apptoken.go shows how to get an application access token. Pretty sure it's the same in theory as getauth.sh so I need to rename one of these and clean up the docs.
envset.go returns true if eBay secret environment variables have been set.
getauth.sh is a Bash shell script using curl that uses your client ID and client secret to do what eBay calls a "client credentials grant". In the rest of the OAuth2 world this is minting a temporary authorization code, which you're later expected to exchange for a real authorization token.