The following command retrieves metadata for an object in a bucket named my-bucket:

aws s3api head-object --bucket my-bucket --key index.html

Output:

{
    "AcceptRanges": "bytes",
    "ContentType": "text/html",
    "LastModified": "Thu, 16 Apr 2015 18:19:14 GMT",
    "ContentLength": 77,
    "VersionId": "null",
    "ETag": "\"30a6ec7e1a9ad79c203d05a589c8b400\"",
    "Metadata": {}
}