TweetFollow Us on Twitter

Shell Game: Calling Shell Commands from Applications

Volume Number: 20 (2004)
Issue Number: 4
Column Tag: Programming

Mac OS X Programming Secrets

by Scott Knaster

Shell Game: Calling Shell Commands from Applications

The coolest thing about Mac OS X is the skillful grafting of the lovely and attractive Aqua graphical user interface onto the geeky but sturdy Unix stuff underneath. Applications in OS X run on a Darwinian layer of software that you can access via Unix shells in Terminal, also known as the command line. This gives you the ability to lift the hood on an application and see what's going on underneath. You can observe an application's behavior, change its settings, or even fool around with its workings, if you know what you're doing or you don't mind the possibility of breaking things.

The commands you can run in the shell are very powerful. Sometimes it's handy to use a shell command from within an application, giving you an ideal combination of friendly user interface and utter command line power. In this month's column, we'll explore how to run shell commands from a Cocoa application. By the time you finish this column, you will be a master (or at least someone capable) of using shell commands from your lofty Cocoa programs.

How Did I Get Here?

What are shell commands, and where do they come from? Shell commands are instructions you type in Terminal to make things happen. For example, you type ls when you want a directory listing, kill to end a process, cp to copy a file, and so on.

Shell commands aren't built into the shell, with very few exceptions. They're actually executable programs stored in well-known (to the shell) directories. When you type the name of a command, the shell searches this set of directories for a file with the same name as the command. When it finds an executable with the sought-after name, the shell runs it. If there's no file by that name, you get the disheartening message Command not found.

The list of directories that hold shell commands is kept in a shell variable name PATH, so named because it holds the search path for commands. You can see the value of the PATH variable by typing a shell command: echo "${PATH}". Here's what I got when I typed that command:

[neb:~] scott% echo ${PATH}
/bin:/sbin:/usr/bin:/usr/sbin:/Developer/Tools

The command output shows that the shell will look for commands in 5 directories: /bin, /sbin, /usr/bin, /user/sbin, and /Developer/Tools. If we take a look in those directories, we should see some familiar command names living there. And sure enough, we do. For example, here are the files in /bin:

[neb:~] scott% ls /bin
[         date        expr      mv        rmdir     test
bash      dd          hostname  pax       sh        zsh
cat       df          kill      ps        sleep     zsh-4.1.1
chmod     domainname  ln        pwd       stty
cp        echo        ls        rcp       sync
csh       ed          mkdir     rm        tcsh

There are 33 commands in this directory, and hundreds more in the others. The /usr/bin directory alone includes over 600 commands.

The search path is initialized when the shell starts. The initial value for this and other shell variables is set by a configuration file named .tcshrc. This file is used with the tcsh shell, which is the default shell in OS X 10.3. If you switch to a different shell, such as bash, you'll have a different configuration file. Commands in .tcshrc are executed when the shell starts up. Let's take a look and see what's in .tcshrc:

[neb:~] scott% cat .tcshrc
setenv PATH "${PATH}:/Developer/Tools"

In this case, the .tcshrc file adds /Developer/Tools to the search path. This line gets tacked on to your configuration file when you install Xcode. If you want to add other directories of commands to the search path, you can put more setenv commands in your shell configuration file.

Task It

A shell command is just an executable file. When the system runs an executable, it creates a process. Because Cocoa likes things best when they're objects, Cocoa provides the NSTask class in the Foundation framework for handling processes. A process has an execution environment that includes its current directory, environment variables, and other values. You can create an NSTask object in your application and use it to control a shell command process.

It's going to be darned easy to call our first shell command. We'll only need a few methods of NSTask:

  • setLaunchPath specifies the file path of the command we're going to call from our application.

  • setArguments supplies any arguments that we would normally pass via the command line. Technically, this method isn't always necessary, because some commands don't take any arguments. But most of them do, most of the time.

  • launch executes the command and creates a new process from it.

That's all there is to it. What could be simpler? Of course, you should call alloc and init when you create the NSTask, and release when you're done.

In order to keep our example as basic as possible, we'll choose the open command, which simply opens a file or directory. For our example, we'll have it open the Applications folder to remind us of all the cool apps we're privileged to have on our Macs.

OK, let's make the application. In Xcode, choose New Project and create a Cocoa application. Name it whatever you like - a nice name, like Melanie, or Commando. Double-click MainMenu.nib to open it in Interface Builder. In the application window, add an NSButton object. Shrink the window down and put the button in the center. Name the button "Show Apps". Your screen should look something like Figure 1.


Figure 1. Laying out our modest application in Interface Builder.

NeXT, click the Classes tab, select NSObject, and press return to create a subclass. Rename the subclass AppController. In the Inspector (which you can open with Tools a Show Info), click the Actions tab, then click Add to create an action named doCommand. Create the files for the class (Classes a Create Files for AppController) and make an instance (Classes a Instantiate AppController).

Now it's time for the hookup. Connect the button to the AppController object by Control-dragging from the button to the AppController (in the MainMenu.nib window). Double-click the doCommand action to complete the wiring. Your screen in IB (which is what the cool kids call Interface Builder) should now look like Figure 2.


Figure 2. Connect the button to the application controller.

Turning our short attention span back to Xcode, it's time to write the code. Double-click AppController.m and type in the doCommand method.

#import "AppController.h"
@implementation AppController
- (IBAction)doCommand:(id)sender
{
   NSTask *theProcess;
   theProcess = [[NSTask alloc] init];
   
   [theProcess setLaunchPath:@"/usr/bin/open"];
      // Path of the shell command we'll execute
   [theProcess setArguments:
      [NSArray arrayWithObject:@"/Applications"]];
      // Arguments to the command: the name of the
      // Applications directory
   
   [theProcess launch];
      // Run the command
   
   [theProcess release];
}
@end

First, we create and set up the task object by calling NSTask alloc and init. Then, once we've instantiated the object, we tell OS X where to find the executable by calling setLaunchPath. In this case, it's in /usr/bin. (How did I know it was in that directory and not another? I looked for it. Remember from our earlier exercise that shell commands are generally in one of five directories: /bin, /sbin, /usr/bin, /usr/sbin, or /Developer/Tools. You can find the location of any shell command by calling the, er, shell command which. And to learn the details of using which, you of course go to the shell and type man which. Ah, Unix, where the fun never ends.)

After setting up the launch path, we call setArguments to say what should be passed to the command when it's called. Here, the only argument we need is what should be opened - the Applications folder - so we pass "/Applications". When we call launch, the command runs, the Finder comes to the front, and the Applications folder opens up. We did it!

Increasing Our Openness

The command we used, open, has a bunch of options that make it a very versatile tool, and you can use any of them when you call it from an application. For example, you can open any file just by specifying its path. If the file is a document, it will be opened with the default application. You can also use open to view a web page in the default browser. Just don't forget the protocol, such as http. Do it like this:

   [theProcess setArguments:
      [NSArray arrayWithObject: @"http://www.mactech.com"]];

The open command has an option, -a, that lets you open a file with a program that's not the default. You can use this when you call open from the command line or from an application. In the shell, it would look like this:

   open -a /Applications/TextEdit.app /Users/Scott/grape.doc

In this example, grape.doc will open with TextEdit instead of Microsoft Word as Bill intended. To achieve this, we're passing multiple arguments to open. The technique for calling setArguments is a little different when you have more than one argument. Because setArguments actually takes an NSArray, you have to do something like this:

   [task setArguments:[NSArray arrayWithObjects:
         @"-a", 
         @"/Applications/TextEdit.app",
         @"/Users/Scott/grape.doc", 
         nil]];  

Here we call arrayWithObjects - that's objects, plural - compared to earlier when we called arrayWithObject, singular, with only one parameter. The arguments are separated by commas. And surprisingly, the "-a" is a separate argument from the application name that follows it. Another wacky thing to remember about arrayWithObjects is that you have to add a nil after your last real object.

Users Can Be Choosers

Our example uses the open command to show the Applications folder in the Finder. That's a fine demonstration of the ability to call a shell command from a Cocoa application, but really, how many times can you look at the same folder? Let's make it slightly more interesting by allowing the user to decide what to open.

To do this, we'll go back to Interface Builder and add an NSTextView to the window. We'll name the new text view fileName - a fine, functional name. Then we'll add an outlet to the AppController object for fileName and introduce the objects to each other by Control-dragging. We save our work in IB and then head over to Xcode.

We need to make a simple change to AppController.m. Find the line where we call setArguments and change it to:

   [theProcess setArguments:
      [NSArray arrayWithObject: [fileName string]]];

This line now extracts the text from fileName and uses it to set the arguments for our impending shell command call. Our remodeled application window is pictured in Figure 3.


Figure 3. Our application now allows the user to enter an item to be opened. This product is now almost ready to ship, and t-shirts will be available soon.

As we discussed previously, this technique works when you have one argument, but not multiple arguments. If you're looking for something to do, a fun exercise would be to add another text view to the window and use it to let the user specify the other argument, an application to use for opening the file.

Moving Right Along

We accomplished our goal of calling a shell command from Cocoa, but we only scratched the surface, with neatly trimmed and manicured nails, of what you can achieve with NSTask. Next month we'll continue messing around with NSTask to see what other fun we can have.


Scott Knaster attended Cocoa Bootcamp at Big Nerd Ranch (http://www.bignerdranch.com/classes/cocoa1.shtml). He did not go Cocoa Loco but he liked it very much. Scott counts the days until pitchers and catchers report.

 
AAPL
$102.99
Apple Inc.
+0.52
MSFT
$44.38
Microsoft Corpora
-0.50
GOOG
$532.71
Google Inc.
+6.17

MacTech Search:
Community Search:

Software Updates via MacUpdate

Evernote 5.6.2 - Create searchable notes...
Evernote allows you to easily capture information in any environment using whatever device or platform you find most convenient, and makes this information accessible and searchable at anytime, from... Read more
OmniOutliner 4.1.3 - Organize your ideas...
OmniOutliner is a flexible program for creating, collecting, and organizing information. Give your creativity a kick start by using an application that's actually designed to help you think. It's... Read more
BBEdit 11.0 - Powerful text and HTML edi...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more
Apple Security Update 2014-005 - For OS...
Apple Security Update is recommended for all users and improves the security of Mac OS X. For information on the security content of this update, please visit this website: http://support.apple.com/... Read more
EyeTV 3.6.6 - Watch and record TV on you...
EyeTV brings a rich TV experience to your Mac. Watch live TV on your Mac. Pause, rewind, and record whenever you want. EyeTV gives you powerful control over what you watch and how you watch it. Put... Read more
RapidWeaver 6.0 - Create template-based...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
NTFS 12.0.39 - Provides full read and wr...
Paragon NTFS breaks down the barriers between Windows and OS X. Paragon NTFS effectively solves the communication problems between the Mac system and NTFS, providing full read and write access to... Read more
RestoreMeNot 2.0.3 - Disable window rest...
RestoreMeNot provides a simple way to disable the window restoration for individual applications so that you can fine-tune this behavior to suit your needs. Please note that RestoreMeNot is designed... Read more
Command-C 1.1.5 - Clipboard sharing tool...
Command-C is a revolutionary app which makes easy to share your clipboard between iOS and OS X using your local WiFi network, even if the app is not currently opened. Copy anything (text, pictures,... Read more
Macgo Blu-ray Player 2.10.9.1750 - Blu-r...
Macgo Mac Blu-ray Player can bring you the most unforgettable Blu-ray experience on your Mac. Overview Macgo Mac Blu-ray Player can satisfy just about every need you could possibly have in a Blu-ray... Read more

Latest Forum Discussions

See All

Worldly (Games)
Worldly 1.2 Device: iOS Universal Category: Games Price: $2.99, Version: 1.2 (iTunes) Description: | Read more »
Pro Strategy Football 2014 (Games)
Pro Strategy Football 2014 2014.141001 Device: iOS Universal Category: Games Price: $4.99, Version: 2014.141001 (iTunes) Description: Take the proven strategy of the PSF franchise and add in Casual Play, improved graphics and... | Read more »
Super Glyph Quest (Games)
Super Glyph Quest 1.01 Device: iOS Universal Category: Games Price: $2.99, Version: 1.01 (iTunes) Description: Adventure is back Questers! Combine elemental glyphs together to cast powerful spells and vanquish adorable monsters in... | Read more »
Fighting Fantasy: Caverns of the Snow Wi...
Fighting Fantasy: Caverns of the Snow Witch 1.0 Device: iOS Universal Category: Games Price: $5.99, Version: 1.0 (iTunes) Description: Travel to Northern Allansia’s perilous Icefinger Mountains to defeat the wicked Snow Witch in this... | Read more »
Star Warfare 2: Payback Review
Star Warfare 2: Payback Review By Blake Grundman on October 22nd, 2014 Our Rating: :: ONE-TRICK PONYUniversal App - Designed for iPhone and iPad Unfortunately, it doesn’t take long for Star Warfare 2’s free-firing fun to turn into... | Read more »
TinType by Hipstamatic (Photography)
TinType by Hipstamatic 1.0 Device: iOS iPhone Category: Photography Price: $.99, Version: 1.0 (iTunes) Description: Create hauntingly beautiful, soul capturing portraits with TinType by Hipstamatic. Inspired by daguerreotypes,... | Read more »
The Latest Update for Heroes of Dragon A...
The Latest Update for Heroes of Dragon Age Introduces Daily PvE Challenges Posted by Ellis Spice on October 22nd, 2014 [ permalink ] | Read more »
Bee Brilliant Review
Bee Brilliant Review By Jennifer Allen on October 22nd, 2014 Our Rating: :: SAMEY MATCH-3Universal App - Designed for iPhone and iPad Want even more Match-3 gaming and fancy a bee based twist? Bee Brilliant is that game. Don’t... | Read more »
A New Trailer has Been Revealed for Epic...
A New Trailer has Been Revealed for Epic of Kings Posted by Jessica Fisher on October 22nd, 2014 [ permalink ] Dead Mage Inc. has released a new, action-packed trailer for the upcoming Epic of Kings. | Read more »
Find the Line Review
Find the Line Review By Campbell Bird on October 22nd, 2014 Our Rating: :: INSLIDE THE LINESUniversal App - Designed for iPhone and iPad Tease out beautiful line drawings in this unique, free-to-play puzzle game.   | Read more »

Price Scanner via MacPrices.net

Save with Best Buy’s College Student Deals
Take an additional $50 off all MacBooks and iMacs at Best Buy Online with their College Students Deals Savings, valid through November 1st. Anyone with a valid .EDU email address can take advantage... Read more
iPad Air 2 & iPad mini 3 Best Tablets Yet...
The new iPads turned out to be pretty much everything I’d been hoping for and more than I’d expected.”More” particularly in terms of a drinking-from-a-firehose choice of models and configurations,... Read more
Drafts 4 Reinvents iOS Productivity App
N Richland Hills, Texas based Agile Tortoise has announced the release of Drafts 4 for iPhone and iPad. Drafts is a quick capture note taking app with flexible output actions. Drafts 4 scales from... Read more
AT&T accepting preorders for new iPads fo...
AT&T Wireless is accepting preorders for the new iPad Air 2 and iPad mini 3, cellular models, for $100 off MSRP with a 2-year service agreement: - 16GB iPad Air 2 WiFi + Cellular: $529.99 - 64GB... Read more
Apple offering refurbished Mac Pros for up to...
The Apple Store is offering Apple Certified Refurbished 2013 Mac Pros for up to $600 off the cost of new models. An Apple one-year warranty is included with each Mac Pro, and shipping is free. The... Read more
Select MacBook Airs $100 off MSRP, free shipp...
B&H Photo has 2014 a couple of MacBook Airs on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only. They also include free copies of Parallels Desktop and LoJack for... Read more
13-inch 2.5GHz MacBook Pro on sale for $100 o...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $999.99 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more
Strong iPhone, Mac And App Store Sales Drive...
Apple on Monday announced financial results for its fiscal 2014 fourth quarter ended September 27, 2014. The Company posted quarterly revenue of $42.1 billion and quarterly net profit of $8.5 billion... Read more
Apple Posts How-To For OS X Recovery
OS X 10.7 Lion and later include OS X Recovery. This feature includes all of the tools you need to reinstall OS X, repair your disk, and even restore from a Time Machine backup. OS X Recovery... Read more
Mac OS X Versions (Builds) Supported By Vario...
Apple Support has posted a handy resource explaining which Mac OS X versions (builds) originally shipped with or are available for your computer via retail discs, downloads, or Software Update. Apple... Read more

Jobs Board

Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work Read more
*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Position Opening at *Apple* - Apple (United...
…customers purchase our products, you're the one who helps them get more out of their new Apple technology. Your day in the Apple Store is filled with a range of Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.