feat: add "outdated" command to list plugins with available updates

Adds "kubectl krew outdated" command that compares installed plugin
versions against the local index and displays a table of plugins
with newer versions available, without performing any upgrades.

Features:
- Shows PLUGIN, INSTALLED, AVAILABLE columns in terminal
- Outputs only plugin names when piped (for scripting)
- Skips detached (manifest-installed) plugins
- Gracefully handles missing index entries and unparseable versions

Author: Moha0x1

cmd/krew/cmd/outdated.go

@@ -0,0 +1,116 @@
+// Copyright 2019 The Kubernetes 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 cmd
+
+import (
+	"fmt"
+	"os"
+	"sort"
+	"strings"
+
+	"github.com/pkg/errors"
+	"github.com/spf13/cobra"
+	"k8s.io/klog/v2"
+
+	"sigs.k8s.io/krew/internal/index/indexscanner"
+	"sigs.k8s.io/krew/internal/installation"
+	"sigs.k8s.io/krew/internal/installation/semver"
+)
+
+func init() {
+	outdatedCmd := &cobra.Command{
+		Use:   "outdated",
+		Short: "List installed plugins with newer versions available",
+		Long: `List all installed kubectl plugins that have newer versions available
+in the local index. This command does not perform any upgrades.
+
+Use "kubectl krew update" to refresh the index before checking for
+outdated plugins.
+
+To upgrade all outdated plugins, use:
+  kubectl krew upgrade`,
+		RunE: func(_ *cobra.Command, _ []string) error {
+			receipts, err := installation.GetInstalledPluginReceipts(paths.InstallReceiptsPath())
+			if err != nil {
+				return errors.Wrap(err, "failed to find installed plugins")
+			}
+
+			var rows [][]string
+			for _, r := range receipts {
+				indexName := indexOf(r)
+				pluginName := r.Name
+
+				// Skip plugins installed from a manifest (detached)
+				if indexName == "detached" {
+					klog.V(2).Infof("Skipping %q: installed via manifest", pluginName)
+					continue
+				}
+
+				// Load latest version from the index
+				indexPlugin, err := indexscanner.LoadPluginByName(paths.IndexPluginsPath(indexName), pluginName)
+				if err != nil {
+					if os.IsNotExist(err) {
+						klog.V(1).Infof("Skipping %q: plugin no longer exists in index %q", pluginName, indexName)
+						continue
+					}
+					return errors.Wrapf(err, "failed to load index entry for plugin %q", pluginName)
+				}
+
+				curVersion := r.Spec.Version
+				newVersion := indexPlugin.Spec.Version
+
+				curv, err := semver.Parse(curVersion)
+				if err != nil {
+					klog.V(1).Infof("Skipping %q: cannot parse installed version %q", pluginName, curVersion)
+					continue
+				}
+				newv, err := semver.Parse(newVersion)
+				if err != nil {
+					klog.V(1).Infof("Skipping %q: cannot parse index version %q", pluginName, newVersion)
+					continue
+				}
+
+				if semver.Less(curv, newv) {
+					rows = append(rows, []string{displayName(indexPlugin, indexName), curVersion, newVersion})
+				}
+			}
+
+			if len(rows) == 0 {
+				fmt.Fprintln(os.Stderr, "All plugins are up to date.")
+				return nil
+			}
+
+			// Sort by plugin name
+			sort.Slice(rows, func(i, j int) bool {
+				return rows[i][0] < rows[j][0]
+			})
+
+			// Return only names when piped
+			if !isTerminal(os.Stdout) {
+				var names []string
+				for _, row := range rows {
+					names = append(names, row[0])
+				}
+				fmt.Fprintln(os.Stdout, strings.Join(names, "\n"))
+				return nil
+			}
+
+			return printTable(os.Stdout, []string{"PLUGIN", "INSTALLED", "AVAILABLE"}, rows)
+		},
+		PreRunE: checkIndex,
+	}
+
+	rootCmd.AddCommand(outdatedCmd)
+}