Add context use command

Signed-off-by: Christopher Crone <christopher.crone@docker.com>

cli/cmd/context.go

@@ -34,8 +34,10 @@ import (
 	"text/tabwriter"
 
 	"github.com/hashicorp/go-multierror"
+	"github.com/pkg/errors"
 	"github.com/spf13/cobra"
 
+	cliconfig "github.com/docker/api/cli/config"
 	"github.com/docker/api/context/store"
 )
 
@@ -50,6 +52,7 @@ func ContextCommand() *cobra.Command {
 		createCommand(),
 		listCommand(),
 		removeCommand(),
+		useCommand(),
 	)
 
 	return cmd
@@ -106,6 +109,21 @@ func removeCommand() *cobra.Command {
 	}
 }
 
+func useCommand() *cobra.Command {
+	return &cobra.Command{
+		Use:   "use CONTEXT",
+		Short: "Set the default context",
+		Args:  cobra.ExactArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			config := cmd.Flag(cliconfig.ConfigFlagName)
+			if config == nil || config.Value.String() == "" {
+				panic(errors.New("no value for config directory"))
+			}
+			return runUse(cmd.Context(), config.Value.String(), args[0])
+		},
+	}
+}
+
 func runCreate(ctx context.Context, opts createOpts, name string, contextType string) error {
 	switch contextType {
 	case "aci":
@@ -162,3 +180,18 @@ func runRemove(ctx context.Context, args []string) error {
 	}
 	return errs.ErrorOrNil()
 }
+
+func runUse(ctx context.Context, configDir string, name string) error {
+	s := store.ContextStore(ctx)
+	// Match behavior of existing CLI
+	if name != store.DefaultContextName {
+		if _, err := s.Get(name, nil); err != nil {
+			return err
+		}
+	}
+	if err := cliconfig.WriteCurrentContext(configDir, name); err != nil {
+		return err
+	}
+	fmt.Println(name)
+	return nil
+}