NSTask with multiple pipe input

I'm trying to use pipes to handle a command that requires multiple inputs, but not quite sure how to do it. Here's a snippet of what I'm trying to do. I know how to handle the first input, but I'm at lost as to piping in the second input -newstdinpass

NSTask *task = [[NSTask alloc] init];
NSPipe *pipe = [NSPipe pipe];

[task setLaunchPath: @"/bin/sh"];
[task setArguments: [NSArray arrayWithObjects: @"-c", @"/usr/bin/hdiutil chpass -oldstdinpass -newstdinpass /path/to/dmg", nil]];
[task setStandardInput:pipe];
[task launch];

[[pipe fileHandleForWriting] writeData:[@"thepassword" dataUsingEncoding:NSUTF8StringEncoding]];
[[pipe fileHandleForWriting] closeFile];

[task waitUntilExit];
[task release];

So I know using hdiutil in this manner is a bit of a hack, but in terms of pipes, am I going about it the correct way?

Thanks.

UPDATE: In case others are wondering about this, a quick solution to my problem is to pass in a null-terminated string as Ken Thomases had pointed out below. Use [[NSString stringWithFormat:@"oldpass\0newpass\0"] dataUsingEncoding:NSUTF8StringEncoding] into the pipe. Now, still need to learn how to bridge multiple NSTasks with pipes...

You can create multiple NSTasks and a bunch of NSPipes and hook them together, or you can use the sh -c trick to feed a shell a command, and let it parse it and set up all the IPC.

Example :

NSTask *task;
task = [[NSTask alloc] init];
[task setLaunchPath: @"/bin/sh"];

NSArray *arguments;
arguments = [NSArray arrayWithObjects: @"-c",
                     @"cat /usr/share/dict/words | grep -i ham | rev | tail -5", nil];
[task setArguments: arguments];
// and then do all the other jazz for running an NSTask.

Reference : http://borkware.com/quickies/one?topic=nstask

Now, as for a "proper" command execution function, here's one I'm usually using...

Code :

/*******************************************************
 *
 * MAIN ROUTINE
 *
 *******************************************************/

- (void)runCommand:(NSString *)cmd withArgs:(NSArray *)argsArray
{
    //-------------------------------
    // Set up Task
    //-------------------------------

    if (task) { [self terminate]; }

    task = [[NSTask alloc] init];
    [task setLaunchPath:cmd];
    [task setArguments:argsArray];

    [task setStandardOutput:[NSPipe pipe]];
    [task setStandardError:[task standardOutput]];

    //-------------------------------
    // Set up Observers
    //-------------------------------

    [PP_NOTIFIER removeObserver:self];
    [PP_NOTIFIER addObserver:self 
                    selector:@selector(commandSentData:) 
                        name: NSFileHandleReadCompletionNotification 
                      object: [[task standardOutput] fileHandleForReading]];

    [PP_NOTIFIER addObserver:self 
                    selector:@selector(taskTerminated:) 
                        name:NSTaskDidTerminateNotification 
                      object:nil];

    //-------------------------------
    // Launch
    //-------------------------------
    [[[task standardOutput] fileHandleForReading] readInBackgroundAndNotify];

    [task launch];
}

/*******************************************************
 *
 * OBSERVERS
 *
 *******************************************************/

- (void)commandSentData:(NSNotification*)n
{
    NSData* d;
    d = [[n userInfo] valueForKey:NSFileHandleNotificationDataItem];

    if ([d length])
    {
        NSString* s = [[NSString alloc] initWithData:d encoding:NSUTF8StringEncoding];

        NSLog(@"Received : %@",s);
    }

    [[n object] readInBackgroundAndNotify]; 
}

- (void)taskTerminated:(NSNotification*)n
{
    [task release];
    task = nil;
}