Refine script (#58)

README.md

@@ -69,8 +69,8 @@ Each file in the `data` directory can be used as a rule in this format: `geosite
 - Install `golang` and `git`
 - Download and install project code: `go get -u -v --insecure github.com/v2fly/domain-list-community`
 - Generate `dlc.dat` (without `datapath` option means to use `data` directory of this repository in `$GOPATH`):
-  - `$(go env GOPATH)/bin/domain-list-community`
-  - `$(go env GOPATH)/bin/domain-list-community --datapath=/path/to/your/custom/data/directory`
+  - `${GOPATH:-$(go env GOPATH)}/bin/domain-list-community`
+  - `${GOPATH:-$(go env GOPATH)}/bin/domain-list-community --datapath=/path/to/your/custom/data/directory`
 
 ## Structure of data