Appending to the end of a file with NSMutableString

I guess you could do a couple of things:

NSFileHandle *fileHandle = [NSFileHandle fileHandleForWritingAtPath:aPath];
[fileHandle seekToEndOfFile];
[fileHandle writeData:[textToWrite dataUsingEncoding:NSUTF8StringEncoding]];
[fileHandle closeFile];

Note that this will append NSData to your file -- NOT an NSString. Note that if you use NSFileHandle, you must make sure that the file exists before hand. fileHandleForWritingAtPath will return nil if no file exists at the path. See the NSFileHandle class reference.

Or you could do:

NSString *contents = [NSString stringWithContentsOfFile:filepath];
contents = [contents stringByAppendingString:textToWrite];
[contents writeToFile:filepath atomically:YES encoding: NSUnicodeStringEncoding error:&err];

I believe the first approach would be the most efficient, since the second approach involves reading the contents of the file into an NSString before writing the new contents to the file. But, if you do not want your file to contain NSData and prefer to keep it text, the second option will be more suitable for you.

[Update] Since stringWithContentsOfFile is deprecated you can modify second approach:

NSError* error = nil;
NSString* contents = [NSString stringWithContentsOfFile:filepath
                                               encoding:NSUTF8StringEncoding
                                                  error:&error];
if(error) { // If error object was instantiated, handle it.
    NSLog(@"ERROR while loading from file: %@", error);
    // …
}
[contents writeToFile:filepath atomically:YES
                                 encoding:NSUnicodeStringEncoding
                                    error:&err];

See question on stackoverflow

Initially I thought that using the FileHandler method in the accepted answer that I was going to get a bunch of hex data values written to my file, but I got readable text which is all I need. So based off the accepted answer, this is what I came up with:

-(void) writeToLogFile:(NSString*)content{
    content = [NSString stringWithFormat:@"%@\n",content];

    //get the documents directory:
    NSString *documentsDirectory = [NSHomeDirectory() stringByAppendingPathComponent:@"Documents"];
    NSString *fileName = [documentsDirectory stringByAppendingPathComponent:@"hydraLog.txt"];

    NSFileHandle *fileHandle = [NSFileHandle fileHandleForWritingAtPath:fileName];
    if (fileHandle){
        [fileHandle seekToEndOfFile];
        [fileHandle writeData:[content dataUsingEncoding:NSUTF8StringEncoding]];
        [fileHandle closeFile];
    }
    else{
        [content writeToFile:fileName
                  atomically:NO
                    encoding:NSUTF8StringEncoding
                       error:nil];
    }
}

This way if the file doesn't yet exist, you create it. If it already exists then you only append to it. Also, if you go into the plist and add a key under the information property list UIFileSharingEnabled and set the value to true then the user can sync with their computer and see the log file through iTunes.