AWS S3: How to check if a file exists in a bucket using bash

Following to @DaveMaple & @MichaelGlenn answers, here is the condition I'm using:

aws s3api head-object --bucket <some_bucket> --key <some_key> || not_exist=true
if [ $not_exist ]; then
  echo "it does not exist"
else
  echo "it exists"
fi

Last time I saw performance comparisons getObjectMetadata was the fastest way to check if an object exists. Using the AWS cli that would be the head-object method, example:

aws s3api head-object --bucket www.codeengine.com --key index.html

which returns:

{
    "AcceptRanges": "bytes",
    "ContentType": "text/html; charset=utf-8",
    "LastModified": "Sun, 08 Jan 2017 22:49:19 GMT",
    "ContentLength": 38106,
    "ContentEncoding": "gzip",
    "ETag": "\"bda80810592763dcaa8627d44c2bf8bb\"",
    "StorageClass": "REDUCED_REDUNDANCY",
    "CacheControl": "no-cache, no-store",
    "Metadata": {}
}