feat(mustEnv): add mustEnv function

docs/index.md

@@ -17,7 +17,7 @@ The Sprig library provides over 70 template functions for Go's template language
 - [Flow Control Functions](flow_control.md): `fail`
 - Advanced Functions
   - [UUID Functions](uuid.md): `uuidv4`
-  - [OS Functions](os.md): `env`, `expandenv`
+  - [OS Functions](os.md): `env`, `expandenv`, `mustEnv`
   - [Version Comparison Functions](semver.md): `semver`, `semverCompare`
   - [Reflection](reflection.md): `typeOf`, `kindIs`, `typeIsLike`, etc.
   - [Cryptographic and Security Functions](crypto.md): `derivePassword`, `sha256sum`, `genPrivateKey`, etc.

docs/os.md

@@ -22,3 +22,15 @@ To substitute environment variables in a string, use `expandenv`:
 ```
 expandenv "Your path is set to $PATH"
 ```
+
+## mustEnv
+
+`mustEnv` function reads an environment variable, and returns an error if that env var does not exist.
+
+```
+# works
+mustEnv "HOME"
+
+# errors
+mustEnv "INVALID"
+```

functions.go

@@ -88,7 +88,7 @@ var nonhermeticFunctions = []string{
 	// OS
 	"env",
 	"expandenv",
-
+	"mustEnv",
 	// Network
 	"getHostByName",
 }
@@ -269,7 +269,7 @@ var genericMap = map[string]interface{}{
 	// OS:
 	"env":       os.Getenv,
 	"expandenv": os.ExpandEnv,
-
+	"mustEnv": mustEnv,
 	// Network:
 	"getHostByName": getHostByName,
 

os.go

@@ -0,0 +1,15 @@
+package sprig
+
+import (
+	"fmt"
+	"os"
+)
+
+func mustEnv(env string) (string, error){
+	v, ok := os.LookupEnv(env)
+	if !ok {
+		return v, fmt.Errorf("env var \"%s\" does not exist", env)
+	}
+
+	return v, nil
+}

os_test.go

@@ -0,0 +1,20 @@
+package sprig
+
+import (
+	"testing"
+)
+
+func TestMustEnv(t *testing.T) {
+	tpl := `{{ mustEnv "INVALID" }}`
+	if err := runt(tpl, "foo"); err == nil {
+		t.Errorf("expected error, got: %v", err)
+	}
+
+	t.Setenv("TMP", "OK")
+
+	tpl = `{{ mustEnv "TMP" }}`
+	if err := runt(tpl, "OK"); err != nil {
+		t.Error(err)
+	}
+
+}