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.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

SpamSieve 2.9.38 - Robust spam filter fo...
SpamSieve is a robust spam filter for major email clients that uses powerful Bayesian spam filtering. SpamSieve understands what your spam looks like in order to block it all, but also learns what... Read more
TeamViewer 15.0.8397 - Establish remote...
TeamViewer gives you remote control of any computer or Mac over the Internet within seconds or can be used for online meetings. Find out why more than 200 million users trust TeamViewer! Free for non... Read more
SteerMouse 5.4.3 - Powerful third-party...
SteerMouse is an advanced driver for USB and Bluetooth mice. SteerMouse can assign various functions to buttons that Apple's software does not allow, including double-clicks, modifier clicks,... Read more
Toast Titanium 18.2.1 - The ultimate med...
Roxio Toast Titanium, the leading DVD burner for Mac, makes burning even better, adding Roxio Secure Burn to protect your files on disc and USB in Mac- or Windows-compatible formats. Get more style... Read more
HoudahSpot 5.0.11 - Advanced file-search...
HoudahSpot is a versatile desktop search tool. Use HoudahSpot to locate hard-to-find files and keep frequently used files within reach. HoudahSpot will immediately feel familiar. It works just the... Read more
ClipGrab 3.8.6 - Download videos from Yo...
ClipGrab is a free downloader and converter for YouTube, Vimeo, Facebook and many other online video sites. It converts downloaded videos to MPEG4, MP3 or other formats in just one easy step Version... Read more
ExpanDrive 7.4.0 - Access cloud storage...
ExpanDrive builds cloud storage in every application, acts just like a USB drive plugged into your Mac. With ExpanDrive, you can securely access any remote file server directly from the Finder or... Read more
Adobe Dreamweaver CC 2020 20.0 - Build w...
Dreamweaver CC 2020 is available as part of Adobe Creative Cloud for as little as $20.99/month (or $9.99/month if you're a previous Dreamweaver customer). Adobe Dreamweaver CC 2020 allows you to... Read more
Eye Candy 7.2.3.85 - 30 professional Pho...
Eye Candy renders realistic effects that are difficult or impossible to achieve in Photoshop alone, such as Fire, Chrome, and the new Lightning. Effects like Animal Fur, Smoke, and Reptile Skin are... Read more
Sparkle Pro 2.8.5 - Visual website creat...
Sparkle Pro will change your mind if you thought building websites wasn't for you. Sparkle is the intuitive site builder that lets you create sites for your online portfolio, team or band pages, or... Read more

Latest Forum Discussions

See All

Pre-register for Hello Kitty AR: Kawaii...
Hello Kitty — the cute cat that launched a multi-billion-pound franchise — has been brought to life… sort of. Sanrio has teamed up with the Bublar Group to create a new mobile game that uses AR tech to turn the real world into Hello Kitty’s... | Read more »
Gorgeous and tranquil puzzler Spring Fal...
One-man indie studio SPARSE//GameDev has now launched its tranquil puzzler, Spring Falls. It's described as "a peaceful puzzle game about water, erosion, and watching things grow". [Read more] | Read more »
Black Desert Mobile gets an official rel...
Pearl Abyss has just announced that its highly-anticipated MMO, Black Desert Mobile, will launch globally for iOS and Android on December 11th. [Read more] | Read more »
Another Eden receives new a episode, cha...
Another Eden, WFS' popular RPG, has received another update that brings new story content to the game alongside a few new heroes to discover. [Read more] | Read more »
Overdox guide - Tips and tricks for begi...
Overdox is a clever battle royale that changes things up by adding MOBA mechanics and melee combat to the mix. This new hybrid game can be quite a bit to take in at first, so we’ve put together a list of tips to help you get a leg up on the... | Read more »
Roterra Extreme - Great Escape is a pers...
Roterra Extreme – Great Escape has been described by developers Dig-It Games as a mini-sequel to their acclaimed title Roterra: Flip the Fairytale. It continues that game's tradition of messing with which way is up, tasking you with solving... | Read more »
Hearthstone: Battlegrounds open beta lau...
Remember earlier this year when auto battlers were the latest hotness? We had Auto Chess, DOTA Underlords, Chess Rush, and more all gunning for our attention. They all had their own reasons to play, but, at least from where I'm standing, most... | Read more »
The House of Da Vinci 2 gets a new gamep...
The House of Da Vinci launched all the way back in 2017. Now, developer Blue Brain Games is gearing up to deliver a second dose of The Room-inspired puzzling. Some fresh details have now emerged, alongside the game's first official trailer. [Read... | Read more »
Shoot 'em up action awaits in Battl...
BattleBrew Productions has just introduced another entry into its award winning, barrelpunk inspired, BattleSky Brigade series. Whilst its previous title BattleSky Brigade TapTap provided fans with idle town building gameplay, this time the... | Read more »
Arcade classic R-Type Dimensions EX blas...
If you're a long time fan of shmups and have been looking for something to play lately, Tozai Games may have just released an ideal game for you on iOS. R-Type Dimensions EX brings the first R-Type and its sequel to iOS devices. [Read more] | Read more »

Price Scanner via MacPrices.net

13″ 2.4GHz MacBook Pros available for up to $...
Apple has a full line of Certified Refurbished 2019 13″ 2.4GHz 4-Core Touch Bar MacBook Pros available starting at $1529 and up to $300 off MSRP. Apple’s one-year warranty is included, shipping is... Read more
New at T-Mobile: Switch to T-Mobile, and get...
T-Mobile is offering a free 64GB iPhone 8 for new customers who switch to T-Mobile and open a new line of service. Eligible trade-in required, and discount applied over a 24 month period. The fine... Read more
Xfinity Mobile’s Black Friday Apple savings:...
Take $250 off the purchase of any iPhone at Xfinity Mobile with a new line activation, and transfer of phone number to Xfinity Mobile, through December 8, 2019. This includes Apple’s new iPhone 11... Read more
2019 13″ 1.4GHz MacBook Pros available starti...
Apple has a full line of Certified Refurbished 2019 13″ 1.4GHz 4-Core Touch Bar MacBook Pros available starting at $1099 and up to $230 off MSRP. Apple’s one-year warranty is included, shipping is... Read more
Save up to $350 on a 21″ or 27″ iMac with the...
Apple has Certified Refurbished 2019 21″ & 27″ iMacs available starting at $929 and up to $350 off the cost of new models. Apple’s one-year warranty is standard, shipping is free, and each iMac... Read more
Early Holiday 2019 Sale: B&H again offers...
B&H Photo has 10.2″ iPads on sale again for $30 off Apple’s MSRP, starting at $299, as part of their early Holiday 2019 sale. Overnight shipping is free to many addresses in the US: – 10.2″ 32GB... Read more
Apple iMacs on sale today at B&H Photo fo...
B&H Photo has new 2019 21″ and 27″ 5K iMacs on stock today and on sale for up to $150 off Apple’s MSRP. Overnight shipping is free to many locations in the US. These are the same iMacs sold by... Read more
2018 4 and 6-Core Mac minis on sale today for...
Apple resellers are offering new 2018 4-Core and 6-Core Mac minis for $80-$100 off MSRP for a limited time. B&H Photo has the new 2018 4-Core and 6-Core Mac minis on sale for up to $100 off Apple... Read more
Early Holiday 2019 sale at B&H Photo: 12....
B&H Photo has new 12.9″ iPad Pros on sale for up to $120 off Apple’s MSRP as part of their early Holiday 2019 sale. Overnight shipping is free to many addresses in the US: – 12.9″ 64GB WiFi iPad... Read more
8-Core iMac Pro on sale today for $4499 at B...
B&H Photo has the base 8-Core 3.2GHz 32GB/1TB iMac Pro on sale today for $4499 — $500 off Apple’s MSRP. Shipping is free. Their price is the lowest available for a new iMac Pro from any Apple... Read more

Jobs Board

*Apple* Health Benefit Specialist - Call Cen...
Description ** Apple Health Benefit Specialist - Call Center (MAS 3/MACSC)** **Olympia, WA Multiple Positions** *The ideal candidate for this position will have Read more
Hair Stylist - *Apple* Blossom Mall - JCPen...
Hair Stylist - Apple Blossom Mall Location:Winchester, VA, United States- Apple Blossom Mall 1850 Apple Blossom Dr Job ID:1065040Salon Professionals Job Read more
*Apple* Mobility Pro - Best Buy (United Stat...
**747088BR** **Job Title:** Apple Mobility Pro **Job Category:** Store Associates **Store NUmber or Department:** 000297-Reston-Store **Job Description:** At Best Read more
Nurse Practitioner - Field Based (San Bernard...
Nurse Practitioner - Field Based (San Bernardino, CA, Apple Valley, Hesperia) **Location:** **United States** **Requisition #:** PS30312 **Post Date:** Nov 11, 2019 Read more
Best Buy *Apple* Computing Master - Best Bu...
**747061BR** **Job Title:** Best Buy Apple Computing Master **Job Category:** Store Associates **Store NUmber or Department:** 000647-Kildeer-Store **Job Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.