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)
+ }
+
+}