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Beginning Portable Shell Scripting
Joe MacDonald writes "The earliest UNIX shell I encountered was the Bourne shell on a SPARCStation
2 at my university. As with many students of my generation, prior to that
nearly all of my exposure to command line interfaces was some variant of DOS.
I was quite proficient with the primitive scripting language that was available
on such platforms but I immediately felt far out of my depth in this new
environment. The commands seemed arcane, possibly dangerous, and almost
immediately I regretted stepping into this unfamiliar wilderness without some
sort of guide." Read below for the rest of Joe's thoughts.
Beginning Portable Shell Scripting: From Novice to Professional
author
Peter Seebach
pages
376
publisher
Apress
rating
4/5
reviewer
Joe MacDonald
ISBN
1-4302-1043-5
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.
You can purchase Beginning Portable Shell Scripting from amazon.com. Slashdot welcomes readers' book reviews -- to see your own review here, read the book review guidelines, then visit the submission page.
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.
You can purchase Beginning Portable Shell Scripting from amazon.com. Slashdot welcomes readers' book reviews -- to see your own review here, read the book review guidelines, then visit the submission page.
Don't do it.
Shell scripts have horrible error handling, and quickly become a maintenance nightmare. These days, e.g. Python is installed everywhere you need to go.
Just do this:
def c(s): os.system(c)
and you have mostly covered the area where shell scripts excel. You can still write minimal "shell scripts" inside c().
Unluckily, you still *need* to grok shell scripting to some extent, or at least be able to read them. Just don't write them if you can help it.
Save your wrists today - switch to Dvorak
You're in luck! I have recently heard of a book that could help with your scripting...
One does not write a web server in Bash, one wraps a webserver in it, pipes its output to a log analyzer, restarts it automatically if it crashes, and so on.
The most important part of any UNIX-derived shell langauge is not its syntax or power but the fact it lets you construct large ad-hoc applications out of a toolbox of tens of thousands of pieces.
This is where all other operating systems (that I've ever used, and that's 30-40) have failed.
Any serious developer should know several glue languages, Unix shells being the most flexible and accessible.
My blog
It's true. I work with a guy (rather old himself) that writes on the Korn shell because it's the only shell that is included on pretty much all Unix based OSs, including Linux. (and Solaris, HP-UX, and AIX, which we also use).
First off, in the interests of full disclosure, Joe MacDonald is one of my coworkers.
Anyway... The big surprise to me was the word "Beginning", which somehow showed up in the publisher's cover pages, but which I didn't know about during the writing process. My tech reviewer was Gary V. Vaughan (yes, the autoconf/libtool guy). I bounced material off a number of seasoned expert scripters during the process. Basically, my goal was to write a book that I could use as a reference, and which would teach me something.
I succeeded beyond my wildest dreams. The discovery that printf(1) is essentially universal these days was a complete shock to me; I had no idea it was portable. During my first pass on the regular expressions section, I started by writing down what I believed I knew about sed, awk, etcetera. Then I tested it... and had to revise most of it. A number of things I was used to were GNU or BSD extensions. When Gary sent the chapter back for tech review, he'd flagged most of these things, because he "knew" the same things I did.
So everything there should be pretty thoroughly checked out now -- I realized very early on that this field was full of things "everyone knows". Many of them wrong. We tested things on a field of around 30 different versions of Unix and Linux. We tested them on unusual installs, we tested them on special cases.
Why?
Because portable shell is an incredibly portable language, and sometimes that matters. Because shell is a very powerful language, too. Because sometimes shell is all you have -- and because sometimes shell is more expressive for a task than your other choices. I love me some C, I program in C by preference much of the time -- but there are a lot of tasks I'll do in shell rather than in C. There are similarly many tasks I'd rather write in shell than in perl. Shell is what make uses to run commands, and sometimes you need to write something clever in shell because make doesn't have quite the right feature.
In short, it's something I have found consistently useful, day in and day out, for about twenty years now. I just wish I'd realized how much more there was to learn years ago, I coulda saved a lot of time... :)
And, to answer a question hinted at earlier: Yes, now that this book exists, I keep a copy on my desk. I look stuff up in it about once a week.
My blog: http://www.seebs.net/log/ --- My iPhone/iPad app: http://www.seebs.net/seebsfrac/
Well, there is some truth to the GPP's comment. Linux and Mac OS X don't even agree on how to tell echo not to print a newline or how to enable extended regular expression mode in sed. May heaven help you if you want to do something as esoteric as creating or mounting a filesystem, creating or mounting a disk image/ramdisk, talk to a USB device in any way, get a list of processes in any useful way, etc. There's a very big lack of standardization in a lot of things you might like to do with scripts, in other words. The Single UNIX Spec and POSIX are not quite sufficient, but more annoyingly, most OSes (Linux, *BSD) out there don't even come close to conforming to it, so you end up with this dichotomy between BSD behavior and AT&T behavior.
That said, a lot of things are standardized, and many others can be worked around with clever use of variables (or possibly eval in a few extreme cases). I've written chapters on the subject myself. The big things you need to remember are that $(( $FOO + 3 )) is not portable, nor for ((...)), nor >&, nor anything involving extended regexp except using Perl, that even "the one true awk" is not quite SUS-compliant, GNU awk doubly so, bash triply so, that you should use printf instead of echo for output if you don't want newlines, that signal numbers are not portable (for trap), that proper quoting of arguments is crucial, and that you need to work with the bare minimum base behavior of utilities (using few or no flags) if you expect any hope of portability without needing to make platform-specific changes.
For some quick examples of some interesting portability issues, read some of my comments in the games at shellscriptgames.com or search for the word "compatibility" in Apple's "Shell Scripting Primer". It's a real eye opener to see how many portability problems exist even for fairly simple shell scripts.
Check out my sci-fi/humor trilogy at PatriotsBooks.
Why bother with portable shell scripts, seriously? Everybody has bash installed, and/or zsh that is mostly compatible, and even then you have bash anyway. I understand retro-nostalgia and all that, but necrophilia is overrated
False.
The majority of systems I work on these days and the majority of systems I have worked on since the mid 90's have not had bash installed. That includes systems running FreeBSD, NetBSD, OpenBSD, AIX, Tru64, Solaris, MacOS, and even Linux. Current versions of some of those will usually have bash in a default installation, but some still do not. Companies running stable systems as important parts of their business do not generally upgrade their OS's just for the sake of novelty. Running older systems isn't usually about nostalgia or necrophilia, it's more often about not having any compelling reason to upgrade. There is also a system hygiene practice common on the BSD's of keeping the base system minimal and only adding on what is needed, a practice that helps in keeping systems secure and stable because they are easier to fully understand. This is also common in many virtualization environments, where a running OS instance is likely to exist for a very narrow purpose and intentionally have a stripped-down set of utilities fit to that narrow purpose.