kubectl plugin: Expand environment variables in the command of plugin.yaml

This commit improves kubectl plugins to expand environment variables in
the command of plugin.yaml.

pkg/kubectl/plugins/runner.go

@@ -18,6 +18,7 @@ package plugins
 
 import (
 	"io"
+	"os"
 	"os/exec"
 	"strings"
 
@@ -47,7 +48,7 @@ type ExecPluginRunner struct{}
 // Run takes a given plugin and runs it in a given context using os/exec, returning
 // any error found while running.
 func (r *ExecPluginRunner) Run(plugin *Plugin, ctx RunningContext) error {
-	command := strings.Split(plugin.Command, " ")
+	command := strings.Split(os.ExpandEnv(plugin.Command), " ")
 	base := command[0]
 	args := []string{}
 	if len(command) > 1 {

pkg/kubectl/plugins/runner_test.go

@@ -18,6 +18,7 @@ package plugins
 
 import (
 	"bytes"
+	"os"
 	"testing"
 )
 
@@ -38,8 +39,16 @@ func TestExecRunner(t *testing.T) {
 			command:     "false",
 			expectedErr: "exit status 1",
 		},
+		{
+			name:        "env",
+			command:     "echo $KUBECTL_PLUGINS_TEST",
+			expectedMsg: "ok\n",
+		},
 	}
 
+	os.Setenv("KUBECTL_PLUGINS_TEST", "ok")
+	defer os.Unsetenv("KUBECTL_PLUGINS_TEST")
+
 	for _, test := range tests {
 		outBuf := bytes.NewBuffer([]byte{})