lang: Add getenv function

$x = getenv("NAME")
    $y = defaultenv("NOTEXIST", "defaultvalue")
    $z = hasenv("NAME")
    $a = env()
    $b = maplookup($a, "NAME", "defaultvalue")

lang/funcs/simple/env_func.go

@@ -0,0 +1,84 @@
+// Mgmt
+// Copyright (C) 2013-2018+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package simple // TODO: should this be in its own individual package?
+
+import (
+	"os"
+	"strings"
+
+	"github.com/purpleidea/mgmt/lang/types"
+)
+
+func init() {
+	Register("getenv", &types.FuncValue{
+		T: types.NewType("func(str) str"),
+		V: GetEnv,
+	})
+	Register("defaultenv", &types.FuncValue{
+		T: types.NewType("func(str, str) str"),
+		V: DefaultEnv,
+	})
+	Register("hasenv", &types.FuncValue{
+		T: types.NewType("func(str) bool"),
+		V: HasEnv,
+	})
+	Register("env", &types.FuncValue{
+		T: types.NewType("func() {str: str}"),
+		V: Env,
+	})
+}
+
+// GetEnv gets environment variable by name or returns empty string if non existing.
+func GetEnv(input []types.Value) (types.Value, error) {
+	return &types.StrValue{
+		V: os.Getenv(input[0].Str()),
+	}, nil
+}
+
+// DefaultEnv gets environment variable by name or returns default if non existing.
+func DefaultEnv(input []types.Value) (types.Value, error) {
+	value, exists := os.LookupEnv(input[0].Str())
+	if !exists {
+		value = input[1].Str()
+	}
+	return &types.StrValue{
+		V: value,
+	}, nil
+}
+
+// HasEnv returns true if environment variable exists.
+func HasEnv(input []types.Value) (types.Value, error) {
+	_, exists := os.LookupEnv(input[0].Str())
+	return &types.BoolValue{
+		V: exists,
+	}, nil
+}
+
+// Env returns a map of all keys and their values.
+func Env(input []types.Value) (types.Value, error) {
+	environ := make(map[types.Value]types.Value)
+	for _, keyval := range os.Environ() {
+		if i := strings.IndexRune(keyval, '='); i != -1 {
+			environ[&types.StrValue{V: keyval[:i]}] = &types.StrValue{V: keyval[i+1:]}
+		}
+	}
+	return &types.MapValue{
+		T: types.NewType("{str: str}"),
+		V: environ,
+	}, nil
+}