TweetFollow Us on Twitter

Quoting in the shell

Volume Number: 23 (2007)
Issue Number: 09
Column Tag: MacEnterprise

Quoting in the shell

Dealing with interesting characters in shell programming

By Philip Rinehart, Yale University

QUOTES?

This month, a rather interesting problem arose on the Macenterprise mailing list. How are quotes used and dealt with when scripting for the bash shell? It can be a particularly difficult problem, as OS X allows the use of non-standard characters and spaces in filenames. If quoting is not done properly, unexpected, and even disastrous results may occur. Anyone, remember iTunes 2.0 and destruction of hard drive data? It was a quoting problem! Thus, it is extremely important to quote things properly when shell programming, both for unexpected results, as well as properly sanitizing input.

Single Quotes?

Let's start the process by looking at the use of single quotes in bash shell scripting. The original question was about the use of a directory listing. The directory listing was then piped to a second command, which failed due to spaces in some of the directory names. The specific example:

   ls -F /Applications

Try it. Note how many Applications contain spaces in the name. If Adobe Acrobat 8 Professional were installed, any shell script attempting to perform an action on the path would fail. Why? As the shell interprets spaces as input separators, Adobe, Acrobat, 8 and Professional are all seen as individual paths. In our above example, if the chmod operation were being performed, "Adobe", "Acrobat", "8" and "Professional" would all be modified by chmod. This action would fail naturally, as each of these paths does not exist. Would using single quotes help?

Single quotes treat all information contained between single quotes as exact character data in the bash shell. In practice, single quotes are extraordinarily difficult to use as variables and are not expanded. Even the backslash character is not considered special within single quotes. In our example, this fact means that a variable cannot be used in single quotes when shell scripting. As a result, single quotes aren't going to solve the problem above, so a different solution is required.

Double Quotes?

What about the use of double quotes? Can they solve the problem? Hmm, it get's a bit closer this time. Again, let's use the above list command. When assigned to a variable, multiple words will be treated as a single word. Wow, what a mouthful! Here's a better example. Let's use Adobe Acrobat 8 Professional again. When talking about single quotes, remember the fact that each word is treated separately? With double quoting, the entire word remains intact, and is not split. So, if the variable test is assigned and then echoed out using double quotes, the entire word, Adobe Acrobat 8 Professional, is used. A brief shell script snippet to further reinforce this concept:

#!/bin/bash
test="Adobe Acrobat 8 Professional"
printf "Single word is $test\n"

Note, that when these commands are entered and run that the variable is correctly displayed with printf. Without the quotes, only the word Single is printed. Does this solution work for the problem encountered on the list? Well, not quite. It certainly gets somewhat closer, as spaces are correctly preserved with double quotes, but command substitutions with variables still see each space separated entry as a separate word. If single and double quotes won't solve the problem, what will? Can the field separator be altered, and will it solve the problem?

IFS

IFS? Never heard of it before? IFS stands for Internal Field Separator. It determines how bash interprets word boundaries. By default, the three special characters, space, tab, and newline are used. Using the Adobe Acrobat 8 example, it now should be somewhat clearer as to why each is treated as a separate word. bash reads each space in the name as a field separator. With the default value for IFS, command substitution treats each space as a delimiter. This can be changed. At the top of each shell script, set the IFS variable. Any character can be the field separator, from a semi-colon, space, to a comma. The Macenterprise list came up with the following solution for the problem when a directory listing contains spaces:

   
   IFS="\n"

The author inserted this line at the beginning of the shell script. Now that the IFS separator has been reset, spaces are no longer recognized as valid field separators. The problem is now gone, and the variable is correctly assigned. It is also useful to note that resetting IFS can be used to ensure that the IFS variable has not been improperly set before any shell script is run. As a security precaution when writing a shell script, just as the PATH variable is set, make it a habit to also set the IFS variable. If input is being read in, or parsed, any spoofing or accidental resetting of this variable becomes a non-issue.

USING FIND

Let's return to the original list command that the article opened with. The original question wanted to take a list of directories and then change their permissions. Jeff McCune from The Ohio State University suggested another method on the list. He used the shell find utility to accomplish the same thing in a more efficient and unix-y way. Let's start with the command in its full form:

find /Applications -type d -maxdepth 1 -mindepth 1 -print0 | xargs -0 chmod 750

O.K., that's a bit much! As the find utility may be new to some, let's break it down into its components. The second argument, /Applications, can be any directory that find will operate on. In this case, it is looking in the Applications directory. The next argument, -type, looks for directories, as specified by the option d. After that, the next two arguments instruct the find command not to descend into any of the Application packages. It is accomplished with the maxdepth and mindepth switches. The next option is where the real magic begins. The option, print0, prints the result to standard output, terminating the output with the ASCII NUL character. It will become clear why this option is important in a second. All the results from the command are printed without a terminating newline. Try just the first part of the command before the pipe character, both with -print and -print0. Note the difference, as it is critical to this commands proper function.

Now that the find results are complete, it is time to process them using xargs. xargs takes an argument list, and processes the results with any other command line utility. Remember the use of print0. As the NUL character terminates the output, xargs must use the -0 flag. Without it, xargs does not know how to interpret the results from the find command. This technique is also generally considered faster by Unix gurus, as it only executes the chmod command once, which may not be the case for a shell loop, or by using the –exec switch with the find utility.

As you can see, the problem presented on the list is not as straightforward as one might expect. As always, there is more than one way to solve the problem, but with a full understanding of the problem's intricacies, it should now be trivial to solve. Until next month, see you on the lists!


Philip Rinehart is co-chair of the steering committee leading the Mac OS X Enterprise Project (macenterprise.org) and is the Lead Mac Analyst at Yale University. He has been using Macintosh Computers since the days of the Macintosh SE, and Mac OS X since its Developer Preview Release. Before coming to Yale, he worked as a Unix system administrator for a dot-com company. He can be reached at: philip.rinehart@yale.edu.

The MacEnterprise project is a community of IT professionals sharing information and solutions to support Macs in an enterprise. We collaborate on the deployment, management, and integration of Mac OS X client and server computers into multi-platform computing environments.

 
AAPL
$100.96
Apple Inc.
-0.83
MSFT
$47.52
Microsoft Corpora
+0.84
GOOG
$596.08
Google Inc.
+6.81

MacTech Search:
Community Search:

Software Updates via MacUpdate

Airfoil 4.8.9 - Send audio from any app...
Airfoil allows you to send any audio to AirPort Express units, Apple TVs, and even other Macs and PCs, all in sync! It's your audio - everywhere. With Airfoil you can take audio from any... Read more
WhatRoute 1.13.0 - Geographically trace...
WhatRoute is designed to find the names of all the routers an IP packet passes through on its way from your Mac to a destination host. It also measures the round-trip time from your Mac to the... Read more
Chromium 37.0.2062.122 - 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
Attachment Tamer 3.1.14b9 - Take control...
Attachment Tamer gives you control over attachment handling in Apple Mail. It fixes the most annoying Apple Mail flaws, ensures compatibility with other email software, and allows you to set up how... Read more
Duplicate Annihilator 5.0 - Find and del...
Duplicate Annihilator takes on the time-consuming task of comparing the images in your iPhoto library using effective algorithms to make sure that no duplicate escapes. Duplicate Annihilator detects... Read more
jAlbum Pro 12.2 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code!... Read more
jAlbum 12.2 - Create custom photo galler...
With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly, with pro results Simply drag and drop photos into groups, choose a design... Read more
Quicken 2015 2.0.4 - Complete personal f...
Quicken 2015 helps you manage all your personal finances in one place, so you can see where you're spending and where you can save. Quicken automatically categorizes your financial transactions,... Read more
iMazing 1.0 - Complete iOS device manage...
iMazing (formerly DiskAid) is the ultimate iOS device manager with capabilities far beyond what iTunes offers. With iMazing and your iOS device (iPhone, iPad, or iPod), you can: Copy music to and... Read more
Xcode 6.0.1 - Integrated development env...
Apple Xcode is Apple Computer's integrated development environment (IDE) for OS X. The full Xcode package is free to ADC members and includes all the tools you need to create, debug, and optimize... Read more

Latest Forum Discussions

See All

View Source – HTML, JavaScript and CSS...
View Source – HTML, JavaScript and CSS 1.0 Device: iOS Universal Category: Utilities Price: $.99, Version: 1.0 (iTunes) Description: View Source is an app plus an iOS 8 Safari extension that makes it easy to do one key web developer... | Read more »
Avenged Sevenfold’s Hail To The King: De...
Avenged Sevenfold’s Hail To The King: Deathbat is Coming to iOS on October 16th Posted by Jessica Fisher on September 19th, 2014 [ permalink ] Just in time for Halloween, on October 16 Avenged Sevenfold will be launching | Read more »
Talisman Has Gone Universal – Can Now be...
Talisman Has Gone Universal – Can Now be Played on the iPhone Posted by Jessica Fisher on September 19th, 2014 [ permalink ] | Read more »
Tap Army Review
Tap Army Review By Jennifer Allen on September 19th, 2014 Our Rating: :: SHOOT EM ALLUniversal App - Designed for iPhone and iPad Mindless but fun, Tap Army is a lane-based shooter that should help you relieve some stress.   | Read more »
Monsters! Volcanoes! Loot! Epic Island f...
Monsters! Volcanoes! Loot! | Read more »
Plunder Pirates: Tips, Tricks, Strategie...
Ahoy There, Seadogs: Interested in knowing our thoughts on all this plundering and pirating? Check out our Plunder Pirates Review! Have you just downloaded the rather enjoyable pirate-em-up Plunder Pirates and are in need of some assistance? Never... | Read more »
Goat Simulator Review
Goat Simulator Review By Lee Hamlet on September 19th, 2014 Our Rating: :: THE GRUFFEST OF BILLY GOATSUniversal App - Designed for iPhone and iPad Unleash chaos as a grumpy goat in this humorous but short-lived casual game.   | Read more »
A New and Improved Wunderlist is Here fo...
A New and Improved Wunderlist is Here for iOS 8 Posted by Jessica Fisher on September 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Evernote Update for iOS 8 Adds Web Clipp...
Evernote Update for iOS 8 Adds Web Clipping, Quick Notes, and More Posted by Ellis Spice on September 19th, 2014 [ permalink ] | Read more »
Apple Names Ultimate Productivity Bundl...
Apple Names Ultimate Productivity Bundle by Readdle as the Essential Bundle on the App Store Posted by Jessica Fisher on September 19th, 2014 [ permalink | Read more »

Price Scanner via MacPrices.net

Updated Price Trackers
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers: - 15″ MacBook Pros - 13″ MacBook... Read more
Mac Pros available for up to $260 off MSRP
Adorama has Mac Pros on sale for up to $260 off MSRP. Shipping is free, and Adorama charges sales tax in NY & NJ only: - 4-core Mac Pro: $2839.99, $160 off MSRP - 6-core Mac Pro: $3739.99, $260... Read more
13-inch 2.6GHz/256GB Retina MacBook Pros avai...
B&H Photo has the 13″ 2.6GHz/256GB Retina MacBook Pro on sale for $1379 including free shipping plus NY sales tax only. Their price is $120 off MSRP. Read more
Previous-generation 15-inch 2.0GHz Retina Mac...
B&H Photo has leftover previous-generation 15″ 2.0GHz Retina MacBook Pros now available for $1599 including free shipping plus NY sales tax only. Their price is $400 off original MSRP. B&H... Read more
21″ 2.7GHz iMac available for $1179, save $12...
Adorama has 21″ 2.7GHz Hawell iMacs on sale for $1179.99 including free shipping. Their price is $120 off MSRP. NY and NJ sales tax only. Read more
iOS 8 Adoption Rate Slower than iOS 7, 6, Hit...
Apple began pushing out iOS 8 updates to eligible devices around 1pm ET on September 17, 2014. However, unlike with iOS 7, which boasted a wide variety of differences from its predecessor iOS 6, in... Read more
LIkely Final Definitive OS X 10.9.5 Mavericks...
Apple has released what will almost certainly be the last incremental version number update of OS X 10.9 Mavericks (save for futire security updates) before OS X 10.10 Yosemite is released next month... Read more
Fingerprints, Apple Pay and Identity Theft Wa...
On Sep 9th, CEO Tim Cook unveiled Apple Pay, along with the new iPhone 6 and iWatch. Apple Pay is a newly developed technology that utilizes a near field communication (NFC) to enable customer... Read more
Amazon Introduces Two All-New Kindles
Amazon on Thursday introduced the 7th generation of its Kindle dedicated e-reader device: Kindle Voyage, its top-of-the-line e-reader, and the new $79 Kindle, with a 20% faster processor, twice the... Read more
Save up to $300 on the price of a new Mac wit...
Purchase a new Mac or iPad at The Apple Store for Education and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free,... Read more

Jobs Board

*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
Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and 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
*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
*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.