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.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Dropbox 193.4.5594 - Cloud backup and sy...
Dropbox is a file hosting service that provides cloud storage, file synchronization, personal cloud, and client software. It is a modern workspace that allows you to get to all of your files, manage... Read more
Google Chrome 122.0.6261.57 - Modern and...
Google Chrome is a Web browser by Google, created to be a modern platform for Web pages and applications. It utilizes very fast loading of Web pages and has a V8 engine, which is a custom built... Read more
Skype 8.113.0.210 - Voice-over-internet...
Skype is a telecommunications app that provides HD video calls, instant messaging, calling to any phone number or landline, and Skype for Business for productive cooperation on the projects. This... Read more
Tor Browser 13.0.10 - Anonymize Web brow...
Using Tor Browser you can protect yourself against tracking, surveillance, and censorship. Tor was originally designed, implemented, and deployed as a third-generation onion-routing project of the U.... Read more
Deeper 3.0.4 - Enable hidden features in...
Deeper is a personalization utility for macOS which allows you to enable and disable the hidden functions of the Finder, Dock, QuickTime, Safari, iTunes, login window, Spotlight, and many of Apple's... Read more
OnyX 4.5.5 - Maintenance and optimizatio...
OnyX is a multifunction utility that you can use to verify the startup disk and the structure of its system files, to run miscellaneous maintenance and cleaning tasks, to configure parameters in the... Read more
Hopper Disassembler 5.14.1 - Binary disa...
Hopper Disassembler is a binary disassembler, decompiler, and debugger for 32- and 64-bit executables. It will let you disassemble any binary you want, and provide you all the information about its... Read more
WhatsApp 24.3.78 - Desktop client for Wh...
WhatsApp is the desktop client for WhatsApp Messenger, a cross-platform mobile messaging app which allows you to exchange messages without having to pay for SMS. WhatsApp Messenger is available for... Read more
War Thunder 2.33.0.135 - Multiplayer war...
In War Thunder, aircraft, attack helicopters, ground forces and naval ships collaborate in realistic competitive battles. You can choose from over 1,500 vehicles and an extensive variety of combat... Read more
Iridient Developer 4.2 - Powerful image-...
Iridient Developer (was RAW Developer) is a powerful image-conversion application designed specifically for OS X. Iridient Developer gives advanced photographers total control over every aspect of... Read more

Latest Forum Discussions

See All

A Legitimate Massage Parlor, I Swear – T...
In this week’s Episode of The TouchArcade Show we talk about some of the major new releases on mobile this week including Warframe, we go over all the major news that came out from the Nintendo Direct Partner Showcase on Wednesday, we read our one... | Read more »
TouchArcade Game of the Week: ‘Rainbow S...
I feel like I am in a fever dream right now. What is this game that I’m playing? It’s a Rainbow Six game? But it’s all cutesy, and cartoony, and also kind of psychedelic? How is this a real thing? Well, it’s no fever dream, it is indeed a real thing... | Read more »
SwitchArcade Round-Up: ‘Promenade’, ‘Cho...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for February 23rd, 2024. It’s Friday, so we have to check out the remaining releases of the week. Not so many big ones today, but a healthy crop nonetheless. After summarizing all the... | Read more »
Steam Deck Weekly: Gundam Breaker 4 and...
Welcome to this week’s slightly shorter edition of the Steam Deck Weekly. I was a bit unwell this week so no reviews in this edition, but there is a lot of news and new Steam Deck Verified and Playable games to catch up on. I have something special... | Read more »
The 10 Best Run-And-Gun Games for Ninten...
The year 2024 is a rare one, because it is a year with a brand-new Contra game. Contra: Operation Galuga might be the freshest face on the block when it comes to Nintendo Switch run-and-gun action games, but it’s hardly fighting that war alone.... | Read more »
Version 1.4 of Reverse: 1999 will be lan...
Free up your diary for February 29th, as Bluepoch has announced the impending release of the award-winning Reverse: 1999s Version 1.4 update. The Prisoner in the Cave is an Ancient Greece-themed update with new recruits, gameplay modes, and plenty... | Read more »
Premium Mobile RPG ‘Ex Astris’ From ‘Ark...
Arknights developer Hypergryph’s premium RPG Ex Astris () recently had its release date confirmed, and we finally have an extended gameplay showcase. | Read more »
Apple Arcade Weekly Round-Up: Updates fo...
Following yesterday’s big Hello Kitty Island Adventure update, a few more notable game updates and events have gone live on Apple Arcade. Cypher 007 () has gotten its first content update in a few months taking you to Egypt for five new levels... | Read more »
‘Thunder Ray’ and ‘Hime’s Quest’ Are Now...
Crunchyroll has pushed two new games into the Crunchyroll Game Vault including Purple Tree Studio’s Thunder Ray which was already on iOS before as a premium release. Shaun even reviewed it last year. Read his review here. The second game is Poppy... | Read more »
Adorable Kitty-Collector Sequel ‘Neko At...
Ya’ll. This October will mark the ten-year anniversary of Hit Point launching Neko Atsume, the adorable kitty collecting sim that has become a runaway success and essentially created a sub-genre of cozy pet-collecting life sim games. Sure, the... | Read more »

Price Scanner via MacPrices.net

16-inch M3 Max MacBook Pro on sale for $300 o...
Amazon is offering a $300 instant discount on one of Apple’s 16″ M3 Max MacBook Pros today. Shipping is free: – 16″ M3 Max MacBook Pros (36GB/1TB/Space Black): $3199, $300 off MSRP Their price is the... Read more
Apple M2 Mac minis on sale for $100 off MSRP...
B&H Photo has Apple’s M2-powered Mac minis in stock and on sale for $100 off MSRP this weekend with prices available starting at $499. Free 1-2 day shipping is available to most US addresses: –... Read more
Apple Watch SE on sale for $50 off MSRP this...
Best Buy has all Apple Watch SE models on sale this weekend for $50 off MSRP on their online store. Sale prices available for online orders only, in-store prices may vary. Order online, and choose... Read more
Deal Alert! Apple 15-inch M2 MacBook Airs on...
Looking for the lowest sale price on a new 15″ M2 MacBook Air? Best Buy has Apple 15″ MacBook Airs with M2 CPUs in stock and on sale today for $300 off MSRP on their online store. Prices valid for... Read more
Amazon discounts iPad mini 6 models up to $12...
Amazon is offering Apple’s 8.3″ WiFi iPad minis for $100-$120 off MSRP, including free shipping, for a limited time. Prices start at $399. Amazon’s prices are among the lowest currently available for... Read more
Apple AirPods Pro with USB-C discounted to $1...
Walmart has Apple’s 2023 AirPods Pro with USB-C in stock and on sale for $199.99 on their online store. Their price is $50 off MSRP, and it’s currently one the lowest prices available for new AirPods... Read more
Apple has 14-inch M3 MacBook Pro with 16GB of...
Apple has 14″ M3 MacBook Pros with 16GB of RAM, Certified Refurbished, available for $270-$300 off MSRP. Each model features a new outer case, shipping is free, and an Apple 1-year warranty is... Read more
Save $318-$432 on 16-inch M3 Max MacBook Pros...
Apple retailer Expercom has 16″ M3 Max MacBook Pros on sale for $318-$432 off MSRP when bundled with a 3-year AppleCare+ Protection Plan. Discounts are available for Silver models as well a Space... Read more
New today at Apple: 16-inch M3 Pro/M3 Max Mac...
Apple is now offering 16″ M3 Pro and M3 Max MacBook Pros, Certified Refurbished, starting at $2119 and ranging up to $530 off MSRP. Each model features a new outer case, shipping is free, and an... Read more
Apple is now offering $300-$480 discounts on...
Apple is now offering 14″ M3 Pro and M3 Max MacBook Pros, Certified Refurbished, starting at $1699 and ranging up to $480 off MSRP. Each model features a new outer case, shipping is free, and an... Read more

Jobs Board

Part-time *Apple* and Peach Research Assist...
…and minimum qualifications: + Assist with planting, pruning, and harvesting of apple and peach trees + Conduct regular maintenance tasks to ensure optimal Read more
Housekeeper, *Apple* Valley Villa - Cassia...
Apple Valley Villa, part of a senior living community, is hiring entry-level Full-Time Housekeepers to join our team! We will train you for this position and offer a Read more
Sublease Associate Optometrist- *Apple* Val...
Sublease Associate Optometrist- Apple Valley, CA- Target Optical Date: Feb 22, 2024 Brand: Target Optical Location: Apple Valley, CA, US, 92307 **Requisition Read more
Sublease Associate Optometrist- *Apple* Val...
Sublease Associate Optometrist- Apple Valley, CA- Target Optical Date: Jan 24, 2024 Brand: Target Optical Location: Apple Valley, CA, US, 92307 **Requisition Read more
Housekeeper, *Apple* Valley Village - Cassi...
Apple Valley Village Health Care Center, a senior care campus, is hiring a Part-Time Housekeeper to join our team! We will train you for this position! In this role, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.