openssl s_client -connect
openssl s_client -connect <host>:443
Risk: Safe
What it does
Opens a sealed line like a browser and shows the server's certificate.
When to use it
To read a certificate's names and dates.
Example
$ openssl s_client -connect kedai.example:443 < /dev/null 2>/dev/null | openssl x509 -noout -datesWhen the ID card starts and ends.
$ openssl x509 -noout -checkend 2592000 -in kedai.crtStill good in 30 days? Exit 0 means yes.
Learn it in: HTTPS and certificates · Official docs