Merge pull request #138 from CircleCI-Public/no-docker-error

Improve error message when docker is not present or not started.

cmd/build.go

@@ -211,9 +211,22 @@ You can use 'circleci config process' to pre-process your config into a version
 	return nil
 }
 
+func ensureDockerIsAvailable() error {
+	if _, err := exec.LookPath("docker"); err != nil {
+		return errors.New("could not find `docker` on the PATH; please ensure than docker is installed")
+	}
+
+	dockerRunning := exec.Command("docker", "version").Run() == nil // #nosec
+
+	if !dockerRunning {
+		return errors.New("failed to connect to docker; please ensure that docker is running, and that `docker version` succeeds")
+	}
+
+	return nil
+}
+
 func runExecute(cmd *cobra.Command, args []string) error {
-	err := validateConfigVersion(args)
-	if err != nil {
+	if err := validateConfigVersion(args); err != nil {
 		return err
 	}
 
@@ -223,6 +236,10 @@ func runExecute(cmd *cobra.Command, args []string) error {
 		return errors.Wrap(err, "Could not find pwd")
 	}
 
+	if err = ensureDockerIsAvailable(); err != nil {
+		return err
+	}
+
 	image, err := picardImage()
 
 	if err != nil {

cmd/update.go

@@ -58,6 +58,11 @@ func updateBuildAgent(cmd *cobra.Command, args []string) error {
 
 // Still depends on a function in cmd/build.go
 func findLatestPicardSha() (string, error) {
+
+	if err := ensureDockerIsAvailable(); err != nil {
+		return "", err
+	}
+
 	outputBytes, err := exec.Command("docker", "pull", picardRepo).CombinedOutput() // #nosec
 
 	if err != nil {