TweetFollow Us on Twitter

Mac in the Shell: What Has Really Changed in Leopard?

Volume Number: 24 (2008)
Issue Number: 02
Column Tag: Mac in the Shell

Mac in the Shell: What Has Really Changed in Leopard?

Some differences for us Unixy scripting people

by Edward Marczak

Introduction

The dichotomy strikes me as a bit amazing: on one hand, I don't find too many end-users that can really quantify what's better about Leopard as compared to Tiger. I've upgraded some people that just had to have the latest and greatest and honestly, they haven't noticed much difference. The flip side of this is the tech perspective: take just about everything you understood in Tiger and throw it out the window. The underlying technology in Leopard is radically different in many ways from its predecessor. So, specifically, what is different for us shell-dwellers?

"Real" Unix

Leopard is the first version of OS X that is Unix 03 certified. What does that mean? To understand this requires a short trip back in time (cue Time Machine space backdrop...), though not too far.

Unix and Unix-like operating systems fall into several categories. AT&T/Bell Labs' Unix (UNICS, then UNIX System V1) is the oldest variant and often the reference implementation. "BSD Unix" was originally distributed at Berkeley University (Berkeley Software Distribution) and did things differently than AT&T Unix. Over the years, BSD and AT&T Unix have diverged significantly. In the early 1990's, we began to see the rise of GNU/Linux and yet another method of doing things.

Often, one would find tools from another Unix platform that were missing from (or just better than) the tools on your current platform. Over time, these tools would get ported, but not necessarily in a way consistent with the target platform. Standards have emerged, such as POSIX, that generally smooth over these cross-platform differences, but not everyone follows the standards.

While OS X has, from the beginning, had a BSD layer on top of its Mach kernel, and typically followed BSD semantics, Leopard changes all of this. As of OS X v10.5, OS X follows AT&T semantics for most command-line utilities.

Where Does it Hurt?

OK, Apple changed this over, so what? Well, like any target upgrade, you need to ensure that long-time scripts still work. I personally got bitten by the change as the sort utility no longer uses the GNU semantics that I had gotten used to. Let's take a look there first.

I had a script that used sort in the most simple way: to sort data on a single column. To do this I used the now-deprecated plus-notation:

generate_data | sort +4 > final_file.txt

Along comes Leopard which acted like I was insane. "Plus notation? What are you talking about? That doesn't exist." A quick trip to the man page revealed that this is true:

SYNOPSIS sort [OPTION]... [FILE]...

...with no plus signs in the man page anywhere. However, a quick trip back to Tiger showed that I wasn't crazy:

SYNOPSIS
       sort  [-cmus]  [-t  separator] [-o output-file] [-T tempdir] [-bdfiMnr]
       [+POS1 [-POS2]] [-k POS1[,POS2]] [file...]
       sort {-help,-version}

Ah ha! Of course, running under Leopard, I still needed to update my script. Naturally, there had to be a hurdle, no matter how small: the plus notation counts fields from zero like a computer, and the "-k" switch counts from 1 like a human. So, my incantation now had to read:

generate_data | sort -k 5 > final_file.txt

In any case, the issue was now fixed.

Portability

While the case I just gave was easily solved, be aware that all of these differences can cause issues when writing scripts that will deployed cross-platform -- whether the platforms in question are intra-OS X or beyond those bounds to Solaris, Linux, etc.

A command as seemingly simple as chown may trip you up if your script is to run on different OSes. Typically, if you use chown on a symlink, the symlink itself remains unmodified, while the target of the symlink is altered. This can be changed with the -P flag. In AT&T semantics, issuing the following commands alter the symlink:

ln -s file.txt test.txt
chown -RP user test.txt

In BSD, the owner of the symlink is not altered. Yes, this includes 10.4 and earlier.

Another one that has caught many is echo. To prompt for input, it has been nice to fall back on the "-n" BSDism that removes suppresses echo from printing a newline character. However, under Leopard, this seemingly broke. Frustratingly, visiting the man page for echo showed that "-n" was still valid and should do what you'd expect. What was going on?

Technically, echo is a shell built-in. Prior to 10.5, the built-in and the version in /bin happened to have the same behavior. 10.5 brings the built-in up to SUSv3 conformance using AT&T semantics, meaning, no "-n" switch. Of course, the shell executes built-in before looking along $PATH. Now, anyone who simply specified "echo" (just about everyone, myself included), and needs the "-n" switch got a surprise. There's a few ways to handle this. The easiest is to call the external echo in /bin, as it still supports "-n". This may actually be your best choice in many cases as there are many tutorials and documents that assume the availability of the "-n" switch.

Another option is to AT&T-ify it. This requires you to drop the "-n" flag and finish off the string with a "\c" character:

echo "Enter number of seconds: \c"

This is perfect, if you're going all-Leopard.

The best choice may be to drop the use of echo altogether. printf is a more robust and universal command. On all systems that I can think of, printf will not print a newline at all, and requires a '\n' character to do so. For cross-platform scripting, printf is the way to go.

In fact, printf is even more flexible than echo, mimicking some of C's formatting functionality. Of course, note that printf comes in both a built-in and external version.

Finally, keep in mind that sometimes, it's simply return codes that differ between platforms. So, while some command may do the same thing, in exactly the same way:

file_pouncer -k -d 7 -y "*txt"

success and failure (or error) may be defined differently in $?. Some programs will silently ignore errors, returning a zero. Remember to test these scenarios between destination platforms.

A final example of this: we've been accustomed to BSD-based 'cp' to copy files. Under BSD, cp will march through, despite errors and copy as much as possible. AT&T semantics have cp abort after the first error. Subtle, and something that will only crop up under a failure condition.

Conclusion

Of course, this doesn't begin to cover what has really changed in Leopard, as there is so much that has, on so many different levels. Those of us working in the shell, though, do need to be aware of what the new SUS certification means to us, and how the AT&T semantics affects our scripts. Remember: test, test, test!


Ed Marczak is a married, father-of-two technologist. Outside of learning about life, business and technology -- particularly OS X -- you can find him home with his family in New York. For a few spare hours at night, you can also occasionally find him riding through Azeroth on his Dreadsteed.

 
AAPL
$101.19
Apple Inc.
+0.61
MSFT
$45.22
Microsoft Corpora
-0.01
GOOG
$583.16
Google Inc.
-0.21

MacTech Search:
Community Search:

Software Updates via MacUpdate

calibre 2.0.0 - Complete e-library manag...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital... Read more
Apple iMovie 10.0.5 - Edit personal vide...
With an all-new design, Apple iMovie lets you enjoy your videos like never before. Browse your clips more easily, instantly share your favorite moments, and create beautiful HD movies and Hollywood-... Read more
Apple Keynote 6.2.2 - Apple's prese...
Apple Keynote makes it simple to create and deliver beautiful presentations. Powerful tools and dazzling effects bring your ideas to life. You can work seamlessly between Mac and iOS devices. And... Read more
Apple Numbers 3.2.2 - Apple's sprea...
With Apple Numbers, sophisticated spreadsheets are just the start. The whole sheet is your canvas. Just add dramatic interactive charts, tables, and images that paint a revealing picture of your data... Read more
OpenOffice 4.1.1 - Free and open-source...
OpenOffice.org is both an Open Source product and a project. The product is a multi-platform office productivity suite. It includes the key desktop applications, such as a word processor,... Read more
Pages 5.2.2 - Apple's word processo...
Apple Pages is a powerful word processor that gives you everything you need to create documents that look beautiful. And read beautifully. It lets you work seamlessly between Mac and iOS devices. And... Read more
Quicken 2015 2.0.1 - Complete personal f...
The new Quicken 2015 helps you manage all your personal finances in one place, so you can see where you're spending and where you can save. Quicken automatically categorizes your financial... Read more
CleanMyMac 2.2.7 - Delete files that was...
CleanMyMac makes space for the things you love. Sporting a range of ingenious new features, CleanMyMac 2 lets you safely and intelligently scan and clean your entire system, delete large, unused... Read more
MacFamilyTree 7.2.4 - Create and explore...
MacFamilyTree gives genealogy a facelift: it's modern, interactive, incredibly fast, and easy to use. We're convinced that generations of chroniclers would have loved to trade in their genealogy... Read more
Videobox 4.1.1 - Download Flash video th...
Videobox allows you to quickly and easily download Flash video from most all of the popular video sites on the internet. Videobox will convert the video into a native Quicktime format so it's ready... Read more

Latest Forum Discussions

See All

The Outcast Review
The Outcast Review By Nadia Oxford on August 22nd, 2014 Our Rating: :: HANDS OFF. WAY OFF.Universal App - Designed for iPhone and iPad It’s easy to see what The Outcast is trying for, but its execution needs a lot of work.   | Read more »
Brain+ Review
Brain+ Review By Nadia Oxford on August 22nd, 2014 Our Rating: :: DIM BULBUniversal App - Designed for iPhone and iPad Brain+ is just another entry in an over-saturated brain-training marketplace – and not a particularly fun entry... | Read more »
The Witcher Battle Arena – New Gameplay...
The Witcher Battle Arena – New Gameplay Trailer Revealed Posted by Jessica Fisher on August 22nd, 2014 [ permalink ] Based in the Witcher universe, | Read more »
Max Gentlemen Review
Max Gentlemen Review By Jennifer Allen on August 22nd, 2014 Our Rating: :: OUTSTAYING ITS WELCOMEiPhone App - Designed for the iPhone, compatible with the iPad Max Gentlemen seems pretty quirky initially but that appeal wears thin... | Read more »
Club Penguin Gets Frozen All Over in Lat...
Club Penguin Gets Frozen All Over in Latest Takeover Event Posted by Ellis Spice on August 22nd, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Doctor Who: Legacy Review
Doctor Who: Legacy Review By Jennifer Allen on August 22nd, 2014 Our Rating: :: RUN OF THE MILLUniversal App - Designed for iPhone and iPad Doctor Who: Legacy is an ok match-3 puzzle game, but it lacks some much needed personality... | Read more »
Pentumble (Games)
Pentumble 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: *** Special Launch Price! 33% Off! *** Pentumble combines the joy of a free-flowing platformer with beautiful graphics and levels... | Read more »
DotEmu Summer Sale Cuts Several Games’ P...
DotEmu Summer Sale Cuts Several Games’ Prices to $0.99 Posted by Ellis Spice on August 21st, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
King Announces its Newest “Saga” – Diamo...
King Announces its Newest “Saga” – Diamond Digger Saga Posted by Jessica Fisher on August 21st, 2014 [ permalink ] Diamond Digger Saga, by King Digital Entertainment, promises to be another addictive, adorable game in the “Saga” | Read more »
Spacetime Studios’ Greg Mueller Tells Al...
| Read more »

Price Scanner via MacPrices.net

Leftover 15-inch 2.0GHz Retina MacBook Pros a...
B&H Photo has leftover previous-generation 15″ 2.0GHz Retina MacBook Pros now available for $1599 including free shipping plus NY sales tax only. Their price is $400 off original MSRP. B&H... Read more
Pro.Calendar – New Productivity App for iPad...
Austin, Texas based mobile business and productivity app developer LightArrow, Inc. has announced Pro.Calendar, a powerful and intuitive calendar app with eight versatile calendar options including... Read more
SanDisk Ultra II SSD — Supercharge Your Syste...
SanDisk Corporation has announced the new SanDisk Ultra II SSD with enhanced SSD Dashboard. The new drive is designed to deliver a cost-effective and easy upgrade solution for PC owners looking to... Read more
Samsung and Barnes & Noble Introduce New...
Samsung Electronics America and NOOK Media, a subsidiary of Barnes & Noble, Inc. have announced the introduction of the new Samsung Galaxy Tab 4 NOOK, a 7-inch tablet combining Samsung’s leading... Read more
21-inch iMacs on sale for up to $150 off MSRP
B&H Photo has 21″ iMacs on sale for up to $150 off MSRP including free shipping plus NY sales tax only. B&H will also include a free copy of Parallels Desktop software: - 21″ 2.7GHz iMac: $... Read more
27-inch 3.2GHz iMac on sale for $1698, save $...
Abt has the 27″ 3.2GHz iMac on sale for $1698 including free shipping. Their price is $101 off MSRP. Read more
Mac Backup Guru 2.0 Drive Backup/Cloneing Uti...
Mac Backup Guru developer MacDaddy has released Mac Backup Guru 2.0, offering new and enhanced advanced features, such as bootable backups, synchronised volumes and folders, and a Snapshot mode that... Read more
Operate GE’s New Free-Standing KItchen Range...
Think you accidentally left the oven on? Switch it off while on the go. The new free-standing Profile™ Series gas and electric ranges are GE’s second cooking appliances, following their wall oven, to... Read more
Parallels Announces Parallels Desktop 10 for...
The no. 1-selling software for running Windows applications on a Mac becomes an even easier choice for millions of consumers and IT professionals worldwide with the launch of the most powerful... Read more
Apple now offering certified refurbished 2014...
 The Apple Store is now offering Apple Certified Refurbished 2014 MacBook Airs for up to $180 off the cost of new models. An Apple one-year warranty is included with each MacBook, and shipping is... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** As more and more people discover Apple , they visit our stores seeking ways to incorporate our products into their lives. It's your job, as a Store Read more
Position Opening at *Apple* - Apple (United...
…Summary** As a Specialist, you help create the energy and excitement around Apple products, providing the right solutions and getting products into customers' hands. You Read more
Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and Read more
*Apple* Solutions Consultant (ASC)- Retail S...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.