Completely unclear how to change a single metadata value without screwing up Content-Type

[zehawki]

Goal: I want to change the Cache-Control key to some value from its existing, eg max-age=xyz, without touching anything else, and esp without having Content-Type getting screwed.

Something so simple cannot be performed on the aws console, this used to be allowed earlier, but now one needs to go thru some convoluted copy step (https://docs.aws.amazon.com/AmazonS3/latest/userguide/add-object-metadata.html). Anyway - the net result of that is that the Content-Type turns to binary/octet-stream from image/jpeg or whatever after copy.

Fine, I thought this was an issue only on the console, and turned to the CLI. (The AWS userguide links to v1 of the CLI docs, while the current version is v2 - which leads to still more headache)

The v2 CLI is equally unhelpful - there is no adequate documentation as given here https://awscli.amazonaws.com/v2/documentation/api/latest/reference/s3/cp.html

The earlier --metadata-directive REPLACE has been removed and there is a new --copy-props. However its quite unclear how to use this to change a single metadata while leaving everything else the same. I cannot imagine that this is some complex usecase - why is this simple thing not supported? If it is supported - where is the helpful documentation around it?

[uldyssian-sh]

You can do this, but S3 requires a COPY for “metadata edits”, and when you use a REPLACE-style copy you must explicitly preserve any headers you don’t want to change (especially Content-Type). If you don’t, S3/CLI may fall back to application/octet-stream.

The most reliable approach is to use s3api copy-object (same-key self copy), read the current Content-Type first, then set only Cache-Control while re-supplying Content-Type.

Example (bash/macOS/Linux):

BUCKET="my-bucket"
KEY="path/to/object.jpg"
NEW_CC="max-age=31536000,public"

CT=$(aws s3api head-object --bucket "$BUCKET" --key "$KEY" --query ContentType --output text)

aws s3api copy-object
--bucket "$BUCKET"
--key "$KEY"
--copy-source "$BUCKET/$KEY"
--metadata-directive REPLACE
--content-type "$CT"
--cache-control "$NEW_CC"

Why this works

S3 only changes metadata via a COPY operation.
Using --metadata-directive REPLACE means “you are providing the metadata/headers now”.
Providing --content-type preserves the existing Content-Type, while --cache-control updates only Cache-Control.

Notes

If the object also uses other important headers (ContentDisposition, ContentEncoding, StorageClass, SSE-KMS, etc.), you must preserve those the same way (fetch via head-object, then pass the corresponding flags on copy-object), otherwise they may reset to defaults during REPLACE.

[uldyssian-sh]

Thanks! If this helps, please consider marking it as “Helpful” or “Accepted” 😊