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


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:

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? 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:


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.


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 ( 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:

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.


Community Search:
MacTech Search:

Software Updates via MacUpdate

Dashlane 5.9.0 - Password manager and se...
Dashlane is an award-winning service that revolutionizes the online experience by replacing the drudgery of everyday transactional processes with convenient, automated simplicity - in other words,... Read more
Cocktail 11.5 - General maintenance and...
Cocktail is a general purpose utility for macOS 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
Backup and Sync 3.40.8921.5350 - File ba...
Backup and Sync (was Google Drive) is a place where you can create, share, collaborate, and keep all of your stuff. Whether you're working with a friend on a joint research project, planning a... Read more
Skype - Voice-over-internet pho...
Skype allows you to talk to friends, family, and co-workers across the Internet without the inconvenience of long distance telephone charges. Using peer-to-peer data transmission technology, Skype... Read more
Hazel 4.2.4 - Create rules for organizin...
Hazel is your personal housekeeper, organizing and cleaning folders based on rules you define. Hazel can also manage your trash and uninstall your applications. Organize your files using a familiar... Read more
ForkLift 3.2 - Powerful file manager: FT...
ForkLift is a powerful file manager and ferociously fast FTP client clothed in a clean and versatile UI that offers the combination of absolute simplicity and raw power expected from a well-executed... Read more
ScreenFlow 7.3 - Create screen recording...
ScreenFlow is powerful, easy-to-use screencasting software for the Mac. With ScreenFlow you can record the contents of your entire monitor while also capturing your video camera, microphone and your... Read more
Time Out 2.4 - Break reminder tool with...
Time Out helps remind you to take work breaks throughout the day. It has two kinds of breaks: a "Normal" break, typically for 10 minutes after 50 minutes of work, so you can move about and relax,... Read more
Smultron 10.1.3 - Easy-to-use, powerful...
Smultron 10 is an elegant and powerful text editor that is easy to use. You can use Smultron 10 to create or edit any text document. Everything from a web page, a note or a script to any single piece... Read more
Tor Browser Bundle 7.5.2 - Anonymize Web...
The Tor Browser Bundle is an easy-to-use portable package of Tor, Vidalia, Torbutton, and a Firefox fork preconfigured to work together out of the box. It contains a modified copy of Firefox that... Read more

Latest Forum Discussions

See All

How PUBG, Fortnite, and the battle royal...
The history of the battle royale genre isn't a long one. While the nascent parts of the experience have existed ever since players first started killing one another online, it's really only in the past six years that the genre has coalesced into... | Read more »
Around the Empire: What have you missed...
Oh hi nice reader, and thanks for popping in to check out our weekly round-up of all the stuff that you might have missed across the Steel Media network. Yeah, that's right, it's a big ol' network. Obviously 148Apps is the best, but there are some... | Read more »
All the best games on sale for iPhone an...
It might not have been the greatest week for new releases on the App Store, but don't let that get you down, because there are some truly incredible games on sale for iPhone and iPad right now. Seriously, you could buy anything on this list and I... | Read more »
Everything You Need to Know About The Fo...
In just over a week, Epic Games has made a flurry of announcements. First, they revealed that Fortnite—their ultra-popular PUBG competitor—is coming to mobile. This was followed by brief sign-up period for interested beta testers before sending out... | Read more »
The best games that came out for iPhone...
It's not been the best week for games on the App Store. There are a few decent ones here and there, but nothing that's really going to make you throw down what you're doing and run to the nearest WiFi hotspot in order to download it. That's not to... | Read more »
Death Coming (Games)
Death Coming Device: iOS Universal Category: Games Price: $1.99, Version: (iTunes) Description: --- Background Story ---You Died. Pure and simple, but death was not the end. You have become an agent of Death: a... | Read more »
Hints, tips, and tricks for Empires and...
Empires and Puzzles is a slick match-stuff RPG that mixes in a bunch of city-building aspects to keep things fresh. And it's currently the Game of the Day over on the App Store. So, if you're picking it up for the first time today, we thought it'd... | Read more »
What You Need to Know About Sam Barlow’s...
Sam Barlow’s follow up to Her Story is #WarGames, an interactive video series that reimagines the 1983 film WarGames in a more present day context. It’s not exactly a game, but it’s definitely still interesting. Here are the top things you should... | Read more »
Pixel Plex Guide - How to Build Better T...
Pixel Plex is the latest city builder that has come to the App Store, and it takes a pretty different tact than the ones that came before it. Instead of being in charge of your own city by yourself, you have to work together with other players to... | Read more »
Fortnite Will Be Better Than PUBG on Mob...
Before last week, if you asked me which game I prefer between Fortnite Battle Royale and PlayerUnknown’s Battlegrounds (PUBG), I’d choose the latter just about 100% of the time. Now that we know that both games are primed to hit our mobile screens... | Read more »

Price Scanner via

Back in stock: 13-inch 2.5GHz MacBook Pro (Ce...
Apple has Certified Refurbished 13″ 2.5GHz MacBook Pros (MD101LL/A) available for $829, or $270 off original MSRP. Apple’s one-year warranty is standard, and shipping is free: – 13″ 2.5GHz MacBook... Read more
Apple restocks Certified Refurbished 2017 13″...
Apple has Certified Refurbished 2017 13″ MacBook Airs available starting at $849. An Apple one-year warranty is included with each MacBook, and shipping is free: – 13″ 1.8GHz/8GB/128GB MacBook Air (... Read more
8-Core iMac Pro on sale for $4699, save $300
Amazon has the 8-core iMac Pro on sale for $4699 including free shipping. Their price is $300 off MSRP, and it’s the currently lowest price available for an iMac Pro. For the latest up-to-date prices... Read more
10″ 512GB WiFi iPad Pros on sale for $849, sa...
B&H Photo has Space Gray and Rose Gold 10.5″ 512GB WiFi iPad Pros on sale for $849. Their price is $150 off MSRP, and it’s the lowest price available for these models, new, from any Apple... Read more
MacBook Pro sale! B&H drops prices on new...
B&H Photo has dropped prices on new 2017 13″ MacBook Pros, with models now on sale for up to $200 off MSRP. Shipping is free, and B&H charges sales tax for NY & NJ residents only. Their... Read more
13″ MacBook Airs on sale for $100-$150 off MS...
B&H Photo has 13″ MacBook Airs on sale for $100-$150 off MSRP. Shipping is free, and B&H charges sales tax for NY & NJ residents only: – 13″ 1.8GHz/128GB MacBook Air (MQD32LL/A): $899, $... Read more
Huge iMac sale! Apple reseller now offering 2...
B&H Photo has new 2017 21″ & 27″ iMacs on sale today for up to $300 off MSRP. Shipping is free, and B&H charges sales tax for NY & NJ residents only: – 27″ 3.8GHz iMac (MNED2LL/A): $... Read more
Sale! 1.4GHz Mac mini for $399, $100 off MSRP
B&H Photo has the 1.4GHz Mac mini on sale for $399 for a limited time. Their price is $100 off MSRP, and it’s the lowest price available for a mini from any Apple reseller: – 1.4GHz Mac mini (... Read more
Sale of the year continues as Apple resellers...
Adorama has new 2017 15″ MacBook Pros on sale for $250-$300 off MSRP. Shipping is free, and Adorama charges sales tax in NJ and NY only: – 15″ 2.8GHz Touch Bar MacBook Pro Space Gray (MPTR2LL/A): $... Read more
iPad sale continues! 9″ WiFi iPads for $50-$8...
Walmart has 9.7″ Apple iPads on sale for $50-$80 off MSRP for a limited time. Sale prices are for online orders only, in-store prices may vary: – 9″ 32GB iPad: $249.99 $80 off – 9″ 128GB iPad: $379 $... Read more

Jobs Board

*Apple* Genius - Technical Customer Service...
Job Description: Job Summary As a Genius at the Apple Store, you maintain customers' trust in Apple as the skilled technical customer service expert, Read more
*Apple* Genius - Technical Customer Service...
Job Description:Job SummaryAs a Genius at the Apple Store, you maintain customers' trust in Apple as the skilled technical customer service expert, Read more
*Apple* Retail Sales Associate / Service Wri...
Job Description iStore is the premier retailer of Apple products and solutions. We're looking for dedicated individuals with a passion to simplify and enhance the Read more
*Apple* Senior Manager - Multi Unit Manageme...
…and opportunities of a continuously evolving business is what makes leadership at the Apple Store a rewarding career. It's your job as a Senior Manager to lead Read more
*Apple* Technician - Visual Dynamics, Inc. (...
Job Description Visual Dynamics, Inc., Brevard's Apple Premier Partner, is looking for a dynamic individual to fill the Apple Technician position at our Viera Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.