$GOPATH #编译工作目录，src目录存放准备编译的软件源代码
$GOROOT #golang 安装目录，尽量保持干净
Q: cannot find package “github.com/go-redis/redis”
# go get github.com/go-redis/redis #默认下载到$GOPATH/src目录
Q: unrecognized import path “golang.org/x/sys/unix” (https fetch: Get https://golang.org/x/sys/unix)
or unrecognized import path “golang.org/x/crypto/md4” (https fetch: Get https://golang.org/x/crypto/md4)
# cd $GOPATH/src/ || cd $GOROOT/src/
# mkdir -p golang.org/x
# cd golang.org/x
# git clone https://github.com/golang/sys
# git clone https://github.com/golang/crypto