Amazon S3 now supports conditional writes | PutObject or CompleteMultipartUpload API

Amazon S3 adds support for conditional writes that can check for the existence of an object before creating it. This capability can help you more easily prevent applications from overwriting any existing objects when uploading data. You can perform conditional writes using PutObject or CompleteMultipartUpload API requests in both general purpose and directory buckets.
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AWS S3 Bucket CLI