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.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

PDFKey Pro 4.3 - Edit and print password...
PDFKey Pro can unlock PDF documents protected for printing and copying when you've forgotten your password. It can now also protect your PDF files with a password to prevent unauthorized access and/... Read more
Kodi 15.0.beta1 - Powerful media center...
Kodi (was XBMC) is an award-winning free and open-source (GPL) software media player and entertainment hub that can be installed on Linux, OS X, Windows, iOS, and Android, featuring a 10-foot user... Read more
DiskCatalogMaker 6.4.12 - Catalog your d...
DiskCatalogMaker is a simple disk management tool which catalogs disks. Simple, light-weight, and fast. Finder-like intuitive look and feel. Super-fast search algorithm. Can compress catalog data... Read more
Macs Fan Control 1.3.0.0 - Monitor and c...
Macs Fan Control allows you to monitor and control almost any aspect of your computer's fans, with support for controlling fan speed, temperature sensors pane, menu-bar icon, and autostart with... Read more
Lyn 1.5.11 - Lightweight image browser a...
Lyn is a lightweight and fast image browser and viewer designed for photographers, graphic artists and Web designers. Featuring an extremely versatile and aesthetically pleasing interface, it... Read more
NeoOffice 2014.11 - Mac-tailored, OpenOf...
NeoOffice is a complete office suite for OS X. With NeoOffice, users can view, edit, and save OpenOffice documents, PDF files, and most Microsoft Word, Excel, and PowerPoint documents. NeoOffice 3.x... Read more
LaunchBar 6.4 - Powerful file/URL/email...
LaunchBar is an award-winning productivity utility that offers an amazingly intuitive and efficient way to search and access any kind of information stored on your computer or on the Web. It provides... Read more
Remotix 3.1.4 - Access all your computer...
Remotix is a fast and powerful application to easily access multiple Macs (and PCs) from your own Mac. Features Complete Apple Screen Sharing support - including Mac OS X login, clipboard... Read more
DesktopLyrics 2.6.6 - Displays current i...
DesktopLyrics is an application that displays the lyrics of the song currently playing in "iTunes" right on your desktop. The lyrics for the song have to be set in iTunes; DesktopLyrics does nothing... Read more
VOX 2.5.1 - Music player that supports m...
VOX is a beautiful music player that supports many filetypes. The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all... Read more

This Week at 148Apps: May 18-22, 2015
May Days at 148Apps How do you know what apps are worth your time and money? Just look to the review team at 148Apps. We sort through the chaos and find the apps you're looking for. The ones we love become Editor’s Choice, standing out above the... | Read more »
Biz Builder Delux (Games)
Biz Builder Delux 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: Ah, there's nothing like the rhythmic bustle of a burgeoning business burg... especially when you're the one building it... | Read more »
Auroch Digital is Bringing Back Games Wo...
| Read more »
Blades of Brim is a New Endless Runner f...
SYBO Games, the minds behind the ever-popular Subway Surfers, have announced their latest project: Blades of Brim. [Read more] | Read more »
Carbo - Handwriting in the Digital Age...
Carbo - Handwriting in the Digital Age 1.0 Device: iOS Universal Category: Productivity Price: $3.99, Version: 1.0 (iTunes) Description: | Read more »
Draggy Dead (Games)
Draggy Dead 1.1 Device: iOS Universal Category: Games Price: $.99, Version: 1.1 (iTunes) Description: Ditch your dead end job and take up a rewarding career in Grave Robbing today!Guide the recently deceased to a fun filled life of... | Read more »
Bad Dinos (Games)
Bad Dinos 1.0.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.0 (iTunes) Description: | Read more »
The Apple Watch isn't Great as a Fi...
| Read more »
Show the World What You See With Stre.am...
Live broadcasting is getting popular on mobile devices, which is why you can now get Stre.am, by Infinite Takes. [Read more] | Read more »
PhotoTime's 2.1 Update Adds Apple W...
The latest PhotoTime update is adding even more functionality to the handy photo organizing app. Yep, including Apple Watch support. [Read more] | Read more »

Price Scanner via MacPrices.net

12-inch MacBook stock status for Monday, May...
The new 12″ Retina MacBooks are still on backorder at The Apple Store with a 3-5 week waiting period. However, a few models are in stock today at Apple resellers. Stock is limited, so act now if you’... Read more
New 27-inch 3.3GHz 5K iMac in stock with free...
Adorama has the new 27″ 3.3GHz 5K iMac in stock today for $1999 including free shipping plus NY & NJ sales tax only. Adorama will include a free copy of Apple’s 3-year AppleCare Protection Plan. Read more
Memorial Day Weekend Sale: New 27-inch 3.3GHz...
Best Buy has the new 27″ 3.3GHz 5K iMac on sale for $1899.99 this weekend. Choose free shipping or free local store pickup (if available). Sale price for online orders only, in-store prices may vary... Read more
OtterBox Maximizes Portability, Productivity...
From the kitchen recipe book to the boarsroom presentation, the OtterBox Agility Tablet System turns tablets into one of the most versatile pieces of handheld technology available. Available now, the... Read more
Launch of New Car App Gallery and Open Develo...
Automatic, a company on a mission to bring the power of the Internet into every car, has announced the launch of the Automatic App Gallery, an app store for nearly every car or truck on the road... Read more
Memorial Day Weekend Sale: 13-inch 1.6GHz Mac...
Best Buy has the new 13″ 1.6GHz/128GB MacBook Air on sale for $849 on their online store this weekend. Choose free shipping or free local store pickup (if available). Sale price for online orders... Read more
Memorial Day Weekend Sale: 27-inch 3.5GHz 5K...
Best Buy has the 27″ 3.5GHz 5K iMac on sale for $2099.99 this weekend. Choose free shipping or free local store pickup (if available). Sale price for online orders only, in-store prices may vary.... Read more
Sale! 16GB iPad mini 3 for $349, save $50
B&H Photo has the 16GB iPad mini 3 WiFi on sale for $349 including free shipping plus NY sales tax only. Their price is $50 off MSRP, and it’s the lowest price available for this model. Read more
Price drop on 2014 15-inch Retina MacBook Pro...
B&H Photo has dropped prices on 2014 15″ Retina MacBook Pros by $200. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799.99 save $200 - 15″ 2.5GHz... Read more
With a Mission to Make Mobile Free, Scratch W...
Scratch Wireless, claiming to be the world’s first truly free mobile service, has announced the availability of a new Scratch-enabled Android smartphone, the Coolpad Arise. The smartphone is equipped... Read more

Jobs Board

Payments Counsel, *Apple* Pay (mobile payme...
**Job Summary** Apple is looking for an atto ey to join Apple 's Legal Department to support Apple Pay. **Key Qualifications** 7+ years of relevant experience Read more
Touch Hardware Design and Integration Enginee...
…Summary** Design, develop, and launch next-generation Touch solutions in the new Apple Watch product category. The Touch team develops cutting-edge Touch solutions and Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
*Apple* TV Live Streaming Frameworks Test En...
**Job Summary** Work and contribute towards the engineering of Apple 's state-of-the-art products involving video, audio, and graphics in Interactive Media Group (IMG) at Read more
*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
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.