Add ps command

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>

main.go

@@ -93,6 +93,7 @@ func main() {
 		killCommand,
 		listCommand,
 		pauseCommand,
+		psCommand,
 		restoreCommand,
 		resumeCommand,
 		specCommand,

ps.go

@@ -0,0 +1,76 @@
+// +build linux
+
+package main
+
+import (
+	"fmt"
+	"os/exec"
+	"strconv"
+	"strings"
+
+	"github.com/codegangsta/cli"
+)
+
+var psCommand = cli.Command{
+	Name:      "ps",
+	Usage:     "ps displays the processes running inside a container",
+	ArgsUsage: `<container-id> <ps options>`,
+	Action: func(context *cli.Context) {
+		container, err := getContainer(context)
+		if err != nil {
+			fatal(err)
+		}
+
+		psArgs := context.Args().Get(1)
+		if psArgs == "" {
+			psArgs = "-ef"
+		}
+
+		output, err := exec.Command("ps", strings.Split(psArgs, " ")...).Output()
+		if err != nil {
+			fatal(err)
+		}
+
+		lines := strings.Split(string(output), "\n")
+		pidIndex, err := getPidIndex(lines[0])
+		if err != nil {
+			fatal(err)
+		}
+
+		pids, err := container.Processes()
+		if err != nil {
+			fatal(err)
+		}
+		fmt.Println(lines[0])
+		for _, line := range lines[1:] {
+			if len(line) == 0 {
+				continue
+			}
+			fields := strings.Fields(line)
+			p, err := strconv.Atoi(fields[pidIndex])
+			if err != nil {
+				fatal(fmt.Errorf("unexpected pid '%s': %s", fields[pidIndex], err))
+			}
+
+			for _, pid := range pids {
+				if pid == p {
+					fmt.Println(line)
+					break
+				}
+			}
+		}
+	},
+}
+
+func getPidIndex(title string) (int, error) {
+	titles := strings.Fields(title)
+
+	pidIndex := -1
+	for i, name := range titles {
+		if name == "PID" {
+			return i, nil
+		}
+	}
+
+	return pidIndex, fmt.Errorf("couldn't find PID field in ps output")
+}