Prompt the user to select a container if they don't provide one

and we can't infer it.

docs/kubectl-log.md

@@ -24,6 +24,7 @@ $ kubectl log -f 123456-7890 ruby-container
 
 ```
   -f, --follow=false: Specify if the logs should be streamed.
+      --interactive=true: If true, prompt the user for input when required. Default true.
 ```
 
 ### Options inherrited from parent commands

docs/man/man1/kubectl-log.1

@@ -21,6 +21,10 @@ Print the logs for a container in a pod. If the pod has only one container, the
 \fB\-f\fP, \fB\-\-follow\fP=false
     Specify if the logs should be streamed.
 
+.PP
+\fB\-\-interactive\fP=true
+    If true, prompt the user for input when required. Default true.
+
 
 .SH OPTIONS INHERITED FROM PARENT COMMANDS
 .PP

pkg/kubectl/cmd/log.go

@@ -17,10 +17,14 @@ limitations under the License.
 package cmd
 
 import (
+	"fmt"
 	"io"
+	"os"
 	"strconv"
 
+	"github.com/GoogleCloudPlatform/kubernetes/pkg/api"
 	"github.com/GoogleCloudPlatform/kubernetes/pkg/kubectl/cmd/util"
+	libutil "github.com/GoogleCloudPlatform/kubernetes/pkg/util"
 	"github.com/spf13/cobra"
 )
 
@@ -32,6 +36,28 @@ $ kubectl log 123456-7890 ruby-container
 $ kubectl log -f 123456-7890 ruby-container`
 )
 
+func selectContainer(pod *api.Pod, in io.Reader, out io.Writer) string {
+	fmt.Fprintf(out, "Please select a container:\n")
+	options := libutil.StringSet{}
+	for ix := range pod.Spec.Containers {
+		fmt.Fprintf(out, "[%d] %s\n", ix+1, pod.Spec.Containers[ix].Name)
+		options.Insert(pod.Spec.Containers[ix].Name)
+	}
+	for {
+		var input string
+		fmt.Fprintf(out, "> ")
+		fmt.Fscanln(in, &input)
+		if options.Has(input) {
+			return input
+		}
+		ix, err := strconv.Atoi(input)
+		if err == nil && ix > 0 && ix <= len(pod.Spec.Containers) {
+			return pod.Spec.Containers[ix-1].Name
+		}
+		fmt.Fprintf(out, "Invalid input: %s", input)
+	}
+}
+
 func (f *Factory) NewCmdLog(out io.Writer) *cobra.Command {
 	cmd := &cobra.Command{
 		Use:     "log [-f] <pod> [<container>]",
@@ -60,11 +86,15 @@ func (f *Factory) NewCmdLog(out io.Writer) *cobra.Command {
 			var container string
 			if len(args) == 1 {
 				if len(pod.Spec.Containers) != 1 {
-					usageError(cmd, "<container> is required for pods with multiple containers")
+					if !util.GetFlagBool(cmd, "interactive") {
+						usageError(cmd, "<container> is required for pods with multiple containers")
+					} else {
+						container = selectContainer(pod, os.Stdin, out)
+					}
+				} else {
+					// Get logs for the only container in the pod
+					container = pod.Spec.Containers[0].Name
 				}
-
-				// Get logs for the only container in the pod
-				container = pod.Spec.Containers[0].Name
 			} else {
 				container = args[1]
 			}
@@ -89,5 +119,6 @@ func (f *Factory) NewCmdLog(out io.Writer) *cobra.Command {
 		},
 	}
 	cmd.Flags().BoolP("follow", "f", false, "Specify if the logs should be streamed.")
+	cmd.Flags().Bool("interactive", true, "If true, prompt the user for input when required. Default true.")
 	return cmd
 }

pkg/kubectl/cmd/log_test.go

@@ -0,0 +1,147 @@
+/*
+Copyright 2014 Google Inc. All rights reserved.
+
+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 (
+	"bytes"
+	"testing"
+
+	"github.com/GoogleCloudPlatform/kubernetes/pkg/api"
+)
+
+func TestSelectContainer(t *testing.T) {
+	tests := []struct {
+		input             string
+		pod               api.Pod
+		expectedContainer string
+	}{
+		{
+			input: "1\n",
+			pod: api.Pod{
+				Spec: api.PodSpec{
+					Containers: []api.Container{
+						{
+							Name: "foo",
+						},
+					},
+				},
+			},
+			expectedContainer: "foo",
+		},
+		{
+			input: "foo\n",
+			pod: api.Pod{
+				Spec: api.PodSpec{
+					Containers: []api.Container{
+						{
+							Name: "foo",
+						},
+					},
+				},
+			},
+			expectedContainer: "foo",
+		},
+		{
+			input: "foo\n",
+			pod: api.Pod{
+				Spec: api.PodSpec{
+					Containers: []api.Container{
+						{
+							Name: "bar",
+						},
+						{
+							Name: "foo",
+						},
+					},
+				},
+			},
+			expectedContainer: "foo",
+		},
+		{
+			input: "2\n",
+			pod: api.Pod{
+				Spec: api.PodSpec{
+					Containers: []api.Container{
+						{
+							Name: "bar",
+						},
+						{
+							Name: "foo",
+						},
+					},
+				},
+			},
+			expectedContainer: "foo",
+		},
+		{
+			input: "-1\n2\n",
+			pod: api.Pod{
+				Spec: api.PodSpec{
+					Containers: []api.Container{
+						{
+							Name: "bar",
+						},
+						{
+							Name: "foo",
+						},
+					},
+				},
+			},
+			expectedContainer: "foo",
+		},
+		{
+			input: "3\n2\n",
+			pod: api.Pod{
+				Spec: api.PodSpec{
+					Containers: []api.Container{
+						{
+							Name: "bar",
+						},
+						{
+							Name: "foo",
+						},
+					},
+				},
+			},
+			expectedContainer: "foo",
+		},
+		{
+			input: "baz\n2\n",
+			pod: api.Pod{
+				Spec: api.PodSpec{
+					Containers: []api.Container{
+						{
+							Name: "bar",
+						},
+						{
+							Name: "foo",
+						},
+					},
+				},
+			},
+			expectedContainer: "foo",
+		},
+	}
+
+	for _, test := range tests {
+		var buff bytes.Buffer
+		container := selectContainer(&test.pod, bytes.NewBufferString(test.input), &buff)
+		if container != test.expectedContainer {
+			t.Errorf("unexpected output: %s for input: %s", container, test.input)
+		}
+	}
+}