Go build: “Cannot find package” (even though GOPATH is set)

It does not work because your foobar.go source file is not in a directory called foobar. go build and go install try to match directories, not source files.

1. Set $GOPATH to a valid directory, e.g. export GOPATH="$HOME/go"
2. Move foobar.go to $GOPATH/src/foobar/foobar.go and building should work just fine.

Additional recommended steps:

1. Add $GOPATH/bin to your $PATH by: PATH="$GOPATH/bin:$PATH"
2. Move main.go to a subfolder of $GOPATH/src, e.g. $GOPATH/src/test
3. go install test should now create an executable in $GOPATH/bin that can be called by typing test into your terminal.