Merge pull request #170 from tgross/gh145_command_env_vars

Interpolate variables in command args

core/app.go

@@ -111,7 +111,8 @@ func (a *App) Run() {
 	if 1 == os.Getpid() {
 		reapChildren()
 	}
-	command, err := utils.ParseCommandArgs(flag.Args())
+	args := getArgs(flag.Args())
+	command, err := utils.ParseCommandArgs(args)
 	if err != nil {
 		log.Errorf("Unable to parse command arguments: %v", err)
 	}
@@ -122,7 +123,7 @@ func (a *App) Run() {
 	}
 	a.handlePolling()
 
-	if len(flag.Args()) != 0 {
+	if len(args) != 0 {
 		// Run our main application and capture its stdout/stderr.
 		// This will block until the main application exits and then os.Exit
 		// with the exit code of that application.
@@ -145,6 +146,22 @@ func (a *App) Run() {
 	select {}
 }
 
+// Render the command line args thru golang templating so we can
+// interpolate environment variables
+func getArgs(args []string) []string {
+	var renderedArgs []string
+	for _, arg := range args {
+		newArg, err := config.ApplyTemplate([]byte(arg))
+		if err != nil {
+			log.Errorf("Unable to render command arguments template: %v", err)
+			renderedArgs = args // skip rendering on error
+			break
+		}
+		renderedArgs = append(renderedArgs, string(newArg))
+	}
+	return renderedArgs
+}
+
 // ToggleMaintenanceMode marks all services for maintenance
 func (a *App) ToggleMaintenanceMode() {
 	a.maintModeLock.RLock()

core/app_test.go

@@ -197,6 +197,21 @@ func TestParseTrailingComma(t *testing.T) {
 	}`, "Do you have an extra comma somewhere?")
 }
 
+func TestRenderArgs(t *testing.T) {
+	flags := []string{"-name", "{{ .HOSTNAME }}"}
+	expected, _ := os.Hostname()
+	if got := getArgs(flags)[1]; got != expected {
+		t.Errorf("Expected %v but got %v for rendered hostname", expected, got)
+	}
+
+	// invalid template should just be returned unchanged
+	flags = []string{"-name", "{{ .HOSTNAME }"}
+	expected = "{{ .HOSTNAME }"
+	if got := getArgs(flags)[1]; got != expected {
+		t.Errorf("Expected %v but got %v for unrendered hostname", expected, got)
+	}
+}
+
 func TestMetricServiceCreation(t *testing.T) {
 
 	jsonFragment := `{

documentation/12-configuration/README.md

@@ -252,9 +252,9 @@ All executable fields, including `services/health`, `preStart`, `preStop`, `post
 
 ### Template configuration
 
-ContainerPilot configuration has template support. If you have an environment variable such as `FOO=BAR` then you can use `{{.FOO}}` in your configuration file and it will be substituted with `BAR`.
+ContainerPilot configuration has template support. If you have an environment variable such as `FOO=BAR` then you can use `{{.FOO}}` in your configuration file or in your command arguments and it will be substituted with `BAR`.
 
-**Example usage**
+**Example usage in a config file**
 
 ```json
 {
@@ -263,4 +263,16 @@ ContainerPilot configuration has template support. If you have an environment va
 }
 ```
 
+**Example usage in a Dockerfile**
+
+```
+ENV APP_PORT=8000
+CMD [ "/usr/local/bin/containerpilot", \
+      "node",\
+      "/usr/local/bin/http-server",\
+      "-p",\
+      "{{ .APP_PORT }}"]
+```
+
+
 **Note**:  If you need more than just variable interpolation, check out the [Go text/template Docs](https://golang.org/pkg/text/template/).

integration_tests/fixtures/app/Dockerfile

@@ -21,6 +21,10 @@ COPY task.sh /task.sh
 # by default use app-consul.json, allows for override in docker-compose
 ENV CONTAINERPILOT=file:///app-with-consul.json
 
+# default port, allows us to override in docker-compose and also test
+# env var interpolation in the command args
+ENV APP_PORT=8000
+
 ENTRYPOINT [ "/bin/containerpilot", \
              "/usr/local/bin/node", \
-             "/usr/local/bin/http-server", "/srv", "-p", "8000"]
+             "/usr/local/bin/http-server", "/srv", "-p", "{{ .APP_PORT }}"]