Getting curl to output HTTP status code

The response 000 indicates that cURL failed to execute for some reason. In such a case, you should test for cURL exit code rather than making assumptions. See the "Exit Codes" section of the curl manpage for a full list of exit codes and their meanings.

Failed DNS resolution (6)

$ curl -w "%{http_code}\n" http://example.invalid/ ; echo "Exit code: $?"
000
curl: (6) Could not resolve host: example.invalid
Exit code: 6

(As answered by ILIV)

Connection refused (7)

$ curl -w "%{http_code}\n" http://localhost:81/ ; echo "Exit code: $?"
000
curl: (7) Failed to connect to localhost port 81: Connection refused
Exit code: 7

Connection timed out (28)

$ curl -w "%{http_code}\n" -m 5 http://10.255.255.1/ ; echo "Exit code: $?"
000
curl: (28) Connection timed out after 5001 milliseconds
Exit code: 28

(As answered by Arun)

Server actually returns 000 for some reason (0)

Start a fake server:

$ nc -l -p 65535 & <<EOF
> HTTP/1.1 000 Fake Status Code
> Content-Length: 0
> Connection: close
>
> EOF

Client request:

$ curl -w "%{http_code}\n" http://localhost:65535/ ; echo "Exit code: $?"
000
Exit code: 0

No idea why this would happen in the real world, but hey. If cURL doesn't get a valid status code at all, it assumes 200.

--fail, as mentioned in the man page, seems to do the job:

$ curl --fail --location http://google.com/nope
$ echo $?
22