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
+}

cmd/flux/envsubst_test.go

@@ -0,0 +1,50 @@
+/*
+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 (
+	"bytes"
+	"os"
+	"testing"
+
+	. "github.com/onsi/gomega"
+)
+
+func TestEnvsubst(t *testing.T) {
+	g := NewWithT(t)
+	input, err := os.ReadFile("testdata/envsubst/file.yaml")
+	g.Expect(err).NotTo(HaveOccurred())
+
+	t.Setenv("REPO_NAME", "test")
+
+	output, err := executeCommandWithIn("envsubst", bytes.NewReader(input))
+	g.Expect(err).NotTo(HaveOccurred())
+
+	expected, err := os.ReadFile("testdata/envsubst/file.gold")
+	g.Expect(err).NotTo(HaveOccurred())
+	g.Expect(output).To(Equal(string(expected)))
+}
+
+func TestEnvsubst_Strinct(t *testing.T) {
+	g := NewWithT(t)
+	input, err := os.ReadFile("testdata/envsubst/file.yaml")
+	g.Expect(err).NotTo(HaveOccurred())
+
+	_, err = executeCommandWithIn("envsubst --strict", bytes.NewReader(input))
+	g.Expect(err).To(HaveOccurred())
+	g.Expect(err.Error()).To(ContainSubstring("variable not set (strict mode)"))
+}

cmd/flux/main_test.go

@@ -31,7 +31,6 @@ import (
 	"text/template"
 	"time"
 
-	"github.com/fluxcd/flux2/v2/internal/utils"
 	"github.com/google/go-cmp/cmp"
 	"github.com/mattn/go-shellwords"
 	"k8s.io/apimachinery/pkg/api/errors"
@@ -40,6 +39,8 @@ import (
 	"k8s.io/client-go/tools/clientcmd"
 	"sigs.k8s.io/controller-runtime/pkg/client"
 	"sigs.k8s.io/controller-runtime/pkg/envtest"
+
+	"github.com/fluxcd/flux2/v2/internal/utils"
 )
 
 var nextNamespaceId int64
@@ -393,6 +394,29 @@ func executeCommand(cmd string) (string, error) {
 	return result, err
 }
 
+// Run the command while passing the string as input and return the captured output.
+func executeCommandWithIn(cmd string, in io.Reader) (string, error) {
+	defer resetCmdArgs()
+	args, err := shellwords.Parse(cmd)
+	if err != nil {
+		return "", err
+	}
+
+	buf := new(bytes.Buffer)
+
+	rootCmd.SetOut(buf)
+	rootCmd.SetErr(buf)
+	rootCmd.SetArgs(args)
+	if in != nil {
+		rootCmd.SetIn(in)
+	}
+
+	_, err = rootCmd.ExecuteC()
+	result := buf.String()
+
+	return result, err
+}
+
 // resetCmdArgs resets the flags for various cmd
 // Note: this will also clear default value of the flags set in init()
 func resetCmdArgs() {
@@ -441,7 +465,7 @@ func resetCmdArgs() {
 	versionArgs = versionFlags{
 		output: "yaml",
 	}
-
+	envsubstArgs = envsubstFlags{}
 }
 
 func isChangeError(err error) bool {

cmd/flux/testdata/envsubst/file.gold

@@ -0,0 +1,10 @@
+apiVersion: source.toolkit.fluxcd.io/v1
+kind: GitRepository
+metadata:
+  name: test
+  namespace: flux-system
+spec:
+  ref:
+    branch: main
+  interval: 5m
+  url: ssh://git@github.com/example/test

cmd/flux/testdata/envsubst/file.yaml

@@ -0,0 +1,10 @@
+apiVersion: source.toolkit.fluxcd.io/v1
+kind: GitRepository
+metadata:
+  name: ${REPO_NAME}
+  namespace: ${REPO_NAMESPACE:=flux-system}
+spec:
+  ref:
+    branch: main
+  interval: 5m
+  url: ssh://git@github.com/example/${REPO_NAME}

go.mod

@@ -19,6 +19,7 @@ require (
 	github.com/fluxcd/notification-controller/api v1.2.4
 	github.com/fluxcd/pkg/apis/event v0.8.0
 	github.com/fluxcd/pkg/apis/meta v1.4.0
+	github.com/fluxcd/pkg/envsubst v1.0.0
 	github.com/fluxcd/pkg/git v0.18.0
 	github.com/fluxcd/pkg/git/gogit v0.18.0
 	github.com/fluxcd/pkg/kustomize v1.9.0
@@ -117,7 +118,6 @@ require (
 	github.com/felixge/httpsnoop v1.0.4 // indirect
 	github.com/fluxcd/pkg/apis/acl v0.2.0 // indirect
 	github.com/fluxcd/pkg/apis/kustomize v1.4.0 // indirect
-	github.com/fluxcd/pkg/envsubst v1.0.0 // indirect
 	github.com/fsnotify/fsnotify v1.7.0 // indirect
 	github.com/go-errors/errors v1.5.1 // indirect
 	github.com/go-fed/httpsig v1.1.0 // indirect