Although they resemble files, objects in Amazon S3 aren't really "files", just like S3 buckets aren't really directories. On a Unix system I can use head to preview the first few lines of a file, no matter how large it is, but I can't do this on a S3. So how do I do a partial read on S3?
S3 files can be huge, but you don't have to fetch the entire thing just to read the first few bytes. The S3 APIs support the HTTP Range: header (see RFC 2616), which take a byte range argument.
Just add a Range: bytes=0-NN header to your S3 request, where NN is the requested number of bytes to read, and you'll fetch only those bytes rather than read the whole file. Now you can preview that 900 GB CSV file you left in an S3 bucket without waiting for the entire thing to download. Read the full GET Object docs on Amazon's developer docs.