Add method GetStringEnv which will interpolate values which look like environmental variables and return them;

go.mod

@@ -6,10 +6,12 @@ require (
 	github.com/magiconair/properties v1.8.0
 	github.com/mitchellh/mapstructure v1.0.0
 	github.com/pelletier/go-toml v1.2.0
+	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/spf13/afero v1.1.2
 	github.com/spf13/cast v1.2.0
 	github.com/spf13/jwalterweatherman v1.0.0
 	github.com/spf13/pflag v1.0.2
+	github.com/stretchr/testify v1.2.2
 	golang.org/x/sys v0.0.0-20180906133057-8cf3aee42992 // indirect
 	golang.org/x/text v0.3.0 // indirect
 	gopkg.in/yaml.v2 v2.2.1

go.sum

@@ -1,3 +1,4 @@
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/fsnotify/fsnotify v1.4.7 h1:IXs+QLmnXW2CcXuY+8Mzv/fWEsPGWxqefPtCP5CnV9I=
 github.com/fsnotify/fsnotify v1.4.7/go.mod h1:jwhsz4b93w/PPRr/qN1Yymfu8t87LnFCMoQvtojpjFo=
@@ -9,6 +10,8 @@ github.com/mitchellh/mapstructure v1.0.0 h1:vVpGvMXJPqSDh2VYHF7gsfQj8Ncx+Xw5Y1KH
 github.com/mitchellh/mapstructure v1.0.0/go.mod h1:FVVH3fgwuzCH5S8UJGiWEs2h04kUh9fWfEaFds41c1Y=
 github.com/pelletier/go-toml v1.2.0 h1:T5zMGML61Wp+FlcbWjRDT7yAxhJNAiPPLOFECq181zc=
 github.com/pelletier/go-toml v1.2.0/go.mod h1:5z9KED0ma1S8pY6P1sdut58dfprrGBbd/94hg7ilaic=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/spf13/afero v1.1.2 h1:m8/z1t7/fwjysjQRYbP0RD+bUIF/8tJwPdEZsI83ACI=
 github.com/spf13/afero v1.1.2/go.mod h1:j4pytiNVoe2o6bmDsKpLACNPDBIoEAkihy7loJ1B0CQ=
 github.com/spf13/cast v1.2.0 h1:HHl1DSRbEQN2i8tJmtS6ViPyHx35+p51amrdsiTCrkg=
@@ -17,6 +20,8 @@ github.com/spf13/jwalterweatherman v1.0.0 h1:XHEdyB+EcvlqZamSM4ZOMGlc93t6AcsBEu9
 github.com/spf13/jwalterweatherman v1.0.0/go.mod h1:cQK4TGJAtQXfYWX+Ddv3mKDzgVb68N+wFjFa4jdeBTo=
 github.com/spf13/pflag v1.0.2 h1:Fy0orTDgHdbnzHcsOgfCN4LtHf0ec3wwtiwJqwvf3Gc=
 github.com/spf13/pflag v1.0.2/go.mod h1:DYY7MBk1bdzusC3SYhjObp+wFpr4gzcvqqNjLnInEg4=
+github.com/stretchr/testify v1.2.2 h1:bSDNvY7ZPG5RlJ8otE/7V6gMiyenm9RtJ7IUVIAoJ1w=
+github.com/stretchr/testify v1.2.2/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=
 golang.org/x/sys v0.0.0-20180906133057-8cf3aee42992 h1:BH3eQWeGbwRU2+wxxuuPOdFBmaiBH81O8BugSjHeTFg=
 golang.org/x/sys v0.0.0-20180906133057-8cf3aee42992/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/text v0.3.0 h1:g61tztE5qeGQ89tm6NTjjM9VPIm088od1l6aSorWRWg=

util.go

@@ -17,6 +17,7 @@ import (
 	"runtime"
 	"strings"
 	"unicode"
+	"regexp"
 
 	"github.com/spf13/afero"
 	"github.com/spf13/cast"
@@ -219,3 +220,19 @@ func deepSearch(m map[string]interface{}, path []string) map[string]interface{}
 	}
 	return m
 }
+// We look up a environmental variable if it looks like '${HOME}'
+func lookupEnvByValue(s string) string {
+	match, err := regexp.MatchString("\\${[A-Za-z_-]+}", s)
+	if (err != nil) {
+		fmt.Println(match, err)
+	}
+	if (match) {	
+		re := regexp.MustCompile("(\\$|{|}|)")
+		clean := re.ReplaceAllString(s, "")
+		env := os.Getenv(clean)
+		if ( env != "" ) {
+			return  env
+		}
+	}
+	return s
+}

viper.go

@@ -715,6 +715,12 @@ func (v *Viper) GetString(key string) string {
 	return cast.ToString(v.Get(key))
 }
 
+// GetStringEnv returns the value of the environmental variable associated with the key as a string.
+func GetStringEnv(key string) string { return lookupEnvByValue(v.GetString(key)) }
+func (v *Viper) GetStringEnv(key string) string {
+	return lookupEnvByValue(cast.ToString(v.Get(key)))
+}
+
 // GetBool returns the value associated with the key as a boolean.
 func GetBool(key string) bool { return v.GetBool(key) }
 func (v *Viper) GetBool(key string) bool {

viper_test.go

@@ -48,6 +48,11 @@ eyes : brown
 beard: true
 `)
 
+var yamlExampleWithEnv = []byte(`
+home: '${HOME_STUFF}'
+undefined: '${UNDEFINED}'
+`)
+
 var yamlExampleWithExtras = []byte(`Existing: true
 Bogus: true
 `)
@@ -437,6 +442,21 @@ func TestSetEnvKeyReplacer(t *testing.T) {
 	assert.Equal(t, "30s", Get("refresh-interval"))
 }
 
+func TestGetStringEnv(t *testing.T) {
+	Reset()	
+	home := "."
+	os.Setenv("HOME_STUFF", home)
+	v := New()
+	v.SetConfigType("yaml")
+	v.ReadConfig(bytes.NewBuffer(yamlExampleWithEnv))
+	value := v.GetStringEnv("home")
+	// defined env variables are interpolated
+	assert.Equal(t, ".", value)
+	// undefined env variables are untouched
+	undefined_value := v.GetStringEnv("undefined")
+	assert.Equal(t, "${UNDEFINED}", undefined_value)
+}
+
 func TestAllKeys(t *testing.T) {
 	initConfigs()