diff --git a/src/main/java/io/codeka/gaia/runner/StackCommandBuilder.java b/src/main/java/io/codeka/gaia/runner/StackCommandBuilder.java
new file mode 100644
index 000000000..331f3ae68
--- /dev/null
+++ b/src/main/java/io/codeka/gaia/runner/StackCommandBuilder.java
@@ -0,0 +1,73 @@
+package io.codeka.gaia.runner;
+
+import io.codeka.gaia.bo.Settings;
+import io.codeka.gaia.bo.Stack;
+import io.codeka.gaia.bo.TerraformModule;
+import io.codeka.gaia.bo.backend.Backend;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.stereotype.Component;
+
+/**
+ * A builder class to create stack commands
+ */
+@Component
+public class StackCommandBuilder {
+
+    private Settings settings;
+
+    @Autowired
+    StackCommandBuilder(Settings settings) {
+        this.settings = settings;
+    }
+
+    /**
+     *
+     * @return
+     */
+    String buildApplyScript(Stack stack, TerraformModule module){
+        var backendGenCommand = String.format("echo \"terraform {\n" +
+                "    backend \\\"http\\\" {\n" +
+                "\t\taddress=\\\""+settings.getExternalUrl()+"/api/state/%s\\\"\n" +
+                "\t}\n" +
+                "}\n\" > backend.tf", stack.getId());
+
+        String applyCommand = buildApplyCommand(stack, module);
+
+        System.out.println(applyCommand);
+
+        var commands = new String[]{
+                "set -ex",
+                String.format("git clone %s module", module.getGitRepositoryUrl()),
+                "cd module",
+                "echo 'generating backend configuration'",
+                backendGenCommand,
+                "cat backend.tf",
+                "terraform version",
+                "terraform init",
+                applyCommand
+        };
+
+        // generate a backend.tf config !
+
+        return String.join("\n", commands);
+    }
+
+    String buildApplyCommand(Stack stack, TerraformModule module) {
+        var varFormatString = "-var \"%s=%s\" ";
+        var variablesBuilder = new StringBuilder();
+
+        module.getVariables().forEach(terraformVariable -> {
+
+            var name = terraformVariable.getName();
+            String value = terraformVariable.getDefaultValue();
+            // try getting the value from the stack
+            if(stack.getVariableValues().containsKey(name)){
+                value = stack.getVariableValues().get(name);
+            }
+            variablesBuilder.append(String.format(varFormatString, name, value));
+        });
+
+        return String.format("terraform apply --auto-approve %s", variablesBuilder.toString());
+    }
+
+}
diff --git a/src/main/java/io/codeka/gaia/runner/StackRunner.java b/src/main/java/io/codeka/gaia/runner/StackRunner.java
index 003a6c06d..0f2d41a43 100644
--- a/src/main/java/io/codeka/gaia/runner/StackRunner.java
+++ b/src/main/java/io/codeka/gaia/runner/StackRunner.java
@@ -33,13 +33,16 @@ public class StackRunner {
 
     private Settings settings;
 
+    private StackCommandBuilder stackCommandBuilder;
+
     private Map<String, Job> jobs = new HashMap<>();
 
     @Autowired
-    public StackRunner(DockerClient dockerClient, ContainerConfig containerConfig, Settings settings) {
+    public StackRunner(DockerClient dockerClient, ContainerConfig containerConfig, Settings settings, StackCommandBuilder stackCommandBuilder) {
         this.dockerClient = dockerClient;
         this.containerConfig = containerConfig;
         this.settings = settings;
+        this.stackCommandBuilder = stackCommandBuilder;
     }
 
     @Async
@@ -98,49 +101,7 @@ public void run(Job job, TerraformModule module, Stack stack) {
                 }
             });
 
-            var backendGenCommand = String.format("echo \"terraform {\n" +
-                    "    backend \\\"http\\\" {\n" +
-                    "\t\taddress=\\\""+settings.getExternalUrl()+"/api/state/%s\\\"\n" +
-                    "\t}\n" +
-                    "}\n\" > backend.tf", stack.getId());
-
-            var varFormatString = " -var \"%s=%s\" ";
-            var variablesBuilder = new StringBuilder();
-
-            module.getVariables().forEach(terraformVariable -> {
-
-                var name = terraformVariable.getName();
-                String value = terraformVariable.getDefaultValue();
-                // try getting the value from the stack
-                if(stack.getVariableValues().containsKey(name)){
-                    value = stack.getVariableValues().get(name);
-                }
-                variablesBuilder.append(String.format(varFormatString, name, value));
-            });
-
-            stack.getVariableValues().forEach((s, s2) -> {
-                variablesBuilder.append(String.format(varFormatString, s, s2));
-            });
-
-            var applyCommand = String.format("terraform apply --auto-approve %s", variablesBuilder.toString());
-
-            System.out.println(applyCommand);
-
-            var commands = new String[]{
-                    "set -ex",
-                    String.format("git clone %s module", module.getGitRepositoryUrl()),
-                    "cd module",
-                    "echo 'generating backend configuration'",
-                    backendGenCommand,
-                    "cat backend.tf",
-                    "terraform version",
-                    "terraform init",
-                    applyCommand
-            };
-
-            // generate a backend.tf config !
-
-            var commandScript = String.join("\n", commands);
+            var commandScript = stackCommandBuilder.buildApplyScript(stack, module);
 
             System.err.println("Writing buffer");
             writable.write(ByteBuffer.wrap(commandScript.getBytes()));
diff --git a/src/test/java/io/codeka/gaia/runner/StackCommandBuilderTest.java b/src/test/java/io/codeka/gaia/runner/StackCommandBuilderTest.java
new file mode 100644
index 000000000..8f328e370
--- /dev/null
+++ b/src/test/java/io/codeka/gaia/runner/StackCommandBuilderTest.java
@@ -0,0 +1,85 @@
+package io.codeka.gaia.runner;
+
+import io.codeka.gaia.bo.Settings;
+import io.codeka.gaia.bo.Stack;
+import io.codeka.gaia.bo.TerraformModule;
+import io.codeka.gaia.bo.TerraformVariable;
+import org.junit.jupiter.api.Test;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class StackCommandBuilderTest {
+
+    @Test
+    void buildApplyCommand_shouldGenerateASimpleApplyCommand(){
+        var stackCommandBuilder = new StackCommandBuilder(new Settings());
+
+        var module = new TerraformModule();
+        module.setVariables(Collections.emptyList());
+
+        var stack = new Stack();
+
+        var applyCommand = stackCommandBuilder.buildApplyCommand(stack, module);
+
+        assertEquals("terraform apply --auto-approve ", applyCommand);
+    }
+
+    @Test
+    void buildApplyCommand_shouldGenerateASingleVariableApplyCommand(){
+        var stackCommandBuilder = new StackCommandBuilder(new Settings());
+
+        var module = new TerraformModule();
+        var variable = new TerraformVariable();
+        variable.setName("test");
+        module.setVariables(List.of(variable));
+
+        var stack = new Stack();
+        stack.setVariableValues(Map.of("test", "value"));
+
+        var applyCommand = stackCommandBuilder.buildApplyCommand(stack, module);
+
+        assertEquals("terraform apply --auto-approve -var \"test=value\" ", applyCommand);
+    }
+
+    @Test
+    void buildApplyCommand_shouldGenerateAMultipleVariableApplyCommand(){
+        var stackCommandBuilder = new StackCommandBuilder(new Settings());
+
+        var module = new TerraformModule();
+        var variable = new TerraformVariable();
+        variable.setName("test");
+        var variable2 = new TerraformVariable();
+        variable2.setName("test2");
+        module.setVariables(List.of(variable, variable2));
+
+        var stack = new Stack();
+        stack.setVariableValues(Map.of("test", "value", "test2", "value2"));
+
+        var applyCommand = stackCommandBuilder.buildApplyCommand(stack, module);
+
+        assertEquals("terraform apply --auto-approve -var \"test=value\" -var \"test2=value2\" ", applyCommand);
+    }
+
+    @Test
+    void buildApplyCommand_shouldUseDefaultVariableValues(){
+        var stackCommandBuilder = new StackCommandBuilder(new Settings());
+
+        var module = new TerraformModule();
+        var variable = new TerraformVariable();
+        variable.setName("test");
+        variable.setDefaultValue("defaultValue");
+        module.setVariables(List.of(variable));
+
+        var stack = new Stack();
+        stack.setVariableValues(Collections.emptyMap());
+
+        var applyCommand = stackCommandBuilder.buildApplyCommand(stack, module);
+
+        assertEquals("terraform apply --auto-approve -var \"test=defaultValue\" ", applyCommand);
+    }
+
+}
\ No newline at end of file