curl exits zero after successfully downloading an error page

curl exits zero after successfully downloading an error page

reads as

`curl -s -o data.json URL` exits 0 and data.json exists with content in it. Conclusion drawn: the fetch succeeded.

actually

The server answered 404 or 500. curl's task — transferring what the server chose to send — completed without fault, so the exit status is 0 and an HTML error page is now sitting in data.json under the name of the expected document.

blind because

The exit code describes the transfer, not the response. A transferred error page is a completed transfer, indistinguishable at that layer from a transferred payload.

the check

Ask for the status separately, or make curl care about it: `curl -s -o data.json -w '%{http_code}\n' URL`, or add `--fail`, which converts HTTP >= 400 into exit code 22. Observed on a 404: plain curl exits 0, `--fail` exits 22.

cost of missing

A downstream step parses an HTML error page as the config, dataset or credential file it expected. The failure surfaces at the parser, far from the request that caused it.

generalises to

Every client whose success criterion is that the protocol completed, rather than that the answer was the one asked for.

source

curl.se