Reading live output from shell script and updating progress bar

Question

Hi,

I have a shell script logging a list of numbers (1, 2, and so forth, one per line) to a log file. I need my ObjC app to read the log file every X seconds and then update my progress bar accordingly based on the last number logged in the log file.

What's the best way to do this?

Answer 1

You can create an instance of NSTimer and have it, every X seconds, call another method that's responsible for reading the file and updating your progress bar. That method can use NSString's stringWithContentsOfFile: to read the file into a string, then parse it appropriately.

For example:

// Create the invocation of the method to call
NSUInteger X = 2; // Change depending on how often you want the timer fired
NSMethodSignature *signature = [self methodSignatureForSelector:@selector(read)];
NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];

// Create the timer, adding it to the default run loop
NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:X
                                              invocation:invocation
                                                 repeats:YES];

And later, you define the selector read:

- (void)read {
    NSString *fileContents = [NSString stringWithContentsOfFile:@"aFile.txt"];
    NSArray *lines = [fileContents componentsSeparatedByString:@"\n"];
    NSString *lastLine = [lines lastObject];

    // Your own parsing and updating code here
}

If you need a definite stopping point, you can store timer in an ivar inside your class, then call [timer invalidate]; whenever your parsing code inside read determines you're done with whatever process you're executing.

Relevant docs:

• NSTimer: scheduledTimerWithTimeInterval:invocation:repeats:
• NSInvocation: invocationWithMethodSignature:
• NSObject: methodSignatureForSelector:
• NSString

Answer 2

Why not use NSFileHandle's readInBackgroundAndNotify instead? This way, your code only runs when something really does happen in the file.