S3cmd tools is a free command line tool and client for uploading, retrieving and managing data in Amazon S3 and other cloud storage service providers that use the S3 protocol, such as Google Cloud Storage or DreamHost DreamObjects.
brew install s3cmd
To authenticate with a bucket you'll need its access key and secret key. These will then need to be passed in as arguments each time you call S3cmd. To list what's in a bucket for example
s3cmd --access_key=UJUJUJU34344EDCOLM --secret_key=VBdsfjh875345MUTYguuiDC868689 ls s3://my-s3-bucket
To download a file
s3cmd --access_key=UJUJUJU34344EDCOLM --secret_key=VBdsfjh875345MUTYguuiDC868689 get s3://my-s3-bucket/name-of-file.tar.gz.enc
To upload a file
s3cmd --access_key=UJUJUJU34344EDCOLM --secret_key=VBdsfjh875345MUTYguuiDC868689 put myfile.tar.gz.enc s3://my-s3-bucket
For full details checkout http://s3tools.org/usage