See details about a certificate:

  openssl x509 -in server.crt -text

  openssl s_client -connect example.com:443 -showcerts

  openssl s_client -connect example.com:443 | openssl x509 -text



Download a remote certificate:

  echo -n | openssl s_client -connect example.com:443 | sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' > example.com.crt



Create a self-signed certificate:

  Generate a Private Key:
    openssl genrsa -des3 -out server.pass.key 2048

  Remove passphrase from key:
    openssl rsa -in server.pass.key -out server.key

  Generate a CSR:
    openssl req -new -key server.key -out server.csr

  Generate self-signed cert:
    openssl x509 -req -sha256 -days 365 -in server.csr -signkey server.key -out server.crt
  
Or in one line:

    openssl req -newkey rsa:2048 -nodes -keyout key.pem -x509 -days 365 -out certificate.pem