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Beginning Portable Shell Scripting 186

Posted by samzenpus
from the read-all-about-it dept.
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.

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Beginning Portable Shell Scripting

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  • more than this? (Score:3, Interesting)

    by tritonman (998572) on Wednesday February 11, 2009 @01:19PM (#26816001)
    does it say more than this? http://www.dreamsyssoft.com/unix-shell-scripting/intro-tutorial.php [dreamsyssoft.com]
  • by ultrabot (200914) on Wednesday February 11, 2009 @01:23PM (#26816049)

    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.

    • by ncw (59013) on Wednesday February 11, 2009 @01:28PM (#26816131) Homepage

      I agree!

      My personal limit is 10 lines for a shell script. If is longer than that I convert it to Python.

      Python scripts have the advantage that they work on Windows too, and they have lots of os independent abstractions for file names, processes etc.

      Why learn an arcane language like sh when you can learn a nice well structured language like Python and write better scripts?

      A few years ago I would have used Perl rather than Python, but I'm converted now ;-)

      • Re: (Score:2, Insightful)

        by cmdr_tofu (826352)
        The tradeoff is that your scripts have a huge memory footprint now. I loathe shell script as much as the next person, but find it necessary when I am working in a minimal busybox-type environment. Perl/Ruby it is whenever I have the chance.
        • Oh yes?
          How about counting each time a shell script has to spawn a separate process and python can do the job inside it?
          And what when you stack together 5 such scripts?

          Beyond maintainability, readability, sanity, one can also have some gains in speed and memory usage by using python;

      • Re: (Score:3, Insightful)

        by Tikkun (992269)

        Why learn an arcane language like sh when you can learn a nice well structured language like Python and write better scripts?

        Where I work pretty much everything has bash already (I install cygwin on all the Windows boxes. Of course, Python is usually there too :) ).
        If you already have a bash script (or find one via the Google), changing it is usually simpler than porting it to Python.
        If you work with people that already know bash scripting but don't know Python using the lowest common denominator can be easier than training.
        There is less memory overhead for a simple shell script than there is for a simple Python script.

        This

        • by nevermore94 (789194) on Wednesday February 11, 2009 @04:58PM (#26819631)
          Lol, under 40 lines... Where I have worked for 10 years, we use portable shell scripts to maintain hundreds of database servers on several different UNIX platforms including Linux, AIX, DEC, and even SCO. Our older utility suite which had a full character curses like menu system is comprised of around 54,000 line of shell including comments in 214 scripts. Our new system which has a full web interface is around 64,000 lines of shell including comments in 226 scripts, plus it makes calls out of the older utilities as well. The longest individual script I maintain is currently 2,741 lines long, but it includes many functions from our shell libraries and makes calls out to even more shell utilities. And, it all still works on all of our Linux, AIX, DEC, and SCO servers. Also, as being a Java programmer, I would hate to even try to calculate how many lines of code it would take to try to duplicate all of the functionality in Java.
      • Re: (Score:2, Interesting)

        by medoc (90780)

        One reason may be that Python's startup time is an order of magnitude slower that a shell's, and sometimes it does matter, even for scripts of much more than 10 lines.

    • by $RANDOMLUSER (804576) on Wednesday February 11, 2009 @01:42PM (#26816393)

      Shell scripts have horrible error handling, and quickly become a maintenance nightmare. These days, e.g. Python is installed everywhere you need to go.

      That's exactly what the Perl people said years ago, and we all know how well that worked out (for low maintenance sysadmin-type tasks). I know the sinking feeling I get every time I find a crontab entry pointing to a Perl script.

      • by swillden (191260)

        Shell scripts have horrible error handling, and quickly become a maintenance nightmare. These days, e.g. Python is installed everywhere you need to go.

        That's exactly what the Perl people said years ago, and we all know how well that worked out (for low maintenance sysadmin-type tasks). I know the sinking feeling I get every time I find a crontab entry pointing to a Perl script.

        There is a big difference between Perl and Python. While you can write readable Perl, or write horribly opaque Python, there's a reason that Perl has a reputation as a write-only language and Python has a reputation for being quite readable.

        Then there's the guy I worked with years ago who wrote (writes, I'm sure) all of his scripts in elisp...

      • by Lord Ender (156273) on Wednesday February 11, 2009 @03:57PM (#26818703) Homepage

        That's not what they said years ago. People arguing to use Perl instead of Bash did so because Perl was just far more functional. Perl and Bash both have pretty terrible maintainability, but Perl is a million times more functional.

        Python and Ruby have the functionality of Perl without the maintenance issues inherent in a language which is really a hodge-podge of ancient unix idioms.

        • by dfn_deux (535506) <[moc.liamg] [ta] [015nustad]> on Wednesday February 11, 2009 @07:45PM (#26821547) Homepage

          maintenance issues inherent in a language which is really a hodge-podge of ancient unix idioms.

          What a ridiculous claim, there are no "maintenance issues" with ancient idioms... The very fact that those techniques are ancient shows how incredibly flexible and useful they are. I'd much rather use conventions which are widely accepted and in many cases are required by Posix/SUS/XPG4 than find myself having to hack up my stuff to accommodate broad and pervasive changes such those experienced when moving from python 2.x to python 3.x...

          People who are constantly advocating against shell scripts tend to be those who see system administration as something it isn't; namely a low level development job. When in reality a sys admin uses shell scripts to glue together existing products of developers in order to manage administrative tasks. If I were an auto mechanic no one would propose that I learn to master a casting foundry and a milling machine in order to work on cars, those are clearly manufacturing/development tools AND certainly no good mechanic would suggest that using a wrench to fasten a nut to a bolt is "a hodge podge of ancient idioms" which should be replaced with whatever flavour of the week fastening system and power tool happens to be popular at the moment.

          Sure there are some arcane aspects to shell scripting, but when I learned Unix in college they taught a thing called "the unix philosophy" which basically said that you should always use the smallest tools for the job, leverage the pipes/redirection, and build to a usable script which doesn't replicate existing functionality of ubiquitous tools. Seems like these days every python/perl wizard around fancies themselves an administrator and yet they waste a large portion of their time rewriting tried and true unixisms; sort, wc, cut, paste, tee, etc...

          Also, get off my lawn!

      • by oliderid (710055)

        I know the sinking feeling I get every time I find a crontab entry pointing to a Perl script.

        For the younger folks out there, the feeling is similar too:
        mono /script/all_your_code_belong_to_us.exe

    • Re: (Score:2, Interesting)

      by Etherized (1038092)

      I can't agree more. I switched over to python for all non-trivial scripting a couple of years ago, and I find it much more pleasant. I even sometimes use iPython instead of bash when I know I'll need to do something complex interactively.

      By the way - if you like using python to control systems, you might also enjoy the func project [fedorahosted.org].

    • by JamesP (688957)

      To me it's mixed feelings there

      Even though python is very easy, you can't beat the ease of use of find / sed / others

      And in python you would need to go to popen/pclose usually, to get the output and return values

      • try this...

        from commands import getoutput as cmd
        foo = cmd("echo bar")

        I often find myself writing small, special-purpose wrappers for some commands, like ps or find. I liked the MS PowerShell's concept of piping objects instead of text. Imagine... "ps | grep name=spam | kill". Yeah, sh sucks.
    • by digitalhermit (113459) on Wednesday February 11, 2009 @01:54PM (#26816623) Homepage

      Python is nice, but hardly installed everywhere. It's available on Linux certainly, but not always on AIX or Solaris. Yes, it is just an installation away, but many of the systems I maintain require change management procedures to even chmod a file.

      Shell scripts do have decent error handling for what they need to do. With traps and proper usage of error codes, they are not much different from lower level languages.

      I'd agree that I now *prefer* to write longer scripts in Python. However, few of the people I work with know Python, or even Perl. They can get around with korn and bourne as these are the default scripting languages on more traditional Unix systems.

      Which comes down to the gist of the issue. Do you write code in a language you prefer or one that can be maintained by the admins? I'd argue that it doesn't matter what language you use. If you write poor code in shell you will likely write poor code in Python too.

    • Re: (Score:3, Interesting)

      by Haeleth (414428)

      These days, e.g. Python is installed everywhere you need to go.

      No it isn't. Not even remotely close.

      And in most cases the response to a request to install an entire programming language would be flat rejection, turning to raucous laughter when they realise you only want it because you don't like any of the several scripting environments that are already available.

      • by ultrabot (200914)

        These days, e.g. Python is installed everywhere you need to go.

        No it isn't. Not even remotely close.

        And in most cases the response to a request to install an entire programming language would be flat rejection, turning to raucous laughter when they realise you only want it because you don't like any of the several scripting environments that are already available.

        Sounds like a rather dysfunctional working environment.

        I had something like this in mind when I said "everywhere you need to go" (as opposed to just saying "everywhere").

    • Re: (Score:2, Informative)

      by fjollberg (1061600)

      > These days, e.g. Python is installed everywhere you need to go.

      Sorry, but no, it isn't.

    • by 0xABADC0DA (867955) on Wednesday February 11, 2009 @04:13PM (#26818961)

      Shell scripts have horrible error handling, and quickly become a maintenance nightmare. These days, e.g. Python is installed everywhere you need to go.

      Python doesn't help much over shell scripts without extra libraries, which may or may not be present on any given system.

      Python has changed incompatibly several times already.

      Python has a large startup overhead:
          20 seconds: 1000x python -c 'print("test")'
          2 seconds: 1000x sh -c 'echo test'

      Python is clumsy to use for gluing several programs together.

      Python is not the same syntax as the shell. If you don't learn the shell then your day-to-day command lines are gimped.

      So Ruby or Python or anything else is better for writing actual programs that do anything complicated, but there are plenty of appropriate uses for shell scripting. Ruby is actually much better... since it has a sensible syntax you could make a rubysh that wouldn't suck.

      • by ultrabot (200914)

        Python doesn't help much over shell scripts without extra libraries, which may or may not be present on any given system.

        What are those extra libraries without which you can't work? Python standard library is absolutely enough for shell script like tasks, both trivial and non-trivial.

        Python has changed incompatibly several times already.

        It's actually quite rare. Python 3.0 was created explicitly to allow breaking of the compatibility. If you write scripts that use the new stuff, of course they won't work on older scripts. But old scripts still work on new python 2.x interpreters.

        Or do you have a concrete example in mind?

        Python has a large startup overhead:

        So a 0.02 second startup time is a problem for you? If you

    • I see nothing wrong with my natural progression:
      1. execute commands once-off in the shell
      2. paste said commands to a file to execute later once I see they work
      3. pretty up the commands
      4. rewrite in Python
      5. rewrite parts in some compiled language if things are too slow
      6. wrap a GUI over parts of it if other people need to use it

      .

      The advantages are rapid prototyping, and many places to stop putting work into the project without losing functionality. If you have a really well defined large project, big design up front ma

  • by Chris_Jefferson (581445) on Wednesday February 11, 2009 @01:24PM (#26816067) Homepage
    While I find anything I want to be stable and distributable I now write in Python, I still can't resist pulling out shell scripting, and a splattering of grep, awk, find, mv and xargs to do 95% of the simple pushing around and chopping up of files I find myself doing every day.

    I find shell scripting have a nasty habit of not working quiet right when moved between Linux, the BSDs and Mac to be safe, and it's always a pain to write scripts that work correctly with spaces in file names.

    Why isn't there (or is there?) a simple python cheat guide, or library, that do the same things as grep, awk, find, mv and xargs?

    • by ultrabot (200914) on Wednesday February 11, 2009 @01:35PM (#26816269)

      Why isn't there (or is there?) a simple python cheat guide, or library, that do the same things as grep, awk, find, mv and xargs?

      re.findall, s.split(), os.walk, shutil.move,
      " ".join

    • by ultrabot (200914)

      Why isn't there (or is there?) a simple python cheat guide, or library, that do the same things as grep, awk, find, mv and xargs?

      Forgot to mention - no need to do it all in raw python. Just call out to shell when you feel like it, if you are sure it can stay unix-only. You can do wonders with some shell invocation + os.popen(...).read()

  • by pieterh (196118) on Wednesday February 11, 2009 @01:26PM (#26816101) Homepage

    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.

    • by dgatwood (11270) on Wednesday February 11, 2009 @02:31PM (#26817263) Journal

      One does not write a web server in Bash

      I accept your challenge. :-D

      But seriously, yeah, you're absolutely right. Ooh, but a basic web server written as a Bourne shell script called by inetd would be so freaking cool....

      Oh, no. Somebody actually did that [debian-adm...ration.org].... Yikes! Now I'm scared.

    • "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."

      What's the advantage supposed to be? Constructing a "large ad-hoc application" doesn't sound like a great idea to me.

      • by seebs (15766) on Wednesday February 11, 2009 @03:56PM (#26818675) Homepage

        One of the points might be that there's a fairly specialized task which takes a person six hours to do, but which is NEARLY all done automatically -- just a bit of hand twiddling.

        Lemme give an example that isn't portable. One of the things I do fairly frequently is take about six large toolchains distributed to me as binaries and source tarballs, and turn them into patches against upstream versions, reorganize them, delete some unused files, create configuration files that refer to the binaries, generate md5sums, and so on.

        This is a task which, if I sit down at 10AM and start typing, is usually done by about 4PM. Testing takes a bit longer, and usually uncovers SOME kind of typo or thing I forgot to do.

        Enter the shell script.

        I tell the script where the files are, and I walk away. An hour later I have the results. Testing is also automated (another script). But testing is also uneventful, because the script never forgets a step, makes a typo, or otherwise screws up.

        By the second time I did this, the script had saved me time. By the third, it had saved me close to a full working day. By now it's closer to a week of my time that wasn't spent messing with this stuff.

        Portability isn't entirely crucial here, you might think? Well, not ENTIRELY crucial, except that when they had me start doing this on a new box running a different variety of Linux, the total time I spent revising the script was 0 minutes.

        • I can see the value of shell scripts for small or possibly medium size ad-hoc applications but not for large ones.

    • by seebs (15766) on Wednesday February 11, 2009 @03:06PM (#26817831) Homepage

      I think "One does not write a web server in Bash" is like "One does not simply walk into Mordor." You're practically daring short people with hairy feet to attempt it.

      But your point is basically good. :)

      • I think "One does not write a web server in Bash" is like "One does not simply walk into Mordor." You're practically daring short people with hairy feet to attempt it.

        Yeah but Linus is already working on the kernel...

    • But it's reasonable, indeed fairly elegant, to write a web server in Tcl. And Tcl was specifically conceived as a glue language. It doesn't hurt that Tcl commands look a lot like interactive shell commands, but expressed in a more regular syntax.

      I'm mentioning this just to point out that these distinctions of granularity are a bit artificial. Yes, glue languages give you composability at the coarse end of the scale. But they're often quite acceptable as programming languages as well.
    • by Eil (82413)

      One does not write a web server in Bash

      Uh oh. Someone should have warned this guy [wikidot.com].

    • by fm6 (162816)

      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.

      Like constructing a Windows app out of ActiveX components?

      No, I'm not saying that ActiveX is as good a glue as the Unix equivalents, or that Windows is as good as Unix. I'm just saying that there's more to Unix than its ability to glue stuff together.

      This is where all other operating systems (that I've ever used, and that's 30-40) have failed.

      I guess your definition of "failure" is "Pieter hates using it". By any other measure, there are are fair number of non-failed OSs that aren't Unix or Unix-like.

  • by Anonymous Coward on Wednesday February 11, 2009 @01:28PM (#26816137)

    I'm sure all of us at one time or another have had a shell script we've relied on for years fail miserably when bringing it to a new environment. The sad fact is, shell scripts were never meant to be programming languages in and of themselves, and I wonder if, knowing what we know now, it isn't overly ego-driven and masochistic to try to take this feature -- tied to a shell which is tied to an operating system -- and promote it beyond its competency.

    So, let's say we take the PSS principles seriously, and abstract away any non-platform-agnostic features you can think of. A few years down the road, you've got PSS all over the shop and you want to upgrade to a different platform nominally supporting your shell of choice. Even if you shake off PSS features you thought could create incompatibilities, you discover the new system buffers differently. Or added a parameter somewhere. Point is, if you went with something like Perl which is designed for cross-compatibility you would have been fine, but now you're all wet.

    Shell scripting is good for what it's meant for, but at the risk of oversimplifying with a tool analogy, I'm concerned that this falls into the trap of "If all you have is a hammer, all your problems look like nails".

    • by seebs (15766)

      You know, that's exactly why a book on it is useful -- because it turns out that you can, in fact, write scripts which port quite nicely.

      Seriously, "buffers differently"? What's THAT supposed to mean?

      That said, I won't deny for a moment that there are things shell isn't a good choice for. In fact, one of the first sections in the book is under the heading "What Shell Scripting Isn't". Because sometimes the best way to write a good program is to pick the right language to begin with. Often, even.

      But some

      • Seriously, "buffers differently"? What's THAT supposed to mean?

        One example is how and when data gets written to disk in the absence of a flush().

        • by seebs (15766)

          And what "flush()" do you expect to see in a shell script?

          You're thinking at a level that generally doesn't apply well to shell scripts. Scripts rarely deal with questions such as "has this been actually written to disk" -- because that's at the wrong level. If you need that information, you shouldn't be writing in shell anyway.

          But normally you don't...

    • by Eil (82413)

      I agree with your overall message, but...

      So, let's say we take the PSS principles seriously, and abstract away any non-platform-agnostic features you can think of. A few years down the road, you've got PSS all over the shop and you want to upgrade to a different platform nominally supporting your shell of choice. Even if you shake off PSS features you thought could create incompatibilities, you discover the new system buffers differently. Or added a parameter somewhere. Point is, if you went with something

  • by seebs (15766) on Wednesday February 11, 2009 @01:59PM (#26816719) Homepage

    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.

    • Saying "I love C but there are things that are better in shell" is completely anachronistic.

      Seriously. The question's been settled for over 20 years.

      And there are other languages, you know. The question is more whether to use Python (for example) instead of shell in some cases, and when.

      • Re: (Score:3, Interesting)

        by seebs (15766)

        I compare C and shell all the time. Sometimes the answer surprises me. e.g., until I knew about printf(1), I sometimes went to C if I needed to pretty-print output. Now sometimes I don't.

        I will happily mix and match multiple languages; one of my first shipping products was written in shell, perl, and C. Each did some things well that the others didn't...

        I tend not to use Ruby for things that I want to be portable, because not everything has Ruby around yet. I tend to avoid Python because it just never

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