openssl-s_client:tldr:c6144

This command is used to establish a secure SSL/TLS connection with a server and retrieve its SSL certificate. Let's break it down:

• openssl is the command-line tool used for various cryptographic operations, including SSL/TLS.
• s_client is a subcommand in OpenSSL that allows the client (your machine) to connect to an SSL/TLS server.
• -connect ${host}:443 specifies the server's address and port to which the client should connect. This command assumes the host variable holds the domain name or IP address of the server. :443 is the default port used for secure HTTPS communication.
• -showcerts tells s_client to display the server's SSL certificate(s) as part of the output.
• </dev/null is a shell redirection that provides an empty input to the s_client command. This is done to prevent any potential interference from user input, as s_client typically waits for user input interactively.

Overall, this command establishes a connection to a server, retrieves the SSL certificate(s) presented by the server, and displays them on the terminal.