Add flux envsubst command

This command can be used to replicate the behavior of the
Flux Kustomization post-build substitutions.

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

cmd/flux/envsubst.go

@@ -0,0 +1,74 @@
+/*
+Copyright 2024 The Flux authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package main
+
+import (
+	"bufio"
+	"fmt"
+
+	"github.com/fluxcd/pkg/envsubst"
+	"github.com/spf13/cobra"
+)
+
+var envsubstCmd = &cobra.Command{
+	Use:   "envsubst",
+	Args:  cobra.NoArgs,
+	Short: "envsubst substitutes the values of environment variables",
+	Long: withPreviewNote(`The envsubst command substitutes the values of environment variables
+in the string piped as standard input and writes the result to the standard output. This command can be used
+to replicate the behavior of the Flux Kustomization post-build substitutions.`),
+	Example: `  # Run env var substitutions on the kustomization build output
+  export cluster_region=eu-central-1
+  kustomize build . | flux envsubst
+
+  # Run env var substitutions and error out if a variable is not set
+  kustomize build . | flux envsubst --strict
+`,
+	RunE: runEnvsubstCmd,
+}
+
+type envsubstFlags struct {
+	strict bool
+}
+
+var envsubstArgs envsubstFlags
+
+func init() {
+	envsubstCmd.Flags().BoolVar(&envsubstArgs.strict, "strict", false,
+		"fail if a variable without a default value is declared in the input but is missing from the environment")
+	rootCmd.AddCommand(envsubstCmd)
+}
+
+func runEnvsubstCmd(cmd *cobra.Command, args []string) error {
+	stdin := bufio.NewScanner(rootCmd.InOrStdin())
+	stdout := bufio.NewWriter(rootCmd.OutOrStdout())
+	for stdin.Scan() {
+		line, err := envsubst.EvalEnv(stdin.Text(), envsubstArgs.strict)
+		if err != nil {
+			return err
+		}
+		_, err = fmt.Fprintln(stdout, line)
+		if err != nil {
+			return err
+		}
+		err = stdout.Flush()
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}