Fix/master: add envOrDefault helper

docs/templating-language.md

@@ -46,6 +46,7 @@ provides the following functions:
   - [containsNone](#containsnone)
   - [containsNotAll](#containsnotall)
   - [env](#env)
+  - [envOrDefault](#envOrDefault)
   - [executeTemplate](#executetemplate)
   - [explode](#explode)
   - [explodeMap](#explodemap)
@@ -1046,6 +1047,20 @@ Reads the given environment variable and if it does not exist or is blank use a
 {{ or (env "CLUSTER_ID") "12345" }}
 ```
 
+### `envOrDefault`
+
+Reads the given environment variable accessible to the current process. If the environment variable is found, the value of that variable will be used. This includes empty values. Otherwise, the default will be used instead.
+
+```liquid
+{{ envOrDefault "CLUSTER_NAME" "Default_Cluster" }}
+```
+
+This function can be chained to manipulate the output:
+
+```liquid
+{{ envOrDefault "CLUSTER_NAME" "Default_Cluster"  | toLower }}
+```
+
 ### `executeTemplate`
 
 Executes and returns a defined template.

template/funcs.go

@@ -80,6 +80,28 @@ func envFunc(env []string) func(string) (string, error) {
 	}
 }
 
+// envWithDefaultFunc returns a function which checks the value of an environment variable.
+// Invokers can specify their own environment, which takes precedences over any
+// real environment variables.
+// If an environment variable is found, the value of that variable will be used. This includes empty values.
+// Otherwise, the default will be used instead.
+func envWithDefaultFunc(env []string) func(string, string) (string, error) {
+	return func(s string, def string) (string, error) {
+		for _, e := range env {
+			split := strings.SplitN(e, "=", 2)
+			k, v := split[0], split[1]
+			if k == s {
+				return v, nil
+			}
+		}
+		val, isPresent := os.LookupEnv(s)
+		if isPresent {
+			return val, nil
+		}
+		return def, nil
+	}
+}
+
 // executeTemplateFunc executes the given template in the context of the
 // parent. If an argument is specified, it will be used as the context instead.
 // This can be used for nested template definitions.

template/template.go

@@ -260,6 +260,7 @@ func funcMap(i *funcMapInput) template.FuncMap {
 		"containsNone":    containsSomeFunc(true, false),
 		"containsNotAll":  containsSomeFunc(false, true),
 		"env":             envFunc(i.env),
+		"envOrDefault":    envWithDefaultFunc(i.env),
 		"executeTemplate": executeTemplateFunc(i.t),
 		"explode":         explode,
 		"explodeMap":      explodeMap,

template/template_test.go

@@ -286,6 +286,23 @@ func TestTemplate_Execute(t *testing.T) {
 			"[dc1 dc2]",
 			false,
 		},
+		{
+			"func_envOrDefault",
+			&NewTemplateInput{
+				Contents: `{{ envOrDefault "SET_VAR" "100" }} {{ envOrDefault "EMPTY_VAR" "200" }} {{ envOrDefault "UNSET_VAR" "300" }}`,
+			},
+			&ExecuteInput{
+				Env: func() []string {
+					return []string{"SET_VAR=400", "EMPTY_VAR="}
+				}(),
+				Brain: func() *Brain {
+					b := NewBrain()
+					return b
+				}(),
+			},
+			"400  300",
+			false,
+		},
 		{
 			"func_file",
 			&NewTemplateInput{