OPEN SSL https show certicate, CURL HTTPS connect, how do CURL validate certificate?

OPEN SSL: 

echo quit | openssl s_client -showcerts -servername server -connect server:443

server replaced by server name

openssl s_client -connect <endpoint hostname or IP address>:<port> -showcerts

https://daniel.haxx.se/blog/2018/11/07/get-the-ca-cert-for-curl/#:~:text=When%20you%20use%20curl%20to,the%20%E2%80%93cacert%20command%20line%20option.

use open ssl to get cert:

openssl s_client -showcerts -connect www.example.com:443 </dev/null

openssl s_client -connect <domain>:443 -showcerts

https://stackoverflow.com/questions/7885785/using-openssl-to-get-the-certificate-from-a-server

When you use curl to communicate with a HTTPS site (or any other protocol that uses TLS), it will by default verify that the server is signed by a trusted Certificate Authority (CA). It does this by checking the CA bundle it was built to use, or instructed to use with the –cacert command line option.

Sometimes you end up in a situation where you don’t have the necessary CA cert in your bundle. It could then look something like this:

https://daniel.haxx.se/blog/2018/11/07/get-the-ca-cert-for-curl/#:~:text=When%20you%20use%20curl%20to,the%20%E2%80%93cacert%20command%20line%20option.

$ curl https://example.com/
curl: (60) SSL certificate problem: self signed certificate
More details here: https://curl.haxx.se/docs/sslcerts.html