How does `aws s3 sync` determine if a file has been updated?

AWS CLI sync:

A local file will require uploading if the size of the local file is different than the size of the s3 object, the last modified time of the local file is newer than the last modified time of the s3 object, or the local file does not exist under the specified bucket and prefix.

--size-only (boolean) Makes the size of each key the only criteria used to decide whether to sync from source to destination.

You want the --size-only option which looks only at the file size not the last modified date. This is perfect for an asset build system that will change the last modified date frequently but not the actual contents of the files (I'm running into this with webpack builds where things like fonts kept syncing even though the file contents were identical). If you don't use a build method that incorporates the hash of the contents into the filename it might be possible to run into problems (if build emits same sized file but with different contents) so watch out for that.

I did manually test adding a new file that wasn't on the remote bucket and it is indeed added to the remote bucket with --size-only.

This article is a bit dated but i'll contribute nonetheless for folks arriving here via google.

I agree with checked answer. To add additional context, AWS S3 functionality is different than standard linux s3 in a number of ways. In Linux, an md5hash can be computed to determine if a file has changed. S3 does not do this, so it can only determine based on size and/or timestamp. What's worse, AWS does not preserve timestamp when transferring either way, so timestamp is ignored when syncing to local and only used when syncing to s3.

According to this - http://docs.aws.amazon.com/cli/latest/reference/s3/sync.html

S3 sync compares the size of the file and the last modified timestamp to see if a file needs to be synced.

In your case, I'd suspect the build system is resulting in a newer timestamp even though the file size hasn't changed?