Reading the output from a shell command in Objective-C

gistfile1.m

- (void) doIt {
  
  // Define the command name and the arguments array (the nil at the end is needed)
  NSArray *arguments = [NSArray arrayWithObjects:@"--some-argument", @"--another", nil];
  NSString *command = @"do-something-shell";
  
  // Setup the NSTask for the command execution
  NSTask *task    = [[NSTask alloc] init];
  task.launchPath = command;
  task.arguments  = arguments;
  
  // Create a NSPipe and assign the new pipe as the task's stdout
  NSPipe *pipe    = [NSPipe pipe];
  task.standardOutput = pipe;
  
  // Get a NSFileHandle from the pipe. This NSFileHandle will provide the stdout buffer
  NSFileHandle *fileHandle = [pipe fileHandleForReading];

  // Setup the current object as an Observer of the data available notification of the file handle
  // We set the selector to "thereIsData:", so thereIsData will be executed when the file handle triggers the
  // notification 
  // (this sentence is verbose)
  [[NSNotificationCenter defaultCenter] addObserver:self 
    selector:@selector(thereIsData:) 
    name:NSFileHandleDataAvailableNotification 
    object:fileHandle];

  // Setup the file handle to work asynchronously and trigger the notification when there is data
  // in the stdout
  [fileHandle waitForDataInBackgroundAndNotify];

  // And finally launch the task, asynchronously
  [task launch];
}

// The notification will be captured and run this method, 
// passing the NSFileHandle as the object property of the notification
- (void)thereIsData:(NSNotification *)notification {
  NSLog(@"%@", [[NSString alloc] initWithData:[notification.object availableData] encoding:NSUTF8StringEncoding]);
  [notification.object waitForDataInBackgroundAndNotify];
}

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