add an example service using EXTRA_CMDLINE

Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au>

docs/os/running-rancheros/server/pxe/index.md

@@ -31,6 +31,20 @@ kernel ${base-url}/vmlinuz rancher.state.dev=LABEL=RANCHER_STATE rancher.state.a
 
 The hidden part of the command line can be accessed with either `sudo ros config get rancher.environment.EXTRA_CMDLINE`, or by using a service file's environment array.
 
+An example service.yml file:
+
+```
+test:
+  image: alpine
+  command: echo "tell me a secret ${EXTRA_CMDLINE}"
+  labels:
+    io.rancher.os.scope: system
+  environment:
+  - EXTRA_CMDLINE
+```
+
+When this service is run, the `EXTRA_CMDLINE` will be set.
+
 
 ### cloud-init Datasources
 

tests/cmdline_test.go

@@ -36,4 +36,23 @@ func (s *QemuSuite) TestElideCmdLine(c *C) {
 	)
 
 	// And then add a service.yml file example.
+	s.CheckCall(c,
+		`echo 'test:
+  image: alpine
+  command: echo "tell me a secret ${EXTRA_CMDLINE}"
+  labels:
+    io.rancher.os.scope: system
+  environment:
+  - EXTRA_CMDLINE
+' > test.yml`)
+	s.CheckCall(c, "sudo mv test.yml /var/lib/rancher/conf/test.yml")
+	s.CheckCall(c, "sudo ros service enable /var/lib/rancher/conf/test.yml")
+	s.CheckCall(c, "sudo ros service up test")
+	s.CheckOutput(c,
+		"test_1 | tell me a secret /init cc.hostname=nope rancher.password=three\n",
+		Equals,
+		"sudo ros service logs test | grep secret",
+	)
+
+	// TODO: add a test showing we have the right password set
 }