Re-add global env variable substitution (#227)

Co-authored-by: Stanislau Arsoba <[email protected]>

Author: Envek

fixtures/substitutions.env

@@ -3,3 +3,4 @@ OPTION_B=${OPTION_A}
 OPTION_C=$OPTION_B
 OPTION_D=${OPTION_A}${OPTION_B}
 OPTION_E=${OPTION_NOT_DEFINED}
+OPTION_F=${GLOBAL_OPTION}

godotenv_test.go

@@ -207,15 +207,21 @@ func TestLoadQuotedEnv(t *testing.T) {
 
 func TestSubstitutions(t *testing.T) {
 	envFileName := "fixtures/substitutions.env"
+
+	presets := map[string]string{
+		"GLOBAL_OPTION": "global",
+	}
+
 	expectedValues := map[string]string{
 		"OPTION_A": "1",
 		"OPTION_B": "1",
 		"OPTION_C": "1",
 		"OPTION_D": "11",
 		"OPTION_E": "",
+		"OPTION_F": "global",
 	}
 
-	loadEnvAndCompareValues(t, Load, envFileName, expectedValues, noopPresets)
+	loadEnvAndCompareValues(t, Load, envFileName, expectedValues, presets)
 }
 
 func TestExpanding(t *testing.T) {

parser.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
+	"os"
 	"regexp"
 	"strings"
 	"unicode"
@@ -264,6 +265,12 @@ func expandVariables(v string, m map[string]string) string {
 		if submatch[1] == "\\" || submatch[2] == "(" {
 			return submatch[0][1:]
 		} else if submatch[4] != "" {
+			if val, ok := m[submatch[4]]; ok {
+				return val
+			}
+			if val, ok := os.LookupEnv(submatch[4]); ok {
+				return val
+			}
 			return m[submatch[4]]
 		}
 		return s