TweetFollow Us on Twitter

Shell Game: Calling Shell Commands from Applications, Part 2

Volume Number: 20 (2004)
Issue Number: 5
Column Tag: Programming

Mac OS X Programming Secrets

by Scott Knaster

Shell Game: Calling Shell Commands from Applications, Part 2

Well, I really hope you saw last month's column, because it was a fun introduction into how to call a shell command from a Cocoa application, which is a neat trick. If you didn't read that column, stick around anyway - we'll recap. This month, we'll extend our knowledge by performing input and output with a shell command from a Cocoa app.

Task Oriented

Last month, we explained how to use Cocoa class NSTask to represent a shell command that we want to execute. We discussed a few methods of NSTask:

setLaunchPath, which specifies the path to the shell command we call from our application.

setArguments, used to pass arguments to the shell command, which you normally do by typing them on the command line itself in Terminal.

launch, which starts the process running.

These methods get us the basic features we need to run a shell command from a Cocoa app. This time, we're going to take a broader look at what you can do with NSTask. When you create and launch a new task, it gets its own little world to run in. Part of this world includes its own copy of environment variables and its own setting for the current directory. NSTask has methods that let you specify these values. Call setEnvironment to change the process's environment variables to its own custom set, which you pass in as a dictionary. Use setCurrentDirectoryPath to pass a string indicating the directory the task should use as the base for relative paths.

When you call a shell command from Terminal, you can set paths for standard input, standard output, and standard error. These let you choose the source of input for the command, specify where the output should go, and indicate what to do with error messages. NSTask has methods for each of these: setStandardInput, setStandardOutput, and setStandardError. When you call these methods, you pass an object, either an NSPipe or an NSFileHandle, as the input or output. In the next section, we'll discuss what on earth these classes are.

Piece Pipe

NSFileHandle is a class that provides a Cocoa-licious way to exercise control over reading and writing files. NSPipe represents a Unix pipe, a communication channel that reads data from one process and writes it to another. In Cocoa, NSPipe is implemented using two NSFileHandles, one for reading and one for writing. You can use an NSPipe together with an NSTask to send data to a shell command's standard input and then get data from its standard output. As an example, we'll create a application that takes a list of words in an NSTextView and alphabetizes the words by calling the sort shell command. The alphabetized list is then displayed in a another NSTextView. This example is based on code from the course material for the awesome Big Nerd Ranch Cocoa Boot Camp (http://www.bignerdranch.com).


Figure 1. This is what our finished application will look like.

First, go to Xcode and create a new Cocoa Application project. We'll begin our app with the part that's easy to do and so hard to describe in text: the Interface Builder portion. In IB, in the main window, add two NSTextView objects, with a button between them. When you run this app, you'll type words in the first text view, then click the button, and the words will appear, sorted, in the second text view.

Now let's make the obligatory AppController class and object. Select NSObject on the Classes tab and choose Classes a Subclass NSObject. Name the new class AppController. Next, open the Info window (Tools a Show Info), go to the Attributes screen, and add two outlets (inText and outText) and one action (sort). Create the source files for Xcode by choosing Classes a Create Files for AppController. Finally, add an instance of your AppController class by choosing Classes a Instantiate AppController.

Now it's time to get our objects to partner up. We want AppController to be connected to both text views, one for input and the other for output. First, Control-drag from the AppController instance to the text view on the left. Double-click inText. Then Control-drag from the AppController to the text view on the right, and double-click outText to connect it. Finally, we need to make the button perform the sort method. Control-drag from the button to AppController, make sure you're on the Target/Action tab of the info window, then double-click sort. Bueno.


Figure 2. Lay out the user interface in Interface Builder, as usual.

Revealing the Code

Now it's time to write the code, so warm up your brain and your typing fingers. All our code will be the implementation for the sort method in AppController.m. First things first:

- (IBAction)sort:(id)sender
{
   NSTask *task;   
      // Task object that will call "sort"
   NSData *sortResult; 
      // Data object for grabbing sorted text
      
   NSFileHandle *fileToWrite;
      // Handle to standard input for pipe
   NSPipe *inputPipe, *outputPipe;
      // The pipes themselves, for input and output
   NSString *typedText;
      // Holds text typed by the user in the text view
   NSString *sortedText;
      // Will hold sorted text after user clicks button
   task = [[NSTask alloc] init];
   inputPipe = [[NSPipe alloc] init];
   outputPipe = [[NSPipe alloc] init];
      // Don't forget to release these later

After declaring and initializing variables, it's time to get the sort command going. We need to create a task and tell it that it's going to execute sort, set up its other values, then send it out into the world, like so:

   [task setLaunchPath:@"/usr/bin/sort"];
      // Set the path to the executable. You can find
      // the path for a shell command by using the
      // command "which" in Terminal, e.g. "which sort".
   [task setStandardOutput: outputPipe];
   [task setStandardInput: inputPipe];
      // Set the standard input and standard output
      // for the task. By assigning these to pipes,
      // we can feed input to the task and read its output.
   [task setArguments: [NSArray arrayWithObject:@"-f"]];
      // Set the arguments for sort. We're setting one
      // argument, -f, which specifies a case-insensitive
      // sort (you know, the way humans like it).
   
   [task launch];
      // The task is all ready to go - start it up!

Now that sort is ready to run, it's time for us to supply it with something to sort. We'll take the text the user has typed into the view on the left and ship it off to the process, using a pipe:

   fileToWrite = [inputPipe fileHandleForWriting];
      // Get a handle to the pipe that we can use
      // for writing.
   typedText = [inText string];
      // Extract the text out of the
      // first text view (inText).
   [fileToWrite writeData:[
      typedText dataUsingEncoding: NSASCIIStringEncoding]];
      // We've got the text out of the field. Now turn around
      // and write it to the task's standard input as ASCII.
      // Mmm...ASCII.
   [fileToWrite closeFile];
      // Close the pipe when we're done writing.

The sort command can now execute, using the text we've written to its standard input. The command proceeds to do its sorting on the input, then writes the sorted result to standard output. We previously hooked up standard output to outputPipe, so now we can deal with the sorted text:

   sortResult = 
    [[outputPipe fileHandleForReading] readDataToEndOfFile];
      // Grab the sorted text by getting a handle for reading
      // the pipe, then read all the data.
   sortedText = [[NSString alloc] initWithData: sortResult
                           encoding: NSASCIIStringEncoding];
      // Encode the text as ASCII and put the result into 
      // sortedText.
   [outText setString: sortedText];
      // Copy the string into the outText view (the one
      // on the right).

At this point, we're basically finished - the sorted text appears in the text view on the right. But just like Mom said, we need to clean up before we're completely done.

   [sortedText release];
   [task release];
   [inputPipe release];
   [outputPipe release];

All About the Process

Now that you've seen how to wrap a shell command in a Cocoa application, you can use shell commands as you need them in your apps. As we saw in this month's installment, you don't have to show any sign of the underlying command if you don't want to - just get control of standard input and standard output, and you can keep the shell command hidden.

For fun projects that use this technique, try writing apps that wrap your own favorite shell commands. Or, for extra credit, you could write a Cocoa app that lets the user type any shell command and its arguments, then executes that command and shows its output. Whatever you do, remember that it's your computer -- you're in command!


Scott Knaster writes all sorts of stuff for all kinds of people to read, but nothing is more fun than messing around with Cocoa and OS X. Scott's latest book is Mac Toys, from Wiley Publishing. Scott eats a nutritious assortment of fruits and vegetables.

 
AAPL
$119.00
Apple Inc.
+1.40
MSFT
$47.75
Microsoft Corpora
+0.28
GOOG
$540.37
Google Inc.
-0.71

MacTech Search:
Community Search:

Software Updates via MacUpdate

Skype 7.2.0.412 - Voice-over-internet ph...
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
HoudahSpot 3.9.6 - Advanced file search...
HoudahSpot is a powerful file search tool built upon MacOS X Spotlight. Spotlight unleashed Create detailed queries to locate the exact file you need Narrow down searches. Zero in on files Save... Read more
RapidWeaver 6.0.3 - Create template-base...
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
iPhoto Library Manager 4.1.10 - Manage m...
iPhoto Library Manager lets you organize your photos into multiple iPhoto libraries. Separate your high school and college photos from your latest summer vacation pictures. Or keep some photo... Read more
iExplorer 3.5.1.9 - View and transfer al...
iExplorer is an iPhone browser for Mac lets you view the files on your iOS device. By using a drag and drop interface, you can quickly copy files and folders between your Mac and your iPhone or... Read more
MacUpdate Desktop 6.0.3 - Discover and i...
MacUpdate Desktop 6 brings seamless 1-click installs and version updates to your Mac. With a free MacUpdate account and MacUpdate Desktop 6, Mac users can now install almost any Mac app on macupdate.... Read more
SteerMouse 4.2.2 - Powerful third-party...
SteerMouse is an advanced driver for USB and Bluetooth mice. It also supports Apple Mighty Mouse very well. SteerMouse can assign various functions to buttons that Apple's software does not allow,... Read more
iMazing 1.1 - Complete iOS device manage...
iMazing (was 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 from... Read more
PopChar X 7.0 - Floating window shows av...
PopChar X helps you get the most out of your font collection. With its crystal-clear interface, PopChar X provides a frustration-free way to access any font's special characters. Expanded... Read more
OneNote 15.4 - Free digital notebook fro...
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

Latest Forum Discussions

See All

Lucha Amigos (Games)
Lucha Amigos 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Forget Ninja Turtles, and meet Wrestlers Turtles! Crazier, Spicier and…Bouncier! Sling carapaces of 7 Luchadores to knock all... | Read more »
Raby (Games)
Raby 1.0.3 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.3 (iTunes) Description: ***WARNING - Raby runs on: iPhone 5, iPhone 5C, iPhone 5S, iPhone 6, iPhone 6 Plus, iPad Mini Retina, iPad Mini 3, iPad 4, iPad Air,... | Read more »
Oddworld: Stranger's Wrath (Games)
Oddworld: Stranger's Wrath 1.0 Device: iOS Universal Category: Games Price: $5.99, Version: 1.0 (iTunes) Description: ** PLEASE NOTE: Oddworld Stranger's Wrath requires at least an iPhone 4S, iPad 2, iPad Mini or iPod Touch 5th gen... | Read more »
Bounce On Back (Games)
Bounce On Back 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »
Dwelp (Games)
Dwelp 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: === 50% off for a limited time, to celebrate release === Dwelp is an elegant little puzzler with a brand new game mechanic. To complete a... | Read more »
Make Way for Fat Chicken, from the Maker...
Make Way for Fat Chicken, from the Makers of Scrap Squad Posted by Jessica Fisher on November 26th, 2014 [ permalink ] Relevant Games has announced they will be releasing their reverse tower defense game, | Read more »
Tripnary Review
Tripnary Review By Jennifer Allen on November 26th, 2014 Our Rating: :: TRAVEL BUCKET LISTiPhone App - Designed for the iPhone, compatible with the iPad Want to create a travel bucket list? Tripnary is a fun way to do exactly that... | Read more »
Ossian Studios’ RPG, The Shadow Sun, is...
Ossian Studios’ RPG, The Shadow Sun, is Now Available for $4.99 Posted by Jessica Fisher on November 26th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Mmmm, Tasty – Having the Angry Birds for...
The very first Angry Birds debuted on iOS back in 2009. When you sit back and tally up the number of Angry Birds games out there and the impact they’ve had on pop culture as a whole, you just need to ask yourself: “How would the birds taste... | Read more »
Rescue Quest Review
Rescue Quest Review By Jennifer Allen on November 26th, 2014 Our Rating: :: PATH BASED MATCH-3Universal App - Designed for iPhone and iPad Guide a wizard to safety by matching gems. Rescue Quest might not be an entirely original... | Read more »

Price Scanner via MacPrices.net

Black Friday: 15% off iTunes Gift Cards
Staples is offering 15% off $50 and $100 iTunes Gift Cards on their online store as part of their Black Friday sale. Click here for more information. Shipping is free. Best Buy is offering $100... Read more
BEVL Releases Dock Tailored for iPhone 6 and...
Seattle based BEVL has released their first product: an iPhone dock that is divergent in build quality, rock-solid function and visual simplicity to complement the iPhone. BEVL is now accepting... Read more
Black Friday: $150 off 13-inch Retina MacBook...
 Best Buy has 13-inch 2.6GHz Retina MacBook Pros on sale for $150 off MSRP on their online store as part of their Black Friday sale. Choose free shipping or free local store pickup (if available).... Read more
Black Friday: $300 off 15-inch Retina MacBook...
 B&H Photo has the new 2014 15″ Retina MacBook Pros on sale for $300 off MSRP as part of their Black Friday sale. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.2GHz Retina... Read more
Black Friday: Up to $140 off MacBook Airs, fr...
 B&H Photo has 2014 MacBook Airs on sale for up to $140 off MSRP as part of their Black Friday sale. Shipping is free, and B&H charges NY sales tax only: - 11″ 128GB MacBook Air: $799 $100... Read more
Black Friday: 13-inch 2.5GHz MacBook Pro on s...
 Best Buy has the 13″ 2.5GHz MacBook Pro on sale for $899.99 on their online store as part of their Black Friday sale. Choose free shipping or free instant local store pickup (if available). Their... Read more
Black Friday: 21-inch 1.4GHz iMac on sale for...
 Best Buy has the 21″ 1.4GHz iMac on sale for $899.99 on their online store as part of their Black Friday sale. Their price is $200 off MSRP. Choose free shipping or free local store pick up. Price... Read more
Black Friday iPad Air 2 sale prices, $100 off...
 Best Buy has iPad Air 2s on sale for $100 off MSRP on their online store for Black Friday. Choose free shipping or free local store pickup (if available). Sale prices available for online orders... Read more
2014 1.4GHz Mac mini on sale for $449, save $...
 B&H Photo has the new 1.4GHz Mac mini on sale for $449.99 including free shipping plus NY tax only. Their price is $50 off MSRP, and it’s the lowest price available for this new model. Adorama... Read more
Early Black Friday pricing on 27-inch 5K iMac...
 B&H Photo continues to offer Black Friday sale prices on the 27″ 3.5GHz 5K iMac, in stock today and on sale for $2299 including free shipping plus NY sales tax only. Their price is $200 off MSRP... Read more

Jobs Board

*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
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* 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* 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
*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
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.