|Beginning Portable Shell Scripting: From Novice to Professional|
|summary||A guide on how to write portable shell scripts.|
It was probably a few weeks after that first, rough introduction that I returned for another round with this strange but somehow seductive tool, armed with a book I'd found and a determination to learn it's secrets. I had no idea then that seventeen years later I'd still be learning new tricks, discovering new features and taking so much pleasure from sharing what I've learned with others. In fact, in those early forays into the realm of shells and scripting, I didn't even really have a strong concept of the separation between the shell and the operating system, so at the time I couldn't have conceived of how much fun I would have in later years discussing and debating the relative strengths and weakness of shells with friends and colleagues, but it is probably my favorite touchstone of computer geek conversation. Discussion of shell features, scripting tricks and semantics almost always result in my learning something new and interesting and having a new tool to add to my collection.
Peter's book, Beginning Portable Shell Scripting, therefore may sound like something intended as a gentle introduction, aimed at the initiate — the sort of text I'd been seeking to carry with me when I first attempted to write what I thought of as "batch files" on that now-ancient UNIX machine — but there's more truth in the subtitle, From Novice to Professional, than one might expect. He writes in an accessible, at times conversational, style and presents detailed technical information alongside a mixture of anecdotes and historical detail that does more than simply serve as a technical reference, it helps the reader understand a great deal about why things are the way they are. It was such an entertaining read that I frequently found myself skipping ahead, reading a section I knew was coming up, then resisting the urge to just keep going from that point. The first of these I encountered on page 18 in which he discusses the relative portability of printf in shell scripts. I knew what he knew, it's clearly non-portable and should be avoided, and thoroughly enjoyed the explanation of how he determined his (and by extension my) assumption was in error. Another on page 108 is the sort of good advice all UNIX users, not just those aiming to write good scripts, should take to heart. Many times, though, I've related precisely the same advice to colleagues to be met with confused stares, so it certainly bears repeating.
This book is a desktop reference in the truest sense of the term for me, it is an interesting, at times laugh-out-loud amusing, discussion of how to write shell scripts that will work on the widest possible range of Bourne-derived and POSIXly correct shells and why this is a desirable goal. In true UNIX tradition, the author doesn't provide simply a set of rules, but guidelines that will help you find your own way through the task of creating portable, maintainable shell scripts.
The real meat of the book begins in Chapter 3 (more on Chapter 2 in a moment) with a discussion of control structures and redirection, the latter being perhaps the defining characteristic of UNIX command line interfaces. I struggled somewhat with trying to decide if redirection would be better discussed after the material on how the shell parses tokens, presented in the first part of Chapter 4, but it does seem that the correct logical grouping is the one presented. It would be easy to get lost, for example, in the semantics of why the same streams of redirection tokens behave differently on different shells, but the key concept in the early chapters is that of many tools, each doing a specific task, working in concert. That objective is achieved quite effectively.
Chapters 5 and 6 go into detail (possibly too much for some, just right in my opinion) on how UNIX executes shells and how shells can spawn other shells, the costs and the benefits and the available alternatives for one to make an informed decision. Frequently there isn't one right answer whether some activity is better done in a script, in a shell function or in a subshell, but the material here will certainly aid in making those determinations. My personal bias being almost always toward writing a shell function — perhaps an indication I've had too much exposure to C programming, perhaps more due to a frugal upbringing and my own sense that spawning a whole new shell to do something is overkill — had me wishing for a larger section on the value of such constructs, but there should be enough there for me to win some converts to my cause.
By far the sections I learned the most from, however, would be Chapter 7: Shell Language Portability and Chapter 8: Utility Portability since I actively avoid exposure to other shells. I have my two preferred options and a third that I will use when presented with no alternative. While this does mean I know "my own" shells very well, it also means that I often bump into the furniture, so to speak, when I find myself using a new shell. These chapters haven't been immediately useful to me, but I know they're the ones that I'll be turning to in the future, I've needed something like them in the not-too-distant past, after all.
The final three chapters assemble the information presented in the earlier sections and suggest a sort of "best practices" approach to writing scripts. Concepts like "degrade gracefully" seem like pretty fundamental ideas when you hear them but I frequently find myself writing functions or scripts that don't do that at all when intended for a limited, usually singular, audience. It may seem like an okay idea when you're doing something for your own use, but when you write a complex function that works then discover a bug in it two or three years late and you have to return to fix it, it can be just as helpful for it to simply fail in an informative way as it would be to have detailed comments explaining the intent and the mechanics.
Truly, there's something here for everyone. In my office I'm considered something of an expert when it comes to complex regular expressions and the subtleties of using them in different editors and tools, but Chapter 2 and Appendix C both had enough new material in them that I found myself frequently making notes in the margins.
I have many, many books in my bookshelf in my office but nearly none on my desk. Beginning Portable Shell Scripting is going to be one of the very few that will be spending a great deal of time lying flat on my desk, in easy arm-reach.
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