How to run NSTask with multiple commands

Question

I'm trying to make a NSTask running a command like this:

ps -clx | grep 'Finder' | awk '{print $2}'

Here is my method

- (void) processByName:(NSString*)name {
    NSTask *task1 = [[NSTask alloc] init];
    NSPipe *pipe1 = [NSPipe pipe];
    [task1 waitUntilExit];
    [task1 setLaunchPath: @"/bin/ps"];
    [task1 setArguments: [NSArray arrayWithObjects: @"-clx", nil]];
    [task1 setStandardOutput: pipe1];

    NSTask *task2 = [[NSTask alloc] init];
    NSPipe *pipe2 = [NSPipe pipe];
    [task2 setLaunchPath: @"/usr/bin/grep"];
    [task2 setArguments: [NSArray arrayWithObjects: @"'Finder'", nil]];
    [task2 setStandardInput:pipe1];
    [task2 setStandardOutput: pipe2];

    NSTask *task3 = [[NSTask alloc] init];
    NSPipe *pipe3 = [NSPipe pipe];
    [task3 setLaunchPath: @"/usr/bin/grep"];
    [task3 setArguments: [NSArray arrayWithObjects: @"'{print $2}'", nil]];
    [task3 setStandardInput:pipe2];
    [task3 setStandardOutput: pipe3];

    NSFileHandle *file = [pipe3 fileHandleForReading];

    [task1 launch];
    [task2 launch];
    [task3 launch];

    NSData *data;
    data = [file readDataToEndOfFile];

    NSString *string;
    string = [[NSString alloc] initWithData: data encoding: NSUTF8StringEncoding];

    NSLog(@"Result: %@", string);
}

But the result is just

Result:

What am I doing wrong?

EDIT

- (void) processByName:(NSString*)name {
    NSTask *task1 = [[NSTask alloc] init];
    NSPipe *pipe1 = [NSPipe pipe];
    [task1 waitUntilExit];
    [task1 setLaunchPath: @"/bin/ps"];
    [task1 setArguments: [NSArray arrayWithObjects: @"-clx", nil]];
    [task1 setStandardOutput: pipe1];

    NSTask *task2 = [[NSTask alloc] init];
    NSPipe *pipe2 = [NSPipe pipe];
    [task2 setLaunchPath: @"/usr/bin/grep"];
    [task2 setArguments: [NSArray arrayWithObjects: @"'Finder'", nil]];
    [task2 setStandardInput:pipe1];
    [task2 setStandardOutput: pipe2];

    NSTask *task3 = [[NSTask alloc] init];
    NSPipe *pipe3 = [NSPipe pipe];
    [task3 setLaunchPath: @"/usr/bin/grep"];
    [task3 setArguments: [NSArray arrayWithObjects: @"'{print $2}'", nil]];
    [task3 setStandardInput:pipe2];
    [task3 setStandardOutput: pipe3];

    NSFileHandle *file = [pipe3 fileHandleForReading];

    [task1 launch];
    [task2 launch];
    [task3 launch];

    [[NSNotificationCenter defaultCenter] addObserverForName:NSTaskDidTerminateNotification
                                                      object:task3
                                                       queue:nil
                                                  usingBlock:^(NSNotification* notification){

                                                      NSData * data = [file readDataToEndOfFile];

                                                      NSString * string;
                                                      string = [[NSString alloc] initWithData: data encoding: NSUTF8StringEncoding];
                                                      NSLog(@"Result: %@", string);
                                                  }];
}

Racine

Wouldn't it make more sense to launch the tasks after you have attached the observer? What if a task completes before the observer is attached? – Pandurate

Answer 1

The tasks run in a separate process from your code, i.e., asychronously. They probably haven't finished (they may not have even launched!) by the time you get to the readDataToEndOfFile two lines later.

If you're already on a background thread here, you can poll their status: while( ![task isRunning]){, or if you're on the main thread, I'd suggest using GCD to put this onto a queue and doing the polling there.

Actually, better than that would be to use notifications:

[task3 launch];

[[NSNotificationCenter defaultCenter] addObserverForName:NSTaskDidTerminateNotification
                                                  object:task3
                                                   queue:nil
                                              usingBlock:^{

    NSData * data = [file readDataToEndOfFile];

    NSString * string;
    string = [[NSString alloc] initWithData: data encoding: NSUTF8StringEncoding];

    NSLog(@"Result: %@", string);
}];

See TN2050: Observing Process Lifetime Without Polling. Each NSTask will send NSTaskDidTerminateNotification when it terminates (you should, ideally, check its return code rather than assuming it ran successfully). You can create a block to be run when task3 sends that notification.

Unwelcome

Could you please provide me with a small example showing how I can do this using GCD? – Racine

You know, I just thought of a better way to do this. Please see my edit. – Unwelcome

Please see my edit. I've added the code, but I still returns an empty string. – Racine

Have you verified that the command string you're running does what you expect if you just enter it to the CL? I see two things that look funny: you're not using the argument to the method, and you're running grep '{print $2}' as the third task -- did you mean awk? – Unwelcome

I cannot get the right result from the code "Edit" part – Multiplier

Answer 2

The following code works for me.

NSTask *task1 = [[NSTask alloc] init];
NSPipe *pipe1 = [NSPipe pipe];
[task1 waitUntilExit];
[task1 setLaunchPath: @"/bin/sh"];
[task1 setArguments: [NSArray arrayWithObjects: @"-c",@"ps -A |grep -m1 Finder | awk '{print $1}'", nil]];
[task1 setStandardOutput: pipe1];
[task1 launch];

NSFileHandle *file = [pipe1 fileHandleForReading];
NSData * data = [file readDataToEndOfFile];

NSString * string;
string = [[NSString alloc] initWithData: data encoding: NSUTF8StringEncoding];
NSLog(@"Result: %@", string);

Multiplier

Answer 3

Almost 8 years later, I think grep binary is'nt in /usr/bin, for me it's in /bin. Also, for your awk command, you set the launch path to grep.

Delladelle