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

BBEdit 11.6.4 - Powerful text and HTML e...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more
BBEdit 11.6.4 - Powerful text and HTML e...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more
beaTunes 4.6.12 - Organize your music co...
beaTunes is a full-featured music player and organizational tool for music collections. How well organized is your music Library? Are your artists always spelled the same way? Any R.E.M. vs REM?... Read more
Tinderbox 7.0.1 - Store and organize you...
Tinderbox is a personal content management assistant. It stores your notes, ideas, and plans. It can help you organize and understand them. And Tinderbox helps you share ideas through Web journals... Read more
FotoMagico 5.4 - Powerful slideshow crea...
FotoMagico lets you create professional slideshows from your photos and music with just a few, simple mouse clicks. It sports a very clean and intuitive yet powerful user interface. High image... Read more
Direct Mail 4.3.9 - Create and send grea...
Direct Mail is an easy-to-use, fully-featured email marketing app purpose-built for OS X. It lets you create and send great looking email campaigns. Start your newsletter by selecting from a gallery... Read more
Tinderbox 7.0.1 - Store and organize you...
Tinderbox is a personal content management assistant. It stores your notes, ideas, and plans. It can help you organize and understand them. And Tinderbox helps you share ideas through Web journals... Read more
Direct Mail 4.3.9 - Create and send grea...
Direct Mail is an easy-to-use, fully-featured email marketing app purpose-built for OS X. It lets you create and send great looking email campaigns. Start your newsletter by selecting from a gallery... Read more
FotoMagico 5.4 - Powerful slideshow crea...
FotoMagico lets you create professional slideshows from your photos and music with just a few, simple mouse clicks. It sports a very clean and intuitive yet powerful user interface. High image... Read more
beaTunes 4.6.12 - Organize your music co...
beaTunes is a full-featured music player and organizational tool for music collections. How well organized is your music Library? Are your artists always spelled the same way? Any R.E.M. vs REM?... Read more

The best sales on the App Store this wee...
The App Store has quite an exciting lineup of discount games this week that range across a variety of genres. It's a great opportunity to catch up on some of the premium games you may have been holding off on -- and some you can even grab for free... | Read more »
The best new games we played this week
Ah, here we are again at the close of another busy week. Don't rest too easy, though. We had a lot of great new releases in mobile games this week, and now you're going to have to spend all weekend playing them. That shouldn't be too much of a... | Read more »
Rollercoaster Tycoon Touch Guide: How to...
| Read more »
Rabbids Crazy Rush Guide: How to unlock...
The Rabbids are back in a new endless running adventure, Rabbids Crazy Rush. It's more ridiculous cartoon craziness as you help the little furballs gather enough fuel (soda) to get to the moon. Sure, it's a silly idea, but everyone has dreams --... | Read more »
Tavern Guardians (Games)
Tavern Guardians 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Tavern Guardians is a Hack-and-Slash action game played in the style of a match-three. You can experience high pace action... | Read more »
Slay your way to glory in idle RPG Endle...
It’s a golden age for idle games on the mobile market, and those addictive little clickers have a new best friend. South Korean developer Ekkorr released Endless Frontier last year, and players have been idling away the hours in the company of its... | Read more »
Tiny Striker: World Football Guide - How...
| Read more »
Good news everyone! Futurama: Worlds of...
Futurama is finding a new home on mobile in TinyCo and Fox Interactive's new game, Futurama: Worlds of Tomorrow. They're really doing it up, bringing on board Futurama creator Matt Groening along with the original cast and writers. TinyCo wants... | Read more »
MUL.MASH.TAB.BA.GAL.GAL (Games)
MUL.MASH.TAB.BA.GAL.GAL 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: ENDLESS UPGRADES. CONSTANT DANGER. ANCIENT WISDOM. BOUNCY BALLS. Launch Sale, 40% OFF for a very limited time!!! MUL.... | Read more »
Dungeon Rushers (Games)
Dungeon Rushers 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Dungeon Rushers is a 2D tactical RPG combining dungeon crawler’s gameplay and turn based fights. Manage your team, loot dusty... | Read more »

Price Scanner via MacPrices.net

13-inch 2.0GHz Apple MacBook Pros on sale for...
B&H has the non-Touch Bar 13″ 2.0GHz MacBook Pros in stock today and on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.0GHz MacBook Pro Space Gray (... Read more
15-inch Touch Bar MacBook Pros on sale for up...
B&H Photo has the new 2016 15″ Apple Touch Bar MacBook Pros in stock today and on sale for up to $150 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.7GHz Touch Bar... Read more
12-inch Retina MacBooks on sale for $1150, $1...
B&H has 12″ 1.1GHz Retina MacBooks on sale for $150 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 12″ 1.1GHz Space Gray Retina MacBook: $1149 $150 off MSRP - 12″ 1.1GHz... Read more
Apple restocks refurbished 11-inch MacBook Ai...
Apple has Certified Refurbished 11″ MacBook Airs (the latest models recently discontinued by Apple), available for up to $170 off original MSRP. An Apple one-year warranty is included with each... Read more
Apple Park Opens to Employees in April With T...
Apple has announced that Apple Park, the company’s new 175-acre campus, will be ready for employees to begin occupying in April. The process of moving more than 12,000 people will take over six... Read more
Manhattan Neighbors for Safer Telecommunicati...
A new education and advocacy group focused on cell phone and wireless risks, Manhattan Neighbors for Safer Telecommunications, launched today at http://www.ManhattanNeighbors.org. Manhattan... Read more
Portable Dual DisplayPort Monitor Dock Enable...
IOGEAR has announced the launch of its USB-C Dual DisplayPort Monitor Portable Dock (GUC3CMST). The dock enables users to easily connect two DisplayPort monitors to a USB-C or Thunderbolt 3 laptop to... Read more
13-inch 2.7GHz Retina MacBook Pro on sale for...
Amazon.com has restocked the 13″ 2.7GHz/128GB Retina MacBook Pro (MF839LL/A) for $200 off MSRP including free shipping: - 13″ 2.7GHz/128GB Retina MacBook Pro: $1099 $200 off MSRP This model tends to... Read more
Apple’s New iPad Ads Don’t Address Pro Users’...
Apple launched a new tranche of iPad Pro TV ads last week addressing actual queries and challenges from the Twitterverse, albeit using actors for the visuals. That’s great. As an iPad fan and heavy... Read more
Free Verbum Catholic Bible Study App For iOS
The Verbum mobile app runs on Logos’ powerful Bible software and is an advanced resource for mobile Catholic study. The Verbum app surrounds the Bible with the Tradition. Verbum comes with 15 free... Read more

Jobs Board

*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions- Chicago...
SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Manager *Apple* Systems Administration - Pu...
Req ID 3315BR Position Title Manager, Apple Systems Administration Job Description The Manager of Apple Systems Administration oversees the administration and Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Manager *Apple* Systems Administration - Pu...
Req ID 3315BR Position Title Manager, Apple Systems Administration Job Description The Manager of Apple Systems Administration oversees the administration and Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.