Expand the template.toml file

Signed-off-by: jolheiser <john.olheiser@gmail.com>

DOCS.md

@@ -11,6 +11,10 @@ A "valid" tmpl template only requires two things
 
 ## template.toml
 
+**NOTE:** The template.toml file will be expanded, though not with the full power of the template itself.  
+The template.toml file will only expand environment variables with syntax `$USER` or `${USER}`.  
+For full documentation on the syntax, see [os.ExpandEnv](https://golang.org/pkg/os/#ExpandEnv).
+
 ```toml
 # Key-value pairs can be simple
 # The user will receive a basic prompt asking them to fill out the variable
@@ -25,7 +29,7 @@ prompt = "The name of the author of this project"
 # help would be extra information (generally seen by giving '?' to a prompt)
 help = "Who will be primarily writing this project"
 # default is the "value" part of the simple pair. This could be a suggested value
-default = "me"
+default = "$USER"
 ```
 
 ## template directory

registry/prompt.go

@@ -2,6 +2,7 @@ package registry
 
 import (
 	"fmt"
+	"io/ioutil"
 	"os"
 	"path/filepath"
 	"sort"
@@ -25,7 +26,15 @@ func prompt(dir string, defaults bool) (templatePrompts, error) {
 		return nil, err
 	}
 
-	tree, err := toml.LoadFile(templatePath)
+	templateBytes, err := ioutil.ReadFile(templatePath)
+	if err != nil {
+		return nil, err
+	}
+
+	// Expand the template with environment variables
+	templateContents := os.ExpandEnv(string(templateBytes))
+
+	tree, err := toml.Load(templateContents)
 	if err != nil {
 		return nil, err
 	}

registry/template_test.go

@@ -12,7 +12,7 @@ var (
 	tmplTemplate = `name = "john olheiser"
 
 [year]
-default = 2020
+default = ${TMPL_TEST}
 
 [package]
 default = "pkg"`
@@ -21,6 +21,12 @@ default = "pkg"`
 )
 
 func testExecute(t *testing.T) {
+	// Set environment variable
+	if err := os.Setenv("TMPL_TEST", "2020"); err != nil {
+		t.Logf("could not set environment: %v", err)
+		t.FailNow()
+	}
+
 	// Get template
 	tmpl, err := reg.GetTemplate("test")
 	if err != nil {