unix command shells comes in -- and also where it is useful to know how to write scripts targeted specifically at the Bourne Shell. If you write your scripts for the Bourne Shell and nothing but the Bourne Shell, chances are far better than equal that your script will run straight out of the mail attachment (don't tell me you're still using boxes to ship things -- come on, get with the program) on any command shell out there.