diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a57bb30..b673462 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -1,24 +1,27 @@
 name: CI
 
-on:
-  push:
-    branches: [ main, dev ]
-  pull_request:
-    branches: [ main, dev ]
+on: ["push", "pull_request"]
 
 jobs:
   build:
     runs-on: ubuntu-latest
+    if: "!contains(github.event.head_commit.message, 'ci skip')"
+
     steps:
-    - uses: actions/checkout@v2
+      - name: Cancel Previous Runs
+        uses: styfle/cancel-workflow-action@0.8.0
+        with:
+          access_token: ${{ github.token }}
+
+      - uses: actions/checkout@v2
 
-    - name: Set up Go
-      uses: actions/setup-go@v2
-      with:
-        go-version: 1.16
+      - name: Set up Go
+        uses: actions/setup-go@v2
+        with:
+          go-version: 1.16
 
-    - name: Build
-      run: go build -v ./...
+      - name: Build
+        run: go build -v ./...
 
-    - name: Test
-      run: go test -v ./...
+      - name: Test
+        run: go test -v ./...
diff --git a/LICENSE b/LICENSE
index d9c14b3..0ce0358 100644
--- a/LICENSE
+++ b/LICENSE
@@ -1,6 +1,6 @@
 MIT License
 
-Copyright (c) 2021 golang-config
+Copyright (c) 2021 gopher-lib
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
diff --git a/README.md b/README.md
index b433151..ee9a15a 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,6 @@
 # [config](https://github.com/golang-config/config): Eloquent configuration for Golang apps.
 
 ## Features:
+
+- Substitutes `$VARIABLE` and `${VARIABLE}` with variables found in a shell environment.
+- By default loads `.env`, but with optional agruments to `config.LoadFile` can load other files.
diff --git a/config.go b/config.go
index acc486c..f70288b 100644
--- a/config.go
+++ b/config.go
@@ -2,55 +2,54 @@ package config
 
 import (
 	"fmt"
+	"io"
 	"os"
 	"path/filepath"
-	"regexp"
 	"strings"
 
 	"github.com/joho/godotenv"
 	"github.com/spf13/viper"
 )
 
-// Load loads configuration from provided file, interpolates
-// environement variables and then unmarshals it into provided struct.
-func Load(rawVal interface{}, filename string, envPath ...string) error {
+func LoadFile(rawVal interface{}, filename string, envPath ...string) error {
 	if len(envPath) > 0 && envPath[0] != "" {
 		if err := godotenv.Load(envPath[0]); err != nil {
-			return fmt.Errorf("failed to load env. file: %v", err)
+			return fmt.Errorf("config: failed to load env. file: %v", err)
 		}
 	} else {
 		// Ignore error as we are loading default env. file.
 		_ = godotenv.Load(".env")
 	}
+	f, err := os.Open(filename)
+	if err != nil {
+		return fmt.Errorf("config: failed to open config file: %v", err)
+	}
+	return Load(f, rawVal, strings.TrimPrefix(filepath.Ext(filename), "."))
+}
 
-	viper.SetConfigName(strings.TrimSuffix(filepath.Base(filename), filepath.Ext(filepath.Base(filename))))
-	viper.SetConfigType("yaml")
-	viper.AddConfigPath(filepath.Dir(filename))
-	err := viper.ReadInConfig() // Find and read the config file
+func Load(in io.Reader, rawVal interface{}, configType string) error {
+	viper.SetConfigType(configType)
+	err := viper.ReadConfig(in)
 	if err != nil {
-		return fmt.Errorf("failed to read in config: %w", err)
+		return fmt.Errorf("config: failed to read in config: %w", err)
 	}
 
-	replaceEnv(viper.AllKeys())
+	for _, key := range viper.AllKeys() {
+		newKey := os.Expand(viper.GetString(key), mapping)
+		viper.Set(key, newKey)
+	}
 
 	err = viper.Unmarshal(&rawVal)
 	if err != nil {
-		return fmt.Errorf("failed to unmarshal config: %w", err)
+		return fmt.Errorf("config: failed to unmarshal config: %w", err)
 	}
 	return nil
 }
 
-var validEnv = regexp.MustCompile(`^\$\{[a-zA-Z_]+[a-zA-Z0-9_]*\}$`)
-
-func replaceEnv(keys []string) {
-	for _, key := range keys {
-		old := viper.GetString(key)
-		var new string
-		if validEnv.MatchString(old) {
-			new = os.Getenv(old[2 : len(old)-1])
-		} else {
-			new = old
-		}
-		viper.Set(key, new)
+// mapping is second argument for os.Expand function.
+func mapping(s string) string {
+	if strings.HasPrefix(s, "$") {
+		return s
 	}
+	return os.Getenv(s)
 }
diff --git a/config_test.go b/config_test.go
index dbf24fe..b46736b 100644
--- a/config_test.go
+++ b/config_test.go
@@ -1,43 +1,67 @@
 package config
 
 import (
+	"os"
 	"reflect"
+	"strings"
 	"testing"
+
+	"github.com/spf13/viper"
 )
 
+func TestLoadFile(t *testing.T) {
+	viper.Reset()
+
+	type db struct {
+		User     string
+		Password string
+	}
+	type config struct {
+		Port             int
+		AuthSecret       string
+		Secret           string
+		Dollar           string
+		DB               db
+		ConnectionString string
+	}
+	var conf config
+	err := LoadFile(&conf, "./testdata/config.testing.yaml", "./testdata/.env.testing")
+	if err != nil {
+		t.Fatal(err)
+	}
+	expected := config{8080, "secret", "", "$dollar", db{"root", "admin"}, "root:admin@tcp(localhost:3306)/core?parseTime=true"}
+	if !reflect.DeepEqual(conf, expected) {
+		t.Errorf("not equal: %v != %v", conf, expected)
+	}
+}
+
 func TestLoad(t *testing.T) {
-	t.Run("simple config with interpolation", func(t *testing.T) {
-		type Config struct {
-			Port    int
-			Secret1 string
-			Secret2 string
-		}
-		var conf Config
-		err := Load(&conf, "testdata/config.testing.yaml", "testdata/.env.testing")
-		if err != nil {
-			t.Fatalf("error loading config: %v\n", err)
-		}
-		expected := Config{5432, "secret-value", ""}
-		if !reflect.DeepEqual(conf, expected) {
-			t.Errorf("not equal: %v != %v", conf, expected)
-		}
-	})
+	t.Run("variable substitution", func(t *testing.T) {
+		viper.Reset()
 
-	t.Run("more complex config with interpolation", func(t *testing.T) {
+		os.Setenv("DB_PASSWORD", "root")
+		const confStr = `
+port: 1234
+db:
+  user: postgres
+  password: ${DB_PASSWORD}
+empty: emp${T}ty
+`
 		type DB struct {
 			User     string
 			Password string
 		}
 		type Config struct {
-			Port int
-			DB   DB
+			Port  int
+			DB    DB
+			Empty string
 		}
 		var conf Config
-		err := Load(&conf, "testdata/config.testing.yaml", "testdata/.env.testing")
+		err := Load(strings.NewReader(confStr), &conf, "yaml")
 		if err != nil {
-			t.Fatalf("error loading config: %v\n", err)
+			t.Fatal(err)
 		}
-		expected := Config{5432, DB{"admin", "root"}}
+		expected := Config{1234, DB{"postgres", "root"}, "empty"}
 		if !reflect.DeepEqual(conf, expected) {
 			t.Errorf("not equal: %v != %v", conf, expected)
 		}
diff --git a/testdata/.env.testing b/testdata/.env.testing
index d9eab58..a936591 100644
--- a/testdata/.env.testing
+++ b/testdata/.env.testing
@@ -1,2 +1,6 @@
-SOME_SECRET=secret-value
-DB_PASSWORD=root
\ No newline at end of file
+AUTH_SECRET=secret
+DB_USER=root
+DB_PASSWORD=admin
+DB_HOST=localhost
+DB_PORT=3306
+DB_NAME=core
\ No newline at end of file
diff --git a/testdata/config.testing.yaml b/testdata/config.testing.yaml
index bfa7b65..4a4265b 100644
--- a/testdata/config.testing.yaml
+++ b/testdata/config.testing.yaml
@@ -1,6 +1,8 @@
-port: 5432
-secret1: ${SOME_SECRET}
-secret2: ${SECRET}
+port: 8080
+authSecret: ${AUTH_SECRET}
+secret: ${SECRET}
+dollar: $$dollar
 db:
-  user: admin
+  user: root
   password: ${DB_PASSWORD}
+connectionString: "${DB_USER}:${DB_PASSWORD}@tcp(${DB_HOST}:${DB_PORT})/${DB_NAME}?parseTime=true"