Add status subcommand to report status of running daemon. (#24856)

* Add status sub-command to report status of running daemon.

* Set exit code based on health status.

* Add changelog.

* Fix format.

(cherry picked from commit 23efd31)

x-pack/elastic-agent/CHANGELOG.next.asciidoc

@@ -79,3 +79,4 @@
 - Add support for Fleet Server running under Elastic Agent {pull}24220[24220]
 - Add CA support to Elastic Agent docker image {pull}24486[24486]
 - Add STATE_PATH, CONFIG_PATH, LOGS_PATH to Elastic Agent docker image {pull}24817[24817]
+- Add status subcommand {pull}24856[24856]

x-pack/elastic-agent/pkg/agent/cmd/common.go

@@ -53,6 +53,7 @@ func NewCommandWithArgs(args []string, streams *cli.IOStreams) *cobra.Command {
 	cmd.AddCommand(newInspectCommandWithArgs(args, streams))
 	cmd.AddCommand(newWatchCommandWithArgs(args, streams))
 	cmd.AddCommand(newContainerCommand(args, streams))
+	cmd.AddCommand(newStatusCommand(args, streams))
 
 	// windows special hidden sub-command (only added on windows)
 	reexec := newReExecWindowsCommand(args, streams)

x-pack/elastic-agent/pkg/agent/cmd/status.go

@@ -0,0 +1,122 @@
+// Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+// or more contributor license agreements. Licensed under the Elastic License;
+// you may not use this file except in compliance with the Elastic License.
+
+package cmd
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"os"
+	"time"
+
+	"gopkg.in/yaml.v2"
+
+	"github.com/spf13/cobra"
+
+	"github.com/elastic/beats/v7/x-pack/elastic-agent/pkg/agent/control/client"
+	"github.com/elastic/beats/v7/x-pack/elastic-agent/pkg/agent/errors"
+	"github.com/elastic/beats/v7/x-pack/elastic-agent/pkg/cli"
+)
+
+type outputter func(io.Writer, *client.AgentStatus) error
+
+var outputs = map[string]outputter{
+	"human": humanOutput,
+	"json":  jsonOutput,
+	"yaml":  yamlOutput,
+}
+
+func newStatusCommand(_ []string, streams *cli.IOStreams) *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "status",
+		Short: "Status returns the current status of the running Elastic Agent daemon.",
+		Long:  `Status returns the current status of the running Elastic Agent daemon.`,
+		Run: func(c *cobra.Command, args []string) {
+			if err := statusCmd(streams, c, args); err != nil {
+				fmt.Fprintf(streams.Err, "Error: %v\n", err)
+				os.Exit(1)
+			}
+		},
+	}
+
+	cmd.Flags().String("output", "human", "Output the status information in either human, json, or yaml (default: human)")
+
+	return cmd
+}
+
+func statusCmd(streams *cli.IOStreams, cmd *cobra.Command, args []string) error {
+	output, _ := cmd.Flags().GetString("output")
+	outputFunc, ok := outputs[output]
+	if !ok {
+		return fmt.Errorf("unsupported output: %s", output)
+	}
+
+	ctx := handleSignal(context.Background())
+	innerCtx, cancel := context.WithTimeout(ctx, 30*time.Second)
+	defer cancel()
+
+	status, err := getDaemonStatus(innerCtx)
+	if err == context.DeadlineExceeded {
+		return errors.New("timed out after 30 seconds trying to connect to Elastic Agent daemon")
+	} else if err == context.Canceled {
+		return nil
+	} else if err != nil {
+		return fmt.Errorf("failed to communicate with Elastic Agent daemon: %s", err)
+	}
+
+	err = outputFunc(streams.Out, status)
+	if err != nil {
+		return err
+	}
+	// exit 0 only if the Elastic Agent daemon is healthy
+	if status.Status == client.Healthy {
+		os.Exit(0)
+	} else {
+		os.Exit(1)
+	}
+	return nil
+}
+
+func humanOutput(w io.Writer, status *client.AgentStatus) error {
+	fmt.Fprintf(w, "Status: %s\n", status.Status)
+	if status.Message == "" {
+		fmt.Fprint(w, "Message: (no message)\n")
+	} else {
+		fmt.Fprintf(w, "Message: %s\n", status.Message)
+	}
+	if len(status.Applications) == 0 {
+		fmt.Fprint(w, "Applications: (none)\n")
+	} else {
+		fmt.Fprint(w, "Applications:\n")
+		for _, app := range status.Applications {
+			fmt.Fprintf(w, "  * %s\t(%s)\n", app.Name, app.Status)
+			if app.Message == "" {
+				fmt.Fprint(w, "    (no message)\n")
+			} else {
+				fmt.Fprintf(w, "    %s\n", app.Message)
+			}
+		}
+	}
+	return nil
+}
+
+func jsonOutput(w io.Writer, status *client.AgentStatus) error {
+	bytes, err := json.MarshalIndent(status, "", "    ")
+	if err != nil {
+		return err
+	}
+	fmt.Fprintf(w, "%s\n", bytes)
+	return nil
+}
+
+func yamlOutput(w io.Writer, status *client.AgentStatus) error {
+	bytes, err := yaml.Marshal(status)
+	if err != nil {
+		return err
+	}
+	fmt.Fprintf(w, "%s\n", bytes)
+	return nil
+}