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

Latest Forum Discussions

See All

Six fantastic ways to spend National Vid...
As if anyone needed an excuse to play games today, I am about to give you one: it is National Video Games Day. A day for us to play games, like we no doubt do every day. Let’s not look a gift horse in the mouth. Instead, feast your eyes on this... | Read more »
Old School RuneScape players turn out in...
The sheer leap in technological advancements in our lifetime has been mind-blowing. We went from Commodore 64s to VR glasses in what feels like a heartbeat, but more importantly, the internet. It can be a dark mess, but it also brought hundreds of... | Read more »
Today's Best Mobile Game Discounts...
Every day, we pick out a curated list of the best mobile discounts on the App Store and post them here. This list won't be comprehensive, but it every game on it is recommended. Feel free to check out the coverage we did on them in the links below... | Read more »
Nintendo and The Pokémon Company's...
Unless you have been living under a rock, you know that Nintendo has been locked in an epic battle with Pocketpair, creator of the obvious Pokémon rip-off Palworld. Nintendo often resorts to legal retaliation at the drop of a hat, but it seems this... | Read more »
Apple exclusive mobile games don’t make...
If you are a gamer on phones, no doubt you have been as distressed as I am on one huge sticking point: exclusivity. For years, Xbox and PlayStation have done battle, and before this was the Sega Genesis and the Nintendo NES. On console, it makes... | Read more »
Regionally exclusive events make no sens...
Last week, over on our sister site AppSpy, I babbled excitedly about the Pokémon GO Safari Days event. You can get nine Eevees with an explorer hat per day. Or, can you? Specifically, you, reader. Do you have the time or funds to possibly fly for... | Read more »
As Jon Bellamy defends his choice to can...
Back in March, Jagex announced the appointment of a new CEO, Jon Bellamy. Mr Bellamy then decided to almost immediately paint a huge target on his back by cancelling the Runescapes Pride event. This led to widespread condemnation about his perceived... | Read more »
Marvel Contest of Champions adds two mor...
When I saw the latest two Marvel Contest of Champions characters, I scoffed. Mr Knight and Silver Samurai, thought I, they are running out of good choices. Then I realised no, I was being far too cynical. This is one of the things that games do best... | Read more »
Grass is green, and water is wet: Pokémo...
It must be a day that ends in Y, because Pokémon Trading Card Game Pocket has kicked off its Zoroark Drop Event. Here you can get a promo version of another card, and look forward to the next Wonder Pick Event and the next Mass Outbreak that will be... | Read more »
Enter the Gungeon review
It took me a minute to get around to reviewing this game for a couple of very good reasons. The first is that Enter the Gungeon's style of roguelike bullet-hell action is teetering on the edge of being straight-up malicious, which made getting... | Read more »

Price Scanner via MacPrices.net

Take $150 off every Apple 11-inch M3 iPad Air
Amazon is offering a $150 discount on 11-inch M3 WiFi iPad Airs right now. Shipping is free: – 11″ 128GB M3 WiFi iPad Air: $449, $150 off – 11″ 256GB M3 WiFi iPad Air: $549, $150 off – 11″ 512GB M3... Read more
Apple iPad minis back on sale for $100 off MS...
Amazon is offering $100 discounts (up to 20% off) on Apple’s newest 2024 WiFi iPad minis, each with free shipping. These are the lowest prices available for new minis among the Apple retailers we... Read more
Apple’s 16-inch M4 Max MacBook Pros are on sa...
Amazon has 16-inch M4 Max MacBook Pros (Silver and Black colors) on sale for up to $410 off Apple’s MSRP right now. Shipping is free. Be sure to select Amazon as the seller, rather than a third-party... Read more
Red Pocket Mobile is offering a $150 rebate o...
Red Pocket Mobile has new Apple iPhone 17’s on sale for $150 off MSRP when you switch and open up a new line of service. Red Pocket Mobile is a nationwide MVNO using all the major wireless carrier... Read more
Switch to Verizon, and get any iPhone 16 for...
With yesterday’s introduction of the new iPhone 17 models, Verizon responded by running “on us” promos across much of the iPhone 16 lineup: iPhone 16 and 16 Plus show as $0/mo for 36 months with bill... Read more
Here is a summary of the new features in Appl...
Apple’s September 2025 event introduced major updates across its most popular product lines, focusing on health, performance, and design breakthroughs. The AirPods Pro 3 now feature best-in-class... Read more
Apple’s Smartphone Lineup Could Use A Touch o...
COMMENTARY – Whatever happened to the old adage, “less is more”? Apple’s smartphone lineup. — which is due for its annual refresh either this month or next (possibly at an Apple Event on September 9... Read more
Take $50 off every 11th-generation A16 WiFi i...
Amazon has Apple’s 11th-generation A16 WiFi iPads in stock on sale for $50 off MSRP right now. Shipping is free: – 11″ 11th-generation 128GB WiFi iPads: $299 $50 off MSRP – 11″ 11th-generation 256GB... Read more
Sunday Sale: 14-inch M4 MacBook Pros for up t...
Don’t pay full price! Amazon has Apple’s 14-inch M4 MacBook Pros (Silver and Black colors) on sale for up to $220 off MSRP right now. Shipping is free. Be sure to select Amazon as the seller, rather... Read more
Mac mini with M4 Pro CPU back on sale for $12...
B&H Photo has Apple’s Mac mini with the M4 Pro CPU back on sale for $1259, $140 off MSRP. B&H offers free 1-2 day shipping to most US addresses: – Mac mini M4 Pro CPU (24GB/512GB): $1259, $... Read more

Jobs Board

All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.