Test a WebSocket using curl.

curl-websocket.sh

curl --include \
     --no-buffer \
     --header "Connection: Upgrade" \
     --header "Upgrade: websocket" \
     --header "Host: example.com:80" \
     --header "Origin: http://example.com:80" \
     --header "Sec-WebSocket-Key: SGVsbG8sIHdvcmxkIQ==" \
     --header "Sec-WebSocket-Version: 13" \
     http://example.com:80/

okay, changing Sec-WebSocket-key with the one provided from browser works (not sure why):

curl --include \
     --no-buffer \
     --header "Connection: Upgrade" \
     --header "Upgrade: websocket" \
     --header "Host: localhost:8080" \
     --header "Origin: http://localhost:8080" \
     --header "Sec-WebSocket-Key: 3tMWYzcHwmObiLR3nWOgsg==" \
     --header "Sec-WebSocket-Version: 13" \
     http://localhost:8080/
HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: YLxhg+bEzrZXCaOyyI5QkRBIdT4=

�       something

EDIT:
after reading the wikipedia, turned out it just a random 16-byte encoded with base64, on Linux, you can generate it with:

head -c 16 /dev/urandom | base64

This was very helpful. Thanks so much. In my case I just needed to confirm that the websocket server could be connected to and the connection would be upgraded, but I see how the headers and other settings can be changed as needed for any particular situation.

Just got to know about this so posting here to help others

WebSocket has been supported by curl as a non-experimental feature since version 8.11.0 (November 6 2024). With the upcoming release of version 8.16.0, we are taking it a step further.

With that release, you can use WebSocket in a pipe from the command line. Like this:

curl --no-progress-meter -T . -N wss://echo.websocket.org/
Request served by 4d896d95b55478dsadsa
Hello!
Hello!
you just echo what I send?
you just echo what I send?
...

This command line contacts the “echo” server run by websocket.org which just sends back what it receives from the client. The command line options are:

-T .: tells curl to upload the data from stdin, in non-blocking fashion.
-N: to avoid buffering of output data, so that even a single character received from the server is on stdout right away.
--no-progress-meter: to avoid curl cluttering stderr with information about how much was sent/received.