TweetFollow Us on Twitter

Mac in the Shell: The Difference Engine

Volume Number: 23 (2007)
Issue Number: 04
Column Tag: Mac in the Shell

Mac in the Shell: The Difference Engine

File differences for non-programmers

by Edward Marczak

Introduction

Far too often, SysAdmins make changes to files, and then are unsure which changes they've made. Or, for people investigating a system for the first time, files may be customized and you need to know how configuration files have been changed from a baseline. There's the venerable Unix tool diff -- probably the most widely used -- to help us find those answers. However, more and more GUI tools have nice ways to point out differences. This month, we look at ways to 'tell the difference'.

Genesis

diff is one of the older Unix utilities around. Originally intended for programmers to find differences in lines of code, it has come to be more generically useful. It can even be used to compare binary files and entire directories of files. Furthermore, a complementary program, patch, can provide an easy way to describe and pass those changes on to others that may require them.

While I extol the virtues of life in the shell, there are just times that a GUI tool works better (OK, emacs users, don't get your knickers in a twist). Most text editors have a way to diff files, and Apple supplies a fantastic differencing tool as part of the Developer Tools install. Let's look at a basic scenario.

Despite the nice GUI that Apple gives us for many tasks, as system administrators, there are still many text files that we need to touch to tap into the full power of the system. Postfix is one subsystem that comes to mind. It has many options and incredible features that are not exposed through the GUI. All of postfix's configuration files live in /etc/postfix, and they're all text (even though they may later be converted to binary or hash files for speed).

So, you start tweaking. And tweaking. And tweaking... and then something breaks. Postfix won't start, or, it does, but stops delivering mail. So, you reach for your backup and get things running again. However, you were tweaking for a reason. You still want to make some change that alters the behavior of postfix. But where did the wheels come off? diff to the rescue! Comparing your updated file with the working version should allow you to see what happened. Let's say, for arguments sake, you were updating the /etc/postfix/main.cf file. Your updated file is now named /etc/postfix/main.cf.updated, and the working version is back in place at /etc/postfix/main.cf.

Easy! Just change into the /etc/postfix directory, and run diff:

diff main.cf.updated main.cf

You'll get some output like this:

398c398
< recipient_delimiter = +
---
> #recipient_delimiter = +

Soooooo... what is that all about? The initial command without options simply compares the two files that you specify. By the way, the OS X man page for diff lists the invocation as "diff [OPTION]... FILES", and the BSD man page similarly states "diff [options] from-file to-file". Some other instructions specifically call the files to be compared "old-file" and "new-file." Better would simply be "left-file" and "right-file." Personally, I always like the newer file on the left (however, see further down where you cannot do this). On to the output: The first line tells you which line(s) would need to be altered to make the files match. In this case, we're told that there's a change ("c") needed on line 398. The "<" points to which file has the differing line, the left ("<") or right (">"). So, we can quickly see that the left file uncommented the recipient delimiter line.

Besides a change, you can also be notified that an add (a) or delete (d) will be required to make the files the same.

So, this is all well and good, but would make for a heck of a short column if that were the end of the story. As the files being compared get longer, and the changes increase, this kind of output can get a little tough to look at. Additionally, the complementary tool to diff is patch, and patch parses a slightly different kind of input: the universal diff. Before we get to that, though, let's look at an option to make diff a little easier on the eyes.

While the in-line view can be useful for a quick idea of what's different, in short files, with few changes, a side-by-side comparison is much more natural. With the -y flag, diff can provide just that. This also has the effect of displaying the entire file, so, piping the output to less is highly recommended (if you're viewing on screen):

$ diff -y -pr -W 70 main.cf.updated main.cf
# Global Postfix configuration   # Global Postfix configuration
# of all 300+ parameters. See   # of all 300+ parameters. See 
# complete list.   # complete list.
...
recipient_delimiter = +    |   #recipient_delimiter = +
...
disable_vrfy_command =     <
smtpd_helo_required = yes  <
(much output snipped for brevity)

From this example, you can see that instead of a single line that describes the modification, you get to see both files side-by-side, and a center column that describes the alteration needed. The pipe ("|") signifies a change, while the greater-than and less-than "pointers" point to a line that exists only in the left-hand file ("<") or right-hand file (">"). Please note that I only used the -W flag to fit the output for print. "W" restricts the maximum width of the output. In other cases, I wouldn't use it at all, as I like to see as much of the line as possible.

Proverbs

diff is such a popular and useful tool that several variations and extensions have cropped up over time, and I'll just touch on some here.

diffstat will output statistics about the differences found. Easier to see in action than describe:

$ diff -u main.cf.updated main.cf | diffstat
 main.cf |   46 +++++++++++++++-------------------------------
 1 files changed, 15 insertions(+), 31 deletions(-)

The output shows the file name, total number of changes to that file, a histogram and summary. Run in batch over many files, particularly source code, the overview provided is incredibly useful.

diff3 extends the concept to three files. This turns out to be a topic unto itself. However, the short version is that diff3 diffs the output of "diff file1 file2" with "diff file1 file3". Check the man page for its options.

bzdiff is really just a convenient way to diff bzip compressed files. All options are passed directly to diff--bzdiff just drops the manual steps needed to compare these files.

If you like printed output, try diffpp (pretty print) as a postscript filter:

enscript -G2re -p maindiffs.ps --filter='diff main.cf.updated main.cf | diffpp main.cf' main.cf.updated

then...

open maindiffs.ps

...and you'll have Preview showing you a beautiful "printed" version of the diff file. You can then save the file from Preview as a PDF.

Also probably a column unto itself, don't forget emacs and vim as diff tools. For a very nice color-output diff, try vimdiff:

vimdiff main.cf.updated main.cf

...gives you the output shown in figure 1.


Figure 1 -- Using vimdiff to highlight differences.

Last, but certainly not least, is a utility that is not "diff-derived", but rather a complementary tool. Mentioned earlier, patch will take a unified diff file and apply it to another file ("patch" it), making the appropriate changes.

Back to our example: main.cf. Let's say you've taken the stock OS X Server postfix main.cf file and altered it to add lines that help fight UBE (Unsolicited Bulk E-mail, a.k.a. "spam"). You have another OS X Server that could use the same exact alterations made to its main.cf. The usage is pretty simple. First, make the appropriate diff file:

diff -u main.cf main.cf.updated > maindiffs.diff

Important: unlike viewing only, creating a diff file for patch is a case where you must have the "from-file" (the old) on the left, and the "to-file" (the new, updated file) on the right. Then, ship the diff file to the other machine. To automatically alter the other file, use patch:

patch /etc/postfix/main.cf < maindiffs.diff

The cool thing is that the diff file is simply text. If you ever have multiple patches that you'd like to apply at once, simply combine them and then apply:

cat DiffFile1 DiffFile2 DiffFile3 > one_big_diff_file.diff

Make your life as easy as possible!

You can even use diff and patch to compare and/or patch entire directories. Let's say you've stored the distribution set of postfix config files in /etc/postfix.dist. You've continued to make changes to many files in /etc/postfix. Now, of course, you want to apply these same changes to another server. Make your diff file:

diff -ruN /etc/postfix.dist /etc/postfix > postfix_cfg.patch

Then, on the second machine you can simply:

cd /etc/postfix
patch < /path/to/postfix_cfg.patch

Done! The entire directory, including subdirectories will be updated.

Exodus

Now, while the diff shell tool solves 90% of my needs, there are some other very nice tools out there. There happen to be more than I can cover here, so, I'm going to touch on three that are popular and/or free.

FileMerge

The nicest GUI tool I've seen comes from Apple. FileMerge is installed as part of the developer tools, and can be found in the /Developer/Applications/Utilities folder. Running FileMerge.app brings up a simplistic window, as seen in figure 2.


Figure 2 -- Initial FileMerge window.

Selecting the files to compare is as easy as dragging-and-dropping the appropriate files on the appropriate panel -- or, you can, of course, use the "Left..." and "Right..." buttons to browse for the files you want. Once your files are selected, click on the "Compare" button and you'll get a three-paned window that looks like that in figure 3.


Figure 3 -- FileMerge.app in action.

Blocks of differences are shaded in grey, and are 'warped' to fit the difference. The shaded blocks are flexible and bend as needed as you scroll through the documents. It's actually an effect that you have to see in action to really appreciate. Another nice touch is the marking in the scroll bar that denotes where changes appear.

To merge changes into a new document, simply select a change by clicking on a grey block, and then choose the appropriate action from the "Actions" drop-down menu. Once you're satisfied with the file that you have in front of you, save it to a new document by choosing "Save Merge" from the File menu (or just Apple-s).


Figure 4 -- FileMerge actions

Like all good utilities, FileMerge can also be called via a shell. Using the opendiff binary simply launches FileMerge itself, but is handy when you're already in a shell, or as a way to integrate FileMerge with another GUI tool that can call shell apps.

TextMate

TextMate, from MacroMates -- basically Allan Odgaard -- is a text editor that can best be compared to emacs or the old DOS Brief Editor (which, was written by Dave Nanian who now runs Shirt Pocket Software and blesses us with SuperDuper!). That's all to say, TextMate unto itself could take up an article...or two...or a book! In fact, a book was just released. More than a text editor, TextMate is a programmer's editor. For anyone banging out code all day, or those trying to learn, it's an excellent tool. As such, it can perform your standard diff and patch operations. In short, it calls the shell tools listed above to perform its magic. Why re-invent the wheel, right? Nicely, you can perform the diff with files in several locations -- even the clipboard, as seen in figure 5.


Figure 5 -- TextMate's diff options

In this example, I have two files loaded into TextMate itself, and simply chose "Selected Files in Project Drawer". That gave me the standard, unified diff output shown in figure 6.


Figure 6 -- TextMate's colored unified diff output

There's a reason Allen and TextMate won an Apple Design Award at last year's WWDC (2006). TextMate can even diff a file against itself since it was first loaded so you can see the changes you've made in a single file (details in the TextMate blog: http://macromates.com/blog/archives/2005/08/11/tips-and-tricks/). With its plug-in architecture, TextMate is beginning to be used for many things, even as a blogging tool. It's a free trial download at http://www.macromates.com, so, check it out.

TextWrangler

I think BareBones Software surprised a lot of people when they shipped the free (as in beer) TextWrangler text editor. It took over for BBEdit Lite as BBEdit's little brother...except free. It's one of those apps that continually impresses, and its diff capabilities don't disappoint. You can compare files that you have loaded into TextWrangler, or any files on disk (or even files that you have loaded from a remote server -- very handy!). Start the diff from the Search menu, and you'll be shown three windows. One window for each file you're comparing, side by side, and a differences window. The differences window, shown in figure 7, keeps the two file windows in sync as you highlight changes.


Figure 7 -- TextWrangler's differences window.

You can apply the changes from one file to the other via the "apply" buttons, or via keyboard shortcuts.

TextWrangler's implementation is a little FileMerge-like visually, but has its own way of altering the files. Overall, TextWrangler is a valuable tool in any OS X user's arsenal, and it doesn't cost you a dime. Find it at http://www.barebones.com.

Revelation

Even though difference utilities started out as a programmer's tool, that doesn't mean that there are no other ways to apply them. I skipped over some other comparison tools such as cmp and comm, as diff is really the most useful for general use -- particularly for SysAdmins. I've even found people using diff for natural language processing: http://dspace.wul.waseda.ac.jp/dspace/bitstream/2065/585/1/interactive-11.pdf. Don't let the 'text' moniker fool you either; you can use these comparison tools on binary files as well. Check the man pages for each, or the documentation/help for the GUI tools and you'll find even more power to exploit.

Media of the month: Ghost in the Shell. How is it that I've never recommended this one to you before? Fantastic animation, a story that's ahead of its time, and a major influence on "The Matrix," it's a must see for anyone involved in tech culture.

WWDC is nigh! Unfortunately for me, I won't be able to stay the entire week, so I'm trying to cram as much into the front as possible. For those of you headed to the conference -- and that should be just about everyone reading this magazine -- I hope to see you there!


Ed Marczak owns and operates Radiotope, a technology consultancy that brings enterprise solutions to small and medium-sized businesses. Outside of this piece of the puzzle, he is Executive Editor of MacTech Magazine, a husband and father, and CTO of WheresSpot, among other things. Find the missing tech piece at www.radiotope.com.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Adium 1.5.10.3 - Popular instant messagi...
Adium is a fast and free instant messaging client which supports AIM, ICQ, Jabber, MSN, Yahoo!, Google Talk, Yahoo! Japan, Bonjour, Gadu-Gadu, Novell Groupwise, SIP/SIMPLE (Text), and Lotus Sametime... Read more
CleanMyMac 3.8.1 - $39.95
CleanMyMac makes space for the things you love. Sporting a range of ingenious new features, CleanMyMac lets you safely and intelligently scan and clean your entire system, delete large, unused files... Read more
BusyCal 3.1.7 - Powerful calendar app wi...
BusyCal is an award-winning desktop calendar that combines personal productivity features for individuals with powerful calendar sharing capabilities for families and workgroups. Its unique features... Read more
Lyn 1.8.9 - Lightweight image browser an...
Lyn is a fast, lightweight image browser and viewer designed for photographers, graphic artists, and Web designers. Featuring an extremely versatile and aesthetically pleasing interface, it delivers... Read more
Tweetbot 2.5 - Popular Twitter client.
Tweetbot is a full-featured OS X Twitter client with a lot of personality. Whether it's the meticulously-crafted interface, sounds and animation, or features like multiple timelines and column views... Read more
Monolingual 1.7.8 - Remove unwanted OS X...
Monolingual is a program for removing unnecesary language resources from OS X, in order to reclaim several hundred megabytes of disk space. If you use your computer in only one (human) language, you... Read more
Dash 4.0.3 - Instant search and offline...
Dash is an API documentation browser and code snippet manager. Dash helps you store snippets of code, as well as instantly search and browse documentation for almost any API you might use (for a full... Read more
Posterino 3.3.6 - Create posters, collag...
Posterino offers enhanced customization and flexibility including a variety of new, stylish templates featuring grids of identical or odd-sized image boxes. You can customize the size and shape of... Read more
Apple Numbers 4.1.1 - 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
Apple Pages 6.1.1 - Apple's word pr...
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

Latest Forum Discussions

See All

Die With Glory (Games)
Die With Glory 1.2.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.2.0 (iTunes) Description: Die with Glory is an epic adventure game where your goal is to die in glorious fashion. You must help Sigurd, a brave old... | Read more »
Get Ike in the new Fire Emblem: Heroes u...
One of the most popular Fire Emblem characters is finally available in a new update to Nintendo'sFire Emblem: Heroes. [Read more] | Read more »
Die With Glory in a new viking adventure...
If you're a fan of classic adventure games you'll do well to pick upDie With Glory, the gorgeous new title from Cloud Castle inc. Die With Glory updatesthe gameplay of the same kind of adventure classics such asMonkey Island for modern, mobile... | Read more »
Get up to speed with everything you need...
In case you haven’t heard, MU Origin just got a colossal new update with new all-server events, battle modes, and systems making their way to the land of MU. Here’s a handy guide to everything you need to know about the latest content. [Read... | Read more »
Minimalist puzzle game, Cuts, free on iO...
If you're looking for a gorgeous puzzle experience on iOS devices, developer Gamebra.in's aesthetically interesting puzzler, Cuts, is discounted to free on the iOS App Store right now. [Read more] | Read more »
Anime tactical RPG, War of Crown, comes...
If you're looking for another tactical RPG fix to go alongside your Fire Emblem Heroes campaigns check out Gamevil's newest, anime-inspired tactics RPG, War of Crown, which comes out tomorrow. [Read more] | Read more »
Fantasy MMORPG MU Origin adds new modes,...
MU Origin, Webzen’s highly popular fantasy MMORPG is getting ready to shake things up for the second time this year, as a new update makes its way to the Google Play and App Store from today. Introducing new systems, modes, and events, the land of... | Read more »
Blizzard is looking to hire a mobile dev...
A new thread on the popular video game rumor forum, NeoGAF, uncovered an interesting job listing over at Blizzard Entertainment. It appears the studio behindStarCraft, World of WarCraft, Hearthstone,andOverwatch is looking to bring on a new hire... | Read more »
Legend of Zelda meets Cooking Mama in ne...
Dungeon Chef is what happens when you mix the RPG elements (and style) of a Legend of Zelda game, with cooking elements. Although, now that The Legend of Zelda: Breath of the Wild also has cookingelements, so maybe the gameplay is not so novel.... | Read more »
ChordFlow (Music)
ChordFlow 1.0.0 Device: iOS Universal Category: Music Price: $6.99, Version: 1.0.0 (iTunes) Description: ChordFlow is a chord sequencer with a unique 4-track polyphonic arpeggiator, extensive chord library, MIDI out and Ableton Link... | Read more »

Price Scanner via MacPrices.net

Digital Paper Tablet Offers Distraction Free...
I typically spend 8-10 hours a day gazing at the screens in my laptops and iPad, as tools of my livelihood, I don’t as a rule use electronic devices for pleasure reading. I subscribe to a daily... Read more
“Today at Apple” Bringing New Educational Ses...
Apple has announced plans to launch dozens of new educational sessions next month in all 495 Apple Stores ranging in topics from photo and video to music, coding, art and design, and more. The hands-... Read more
Smart Finance Free Comprehensive Personal Fin...
Moscow-based indie developer, Alexander Survillo has announced the release and immediate availability of Smart Finance: Personal Finance, Budget & Money 1.1.4, an update to his comprehensive... Read more
12-inch 1.1GHz Retina MacBooks on sale for $1...
B&H has 12″ 1.1GHz Retina MacBooks on sale for $100 off MSRP. Shipping is free, and B&H charges NY & NJ sales tax only: - 12″ 1.1GHz Space Gray Retina MacBook: $1199.99 $100 off MSRP - 12... Read more
13-inch 2.7GHz Retina MacBook Pro on sale for...
B&H Photo has the 13″ 2.7GHz Retina MacBook Pro on sale for $130 off MSRP. Shipping is free, and B&H charges NY & NJ tax only: - 13″ 2.7GHz/128GB Retina MacBook Pro (MF839LL/A): $1169 $... Read more
15-inch 2.2GHz Retina MacBook Pros available...
B&H Photo has the 15″ 2.2GHz Retina MacBook Pro available for $200 off MSRP including free shipping plus NY & NJ sales tax only: - 15″ 2.2GHz Retina MacBook Pro (MJLQ2LL/A): $1799.99 $200 off... Read more
13-inch Touch Bar MacBook Pros on sale for up...
B&H Photo has the 2016 Apple 13″ Touch Bar MacBook Pros in stock today for up to $150 off MSRP. Shipping is free, and B&H charges NY & NJ sales tax only: - 13″ 2.9GHz/512GB Touch Bar... Read more
Apple refurbished Apple TVs available for up...
Apple has Certified Refurbished 32GB and 64GB Apple TVs available for up to $30 off the cost of new models. Apple’s standard one-year warranty is included with each model, and shipping is free: -... Read more
12-inch 1.2GHz Retina MacBooks on sale for up...
B&H has 12″ 1.2GHz Retina MacBooks on sale for up to $160 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 12″ 1.2GHz Space Gray Retina MacBook: $1439.99 $160 off MSRP - 12″ 1... Read more
HyperX Ships Pulsefire FPS Gaming Mouse, Winn...
Your reporter is a longtime fan of gaming mice for general purpose coomnputing use, finding them typically superior in comfort and performance. HyperX, a division of Kingston Technology Company, Inc... Read more

Jobs Board

*Apple* Mac Computer Technician - GeekHampto...
…complex computer issues over the phone and in person? GeekHampton, Long Island's Apple Premium Service Provider, is looking for you! Come work with our crew Read more
Product Manager, *Apple* Platforms - Viacom...
…Product Manager to drive the execution of its iOS and AppleTV experiences. The Apple Platform Product Manager will be a leader in our Agile/Scrum environment and Read more
Geek Squad *Apple* Master Consultation Agen...
**500662BR** **Job Title:** Geek Squad Apple Master Consultation Agent **Location Number:** 000286-Canton-Store **Job Description:** **What does a Geek Squad Read more
*Apple* Mobile Master - Best Buy (United Sta...
**500710BR** **Job Title:** Apple Mobile Master **Location Number:** 000279-North Olmsted-Store **Job Description:** **What does a Best Buy Apple Mobile Master Read more
*Apple* Engineering Specialist - CSRA (Unite...
Apple Engineering Specialist All times are in Eastern Daylight Time Requisition ID Job Locations US DC Washington DC Posted Date Category Engineering Sciences Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.