TweetFollow Us on Twitter

awk for Data Processing

Volume Number: 22 (2006)
Issue Number: 3
Column Tag: Programming

Mac In The Shell

awk for Data Processing

by Edward Marczak

The Complementary Pattern Processor to sed.

sed and awk are typically mentioned in the same sentence. They both have their own strengths and areas where they are most effective. The past few columns have walked though the power of sed, and I hope everyone has put sed into practice. If sed is so great, why do we need awk? sed is a non-interactive editor. It's powerful for unstructured data, and picking out patterns, and making changes in that data. awk excels at pulling, manipulating fields in structured data, and generating output formatted as you specify. You'll encounter both types of data as you work, and now you'll have the best, and most appropriate tools. How can awk help us?

History...Again

When I took one of my very first computer classes, in 7th grade or so, I remember the teacher launching into the history of computing. What?!? History? When are we going to sit down and start typing? Nowadays, I find myself launching into history quite a bit as I write these columns. The benefit is that it frames the present so nicely. A place we couldn't be now without that history. This is a long-winded way of saying I'm going to describe a little bit about the history of awk!

awk appeared in Bell Labs Unix V7 - roughly 1977 - and has been part of the standard distribution since. However, there have been a few revisions and versions of awk. Sometimes, these well-meaning versions have extended awk a little here and there. In 1985, the original authors officially revised the language. I can't possibly cover each and every facet of non-standard awk versions. Since this is MacTech, I'm going to cover Lucent awk, version 20040207, the version distributed with OS X, 10.4. This is the version of awk described in "The AWK Programming Language", 1988, by Al Aho, Peter Weinberger, and Brian Kernighan. (Do we see where the name "awk" comes from now?) Be aware that this version matches the POSIX standard of awk. It does not have every one of the extensions that have shown up over the years.

What is it?

The man page for awk says that it is a "pattern-directed scanning and processing language." The first thing to note is that it is a 'real' programming language, with structure. We've seen flow control and looping in bash and sed before. On a basic level, awk auto-constructs the main loop for you: it loops around each line of input. When it reaches EOF, the loop is broken. Like my Calculus I professor used to drill, "You have to know the rules!" Same goes for any programming language. However, rather than launch into a terse description, let's get right to some examples.

Awk me!

Here's an easy one:

$ awk '{print "Got a line"}' some_file.txt

This will print "Got a line" for each line in some_file.txt - there's that loop. This script has one action: run a print statement for each line of input. Besides running awk against a file, you can also pipe data in. Unlike sed, awk does not print input by default. So, to emulate cat, you could simply:

$ awk '{print}' some_file.txt

or

$ ls -l | awk '{print}'

(but using this to emulate cat would be silly). Again, awk really shines when operating on data with a structure. Comma, tab and other delimited formats are ideal - those have obvious structure. However, with enough practice, you'll start to see structure in non-obvious places.

For anyone that really dug into the sed columns, awk's pattern matching will look very familiar:

$ ls -l | awk '/pcap/ {print}'

We pipe the output of 'ls -l' into awk, where awk will jump into action each time it finds a line with 'pcap' on it. Well, we could have done that with 'ls -l *pcap*', right? Well, yes - but stay with me here. What if we didn't want all of the information that comes with 'ls -l'? Or, perhaps, if we wanted to rearrange that info? The output of ls, with the "-l" switch, happens to be very structured. Let's look at a snippet:

drwxr-x---   5 marczak  marczak  170 Jan 18 17:07 tmp
-rw-r-----    1 marczak  marczak  149 Oct 10 15:17 tw.png
-rw-r-----    1 root     marczak 3114 Nov  8 20:00 ts05.pcap

awk will refer to each of the columns as fields - just like a database. The permissions column is field 1, links column is field 2, and so on, up to field 9, in this example, being the file name. If we wanted to rearrange an 'ls' listing, we could use this:

$ ls -l | awk '/pcap/ {print $9,$5,$1}'
cramdump.pcap 15151 -rw-r-----
dhcp.pcap 16422 -rw-r-----
skypecatch.pcap 43421 -rw-r-----
ssldump.pcap 26070 -rw-r-----
testdump.pcap 12716 -rw-r-----
tsnow.pcap 391174 -rw-r-----

This example combines pattern-matching and the field operator. Again, the output of ls is piped to awk, which only acts when the input line matches "pcap". However, we decide to selectively output only the ninth, fifth and first fields.

Further into the Warren

With sed, we saw that it was good practice to create your script in a separate file - especially if it was a particularly complex script. awk can do the same using the '-f' switch. More conventionally, you may find long awk scripts written like a shell script, utilizing the 'she-bang' notation - #!/usr/bin/awk. Just remember to mark the script executable if you do this.

Another important practice, as pointed out with sed, is to comment your script! With awk, it turns out to be even more important, as you should document the expected input format along with code comments. Any routine that relies on structured data is fragile. When the data isn't perfect, it shatters into a million pieces. So, if you're processing a tab-delimited file, you might start your script with these comments:

# thinner.awk
# Remove un-needed data before injecting into mailing database
# Input: tab delimited file with layout:
# first_name, last_name, phone_num, shoe_size, e-mail, e-mail2, favorite_color

This way, when, three years later, the script stops working the way you'd expect, you can compare the input file against what you need.

awk has some built-in variables that help you move data around. You've seen the field operator - $ - which, I should note, starts at 1. I mean, the first field is actually numbered "1". What happened to programmers counting from zero? The field $0 refers to the entire line of input. A useful built-in that goes along with the field operators is NF.

NF references the number of fields on the current line. A side-effect is that NF will always refer to the last field (or, 'column'). We could rewrite the file listing example above like this:

ls -l | awk '/pcap/ {print $NF,$5,$1}'

Another important built-in is FS - field separator. Let's look at a very practical OS X use for awk - but we'll need to combine a few concepts to get there. By default, FS is set to a space character. As lines come into awk for processing, it splits up fields by string. Unfortunately, this means that a record reading "Name: Catherine O'Hara" is three fields, not two (of course, it's even worse for "James T. Kirk"). You can leave FS alone, making awk split based on a space character. You can also set FS to be any other single character, such as a comma - obviously useful for a CSV file. Finally, you can use a regexp and match multiple characters as a separator.

In addition to pattern matching to find data to process, awk supports two structures that allow for setup and tear-down (aka pre-processing and post-processing). The BEGIN structure runs before any lines are read in. This is ideal for setting variable states before diving in. The END structure runs after all input is processed, and is naturally useful for summing things up. BEGIN is a perfect place to set FS, although FS can even be changed while the script is running.

So, you're running OS X Server, and want to know who's logged on via AFP. awk to the rescue! Run this:

serveradmin command afp:command = getConnectedUsers | awk 'BEGIN {FS = "="} /name/ { print $NF }'

The output of serveradmin is fed to awk, which sets FS to the equal sign in a BEGIN structure. This simply splits the line in two, based on the input. Then we go on to look for 'name' records, and print out the last field using NF. Let's say that you just wanted to find out if one particular user is connected. awk will let you test a field for a match with the tilde operator ("~"). So, if we're only interested in finding out if "jane" was connected via afp, we can easily do this:

serveradmin command afp:command = getConnectedUsers | awk 'BEGIN {FS = "="} $2 ~ 
   /jane/ { print "Jane is connected!" }'

Of course, you can match any regular expression this way. (didn't I tell you learning regexp would let you rule the universe?) You can invert the tilde match with an exclamation point:

awk $2 !~ /barrel/ { print "Not a barrel" }

La Regle du Jeu

I mentioned some rules earlier. What are they, and how does that help us? Like sed, awk processes input in a very specific way.

By default, each incoming line is broken into fields, separated by a space. Lines ("records") are separated by a newline. An awk script is a set of pattern matching rules and actions, with the format:

pattern {action}

Patterns can be one of:

    A regular expression

    A relational expression

    BEGIN

    END

    A pattern range.

The BEGIN pattern runs its action before the first line of input is read. The END pattern runs its action after the last line of input is read and acted upon.

Some other rules about processing: A missing action defaults to "print". A missing pattern always matches. Program lines are terminated by a semicolon or newline. Comments begin with "#" and are not treated as statements. Comments do not need to start at column 1, and will continue until a newline is reached.

If you're thinking, "Hey! awk is pretty powerful and simple!" you'd be right. Like many Unix utilities it focuses on one thing, and does it really, really well. In some ways, it's only as complex as you make it. Of course, I've only laid out a fraction of awk's abilities. One more before I leave off.

Variables and Equations

Like every programming language, awk supports variables, and operations on those variables. Variables are case sensitive, but do not need to be declared or initialized. Like PHP, this allows variables to be loosely typed, and awk will choose the context automatically. The following examples do what you'd expect:

x = 7
y = x+3
a = "Hello, world"
z = $1   # assign the first field to z
print "z = " z
print "a contains " a
print "x = " x

Pretty straight-forward. Variables can be used in the pattern portion of a rule. How about a short example?

BEGIN { FS=":"; x=0 }
$2 ~ /Miguel/ { x = x  + 1 }
END { print "Miguel appears " x " times in the data." }

This fictitious example adds one to "x" for every time that the second field matches /Miguel/. If I claim that variables don't need to be initialized, why did I in this example? Because the auto-typing can sometimes trip you up. If "x" is not initialized, and there are no matches, awk assumes that, due to the context, that "x" is a string. This results in the message, "Miguel appears times in the data." And that's just not very friendly, is it?

In Summary...

Glad I didn't try to rush awk into last month's column. The more that you use both sed and awk, the more you see patterns in data, and tend to go back to these utilities. Despite being created in a time when personal computers (or even larger systems) didn't have their own SQL server running locally, or a powerful spreadsheet program at their disposal, sed and awk still have tremendous usefulness. Next month, I'm going to round out a little more about awk, and tie it into OS X.

Speaking of last month's column, I missed it then, but now realize that it marked "Mac in the Shell's" one-year anniversary! I need to thank David Sobsey, Neil Ticktin, and everyone at the magazine for getting me involved, spurring me along, and keeping me interested. Oh, and Dennis - I loved last month's cover! So, I raise my virtual glass in toast to another great year of MacTech! Cheers!

Finally, now that the dust has settled from MacWorld, I do want to say it was a pleasure meeting with many, many MacTech readers! As always, please feel free to comment, suggest and ask questions. See you next month.


Ed Marczak owns and operates Radiotope, a technology consulting company. More tech tips at the blog: http://www.radiotope.com/writing

 
AAPL
$106.98
Apple Inc.
-0.36
MSFT
$46.05
Microsoft Corpora
-0.57
GOOG
$550.31
Google Inc.
+0.98

MacTech Search:
Community Search:

Software Updates via MacUpdate

Cocktail 8.0.1 - General maintenance and...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
LibreOffice 4.3.3.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
VMware Fusion 7.0.1 - Run Windows apps a...
VMware Fusion allows you to create a Virtual Machine on your Mac and run Windows (including Windows 8.1) and Windows software on your Mac. Run your favorite Windows applications alongside Mac... Read more
OneNote 15.3.2 - Free digital notebook f...
OneNote is your very own digital notebook. With OneNote, you can capture that flash of genius, that moment of inspiration, or that list of errands that's too important to forget. Whether you're at... Read more
Audio Hijack Pro 2.11.4 - Record and enh...
Audio Hijack Pro drastically changes the way you use audio on your computer, giving you the freedom to listen to audio when you want and how you want. Record and enhance any audio with Audio Hijack... Read more
Iridient Developer 3.0.0 beta 3 - Powerf...
Iridient Developer (was RAW Developer) is a powerful image conversion application designed specifically for OS X. Iridient Developer gives advanced photographers total control over every aspect of... Read more
TextWrangler 4.5.11 - Free general purpo...
TextWrangler is the powerful general purpose text editor, and Unix and server administrator's tool. Oh, and also, like the best things in life, it's free. TextWrangler is the "little brother" to... Read more
NeoFinder 6.6 - Catalog your external me...
NeoFinder (formerly CDFinder) rapidly organizes your data, either on external or internal disks, or any other volumes. It catalogs all your data, so you stay in control of your data archive or disk... Read more
Chromium 38.0.2125.111 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. FreeSMUG-Free OpenSource Mac User Group build is... Read more
Default Folder X 4.6.11 - Enhances Open...
Default Folder X attaches a toolbar to the right side of the Open and Save dialogs in any OS X-native application. The toolbar gives you fast access to various folders and commands. You just click... Read more

Latest Forum Discussions

See All

SAS: Zombie Assault 4 Review
SAS: Zombie Assault 4 Review By Jennifer Allen on October 30th, 2014 Our Rating: :: FLAWED SHOOTERUniversal App - Designed for iPhone and iPad Shoot everything that moves in this fun, if flawed, twin-stick shooter.   | Read more »
Naailde the Witch Review
Naailde the Witch Review By Amy Solomon on October 30th, 2014 Our Rating: :: PITCH-PERFECT STORYTELLINGUniversal App - Designed for iPhone and iPad Marvelous storytelling, narration, and moving illustrations make this storybook... | Read more »
1st & Goal Review
1st & Goal Review By Andrew Fisher on October 30th, 2014 Our Rating: :: FOR THE D&D LOVING QBUniversal App - Designed for iPhone and iPad 1st & Goal is a board gamer’s football game, a football fan’s board game, and... | Read more »
French Developer Pated Unveils Seashine
French Developer Pated Unveils Seashine Posted by Ellis Spice on October 30th, 2014 [ permalink ] French one-man studio Pated has unveiled Seashine, “a poetic journey into the abyss.” Players take on the role of a jellyfish strugglin | Read more »
Agents of Storm: Tips, Tricks, and Strat...
Calling all agents: Would you like to see what we thought of this rather pretty base builder? Check out our Agents of Storm review! Have you downloaded Agents of Storm, been bowled over by the graphics, and aren’t quite sure what to do next? Never... | Read more »
Any.DO 2.0 Hopes to Help Manage Producti...
Any.DO 2.0 Hopes to Help Manage Productivity Posted by Ellis Spice on October 30th, 2014 [ permalink ] iPhone App - Designed for the iPhone, compatible with the iPad | Read more »
Base Busters Review
Base Busters Review By Jennifer Allen on October 30th, 2014 Our Rating: :: FUN BUT RESTRICTED MIXUniversal App - Designed for iPhone and iPad Mixing up two forms of tower defense gaming and collectible cards, Base Busters is a fun... | Read more »
Sumptus - Personal Expense Tracker in Yo...
Sumptus - Personal Expense Tracker in Your Pocket 1.0 Device: iOS iPhone Category: Finance Price: $3.99, Version: 1.0 (iTunes) Description: Sumptus - a personal finance assistant in your pocket. It’s an ultimate expense tracker... | Read more »
Meat Factory (Games)
Meat Factory 0.1 Device: iOS Universal Category: Games Price: $.99, Version: 0.1 (iTunes) Description: Meat factory harks back to the old days of the game and watch. Easy to pick up and play, but hard to score big. With cute visuals... | Read more »
Medford Asylum: Paranormal Case - Hidden...
Medford Asylum: Paranormal Case - Hidden Object Adventure (Full) 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Uncover the mystery of Medford City asylum! | Read more »

Price Scanner via MacPrices.net

Apple Regains Momentum As Windows Stutters An...
The latest smartphone sales data from Kantar Worldpanel ComTech, for the three months to March 2014, shows Apple performing strongly in the first quarter of the year, with sales bouncing back in... Read more
Worldwide Smartphone Shipments Increase 25.2%...
New smartphone releases and an increased emphasis on emerging markets drove global smartphone shipments above 300 million units for the second consecutive quarter, according to preliminary data from... Read more
Apple now offering refurbished 2014 15-inch M...
The Apple Store is now offering Apple Certified Refurbished 2014 15″ Retina MacBook Pros for up to $400 off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Apple drops prices on refurbished 2013 Retina...
The Apple Store has dropped prices on 2013 Apple Certified Refurbished 13″ and 15″ Retina MacBook Pros, with Retina models now available starting at $999. Apple’s one-year warranty is standard, and... Read more
New 2.8GHz Mac mini on sale for $949, save $5...
Abt Electronics has the new 2.8GHz Mac mini in stock and on sale for $949.05 including free shipping. Their price is $50 off MSRP, and it’s the lowest price available for this model from any reseller... Read more
Sale! 3.7GHz Quad Core Mac Pro available for...
 B&H Photo has the 3.7GHz Quad Core Mac Pro on sale for $2649 including free shipping plus NY sales tax only. Their price is $350 off MSRP, and it’s the lowest price for this model from any... Read more
Mujjo Steps Up The Game With Refined Touchscr...
Netherlands based Mujjo have just launched their Refined Touchscreen Gloves, stepping up their game. The gloves feature a updated elegant design that takes these knitted gloves to the next level. A... Read more
Sale! Preorder the new 27-inch 5K iMac for $2...
 Abt Electronics has the new 27″ 3.5GHz 5K iMac on sale and available for preorder for $2374.05 including free shipping. Their price is $125 off MSRP, and it’s the lowest price available for this... Read more
Simplex Solutions Inc. Brings Secure Web Surf...
New York based Simplex Solutions Inc. has announced the release and immediate availability of Private Browser 1.0, its revolutionary new secure web browser developed for iPhone, iPad and iPod touch... Read more
Save up to $180 off MSRP with an Apple refurb...
The Apple Store has Apple Certified Refurbished 2014 MacBook Airs available for up to $180 off the cost of new models. An Apple one-year warranty is included with each MacBook, and shipping is free.... Read more

Jobs Board

*Apple* Solutions Consultant (ASC) - Apple I...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
*Apple* Solutions Consultant (ASC) - Apple I...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
*Apple* Solutions Consultant (ASC) - Apple I...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
*Apple* Solutions Consultant - Apple Inc. (U...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
*Apple* Solutions Consultant - Apple Inc. (U...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.