Go 言語の環境変数管理

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/username/.cache/go-build"
GOENV="/home/username/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/username/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
...

$ export GO111MODULE=on

$ go env GO111MODULE
on

$ export -n GO111MODULE

$ go env -w GO111MODULE=auto

$ go env GO111MODULE
auto

$ go env -u GO111MODULE

$ go env | grep GO111MODULE
GO111MODULE=""

$ export GO111MODULE=on

$ go env -w GO111MODULE=auto
warning: go env -w GO111MODULE=... does not override conflicting OS environment variable

$ go env | grep GO111MODULE
GO111MODULE="on"

$ go env GOENV
/home/username/.config/go/env

// UserConfigDir returns the default root directory to use for user-specific
// configuration data. Users should create their own application-specific
// subdirectory within this one and use that.
//
// On Unix systems, it returns $XDG_CONFIG_HOME as specified by
// https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html if
// non-empty, else $HOME/.config.
// On Darwin, it returns $HOME/Library/Application Support.
// On Windows, it returns %AppData%.
// On Plan 9, it returns $home/lib.
//
// If the location cannot be determined (for example, $HOME is not defined),
// then it will return an error.
func UserConfigDir() (string, error) {
	var dir string

	switch runtime.GOOS {
	case "windows":
		dir = Getenv("AppData")
		if dir == "" {
			return "", errors.New("%AppData% is not defined")
		}

	case "darwin":
		dir = Getenv("HOME")
		if dir == "" {
			return "", errors.New("$HOME is not defined")
		}
		dir += "/Library/Application Support"

	case "plan9":
		dir = Getenv("home")
		if dir == "" {
			return "", errors.New("$home is not defined")
		}
		dir += "/lib"

	default: // Unix
		dir = Getenv("XDG_CONFIG_HOME")
		if dir == "" {
			dir = Getenv("HOME")
			if dir == "" {
				return "", errors.New("neither $XDG_CONFIG_HOME nor $HOME are defined")
			}
			dir += "/.config"
		}
	}

	return dir, nil
}