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