$: curl -L -o opa https://openpolicyagent.org/downloads/latest/opa_darwin_amd64
$: curl -L -o opa https://openpolicyagent.org/downloads/latest/opa_linux_amd64
$: chmod 755 ./opa
$: ./opa eval Opa-Expression
$: ./opa run [Expression-File] [Data-File] [--server] # REPL (Read-Eval-Print-Loop) mode