cli: implement ps command

ps command is used by ```docker top``` to show
the processes running inside the container

partially fixes clearcontainers#95

Signed-off-by: Julio Montes <[email protected]>

main.go

@@ -109,6 +109,7 @@ var runtimeCommands = []cli.Command{
 	killCLICommand,
 	listCLICommand,
 	pauseCLICommand,
+	psCLICommand,
 	resumeCLICommand,
 	runCLICommand,
 	startCLICommand,

ps.go

@@ -0,0 +1,82 @@
+// Copyright (c) 2014,2015,2016 Docker, Inc.
+// Copyright (c) 2017 Intel Corporation
+//
+// 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 (
+	"fmt"
+
+	vc "github.com/containers/virtcontainers"
+	"github.com/urfave/cli"
+)
+
+var psCLICommand = cli.Command{
+	Name:      "ps",
+	Usage:     "ps displays the processes running inside a container",
+	ArgsUsage: `<container-id> [ps options]`,
+	Flags: []cli.Flag{
+		cli.StringFlag{
+			Name:  "format, f",
+			Value: "table",
+			Usage: `select one of: ` + formatOptions,
+		},
+	},
+	Action: func(context *cli.Context) error {
+		return ps(context)
+	},
+	SkipArgReorder: true,
+}
+
+func ps(context *cli.Context) error {
+	containerID := context.Args().First()
+	if containerID == "" {
+		return fmt.Errorf("Missing container ID")
+	}
+
+	// Checks the MUST and MUST NOT from OCI runtime specification
+	status, podID, err := getExistingContainerInfo(containerID)
+	if err != nil {
+		return err
+	}
+
+	containerID = status.ID
+
+	// container MUST be running
+	if status.State.State != vc.StateRunning {
+		return fmt.Errorf("Container %s is not running", containerID)
+	}
+
+	var options vc.ProcessListOptions
+
+	// [1:] is to remove command name, ex:
+	// context.Args(): [containet_id ps_arg1 ps_arg2 ...]
+	// psArgs:         [ps_arg1 ps_arg2 ...]
+	//
+	options.Args = context.Args()[1:]
+	if len(options.Args) == 0 {
+		options.Args = []string{"-ef"}
+	}
+
+	options.Format = context.String("format")
+
+	msg, err := vc.ProcessListContainer(containerID, podID, options)
+	if err != nil {
+		return err
+	}
+
+	fmt.Print(string(msg))
+
+	return nil
+}