List all databases with sqlcmd

sqlcmd -s <db server or instance> -q "exec sp_databases"

Examples:
 sqlcmd -s localhost -q "exec sp_databases"
 sqlcmd -s dbserver -u loginname -p password -q "exec sp_databases"
 sqlcmd -s ./MSSQLSERVER -u loginname -p password -q "exec sp_databases"

Deleting a database with sqlcmd

sqlcmd -s <servername> -q "DROP DATABASE <database name>"
OR
sqlcmd -s ./<sqlserver instance name> -q "DROP DATABASE <database name>"

Examples:

sqlcmd -s dbserver -u username -p password -q "DROP DATABASE products"

sqlcmd -s localhost -q "DROP DATABASE products" 

sqlcmd -s ./MSSQLSERVER -q "DROP DATABASE products"