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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 (

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.


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