diff --git a/meson.build b/meson.build
index bfea405..2320cb1 100644
--- a/meson.build
+++ b/meson.build
@@ -23,6 +23,8 @@ snake_lib = static_library('snake',
                              'src/State.cpp',
                              'src/DAG.cpp',
                              'src/SymTable.cpp',
+                             'src/Loader.cpp',
+                             'src/Executor.cpp',
                            ],
                            dependencies: [
                              dependency('sqlite3')
@@ -46,6 +48,7 @@ executable('snake-tests',
              'tests/main.cpp',
              'tests/Lexer.cpp',
              'tests/Parser.cpp',
+             'tests/Interpreter.cpp',
            ],
            dependencies: [
              snake_dep,
diff --git a/src/Executor.cpp b/src/Executor.cpp
new file mode 100644
index 0000000..06ad1c2
--- /dev/null
+++ b/src/Executor.cpp
@@ -0,0 +1,30 @@
+#include "Executor.hpp"
+
+namespace sn
+{
+  /*explicit*/ Executor::Executor()
+  {
+  }
+
+  /*virtual*/ Executor::~Executor()
+  {
+  }
+
+  /*virtual*/ std::string Executor::execute(std::string const& command)
+  {
+    std::stringstream ss;
+
+    FILE* out = popen(command.c_str(), "r");
+    assert(out);
+
+    char buf;
+
+    while ( fread(&buf, sizeof(char), 1, out) )
+      {
+        ss << buf;
+      }
+
+    pclose(out);
+    return ss.str();
+  }
+}
diff --git a/src/Executor.hpp b/src/Executor.hpp
new file mode 100644
index 0000000..a99ee03
--- /dev/null
+++ b/src/Executor.hpp
@@ -0,0 +1,20 @@
+#ifndef sn_EXECUTOR_HPP
+#define sn_EXECUTOR_HPP
+
+#include "commons.hpp"
+
+namespace sn
+{
+  class Executor
+  {
+  public:
+    explicit Executor();
+    virtual ~Executor();
+
+    virtual std::string execute(std::string const& command);
+
+  private:
+  };
+}
+
+#endif
diff --git a/src/Interpreter.cpp b/src/Interpreter.cpp
index 1389a5f..61b50ad 100644
--- a/src/Interpreter.cpp
+++ b/src/Interpreter.cpp
@@ -5,7 +5,12 @@
 
 namespace sn
 {
-  /*explicit*/ Interpreter::Interpreter()
+  /*explicit*/ Interpreter::Interpreter(std::shared_ptr<State> state,
+                                        std::shared_ptr<Loader> loader,
+                                        std::shared_ptr<Executor> executor)
+    : m_state  { state }
+    , m_loader { loader }
+    , m_executor { executor }
   {
   }
 
@@ -17,7 +22,7 @@ namespace sn
   {
     // Process the document ast
     // ------------------------
-    auto node = parse_snakefile();
+    auto node = m_loader->parse_snakefile();
     process(node);
 
     // Get all files
@@ -47,11 +52,9 @@ namespace sn
 
     // Get modified files
     // ------------------
-    auto state = std::make_shared<State>();
+    m_state->init(files);
 
-    state->init(files);
-
-    auto modified_files = state->get_modified_files(files);
+    auto modified_files = m_state->get_modified_files(files);
 
     // Build DAG
     // ---------
@@ -120,7 +123,7 @@ namespace sn
               }
           }
 
-        state->update(file);
+        m_state->update(file);
       }
 
     if (sorted.empty())
@@ -129,38 +132,6 @@ namespace sn
       }
   }
 
-  std::filesystem::path Interpreter::find_snakefile()
-  {
-    auto path = std::filesystem::current_path() / "Snakefile";
-
-    if (!std::filesystem::exists(path))
-      {
-        throw run_error {"Snakefile not found"};
-      }
-
-    return path;
-  }
-
-  std::string Interpreter::load_snakefile()
-  {
-    std::ifstream file {find_snakefile()};
-    assert(file);
-    std::stringstream ss;
-    ss << file.rdbuf();
-    return ss.str();
-  }
-
-  std::shared_ptr<Node> Interpreter::parse_snakefile()
-  {
-    Lexer lexer;
-    lexer.scan(load_snakefile());
-
-    Parser parser;
-    auto node = parser.parse(lexer.all());
-
-    return node;
-  }
-
   std::string Interpreter::get_value(std::shared_ptr<Node> node)
   {
     if (node->type() == NODE_IDENT)
@@ -283,20 +254,7 @@ namespace sn
 
   std::string Interpreter::execute(std::string const& script) const
   {
-    std::stringstream ss;
-
-    FILE* out = popen(script.c_str(), "r");
-    assert(out);
-
-    char buf;
-
-    while ( fread(&buf, sizeof(char), 1, out) )
-      {
-        ss << buf;
-      }
-
-    pclose(out);
-    return ss.str();
+    return m_executor->execute(script);
   }
 
   std::filesystem::path
diff --git a/src/Interpreter.hpp b/src/Interpreter.hpp
index 27b9dd2..3385cc6 100644
--- a/src/Interpreter.hpp
+++ b/src/Interpreter.hpp
@@ -6,6 +6,8 @@
 #include "State.hpp"
 #include "DAG.hpp"
 #include "SymTable.hpp"
+#include "Loader.hpp"
+#include "Executor.hpp"
 
 namespace sn
 {
@@ -19,16 +21,18 @@ namespace sn
   class Interpreter
   {
   public:
-    explicit Interpreter();
+    explicit Interpreter(std::shared_ptr<State> state,
+                         std::shared_ptr<Loader> loader,
+                         std::shared_ptr<Executor> executor);
     virtual ~Interpreter();
 
     void run();
 
-    std::filesystem::path find_snakefile();
-    std::string load_snakefile();
-    std::shared_ptr<Node> parse_snakefile();
-
   private:
+    std::shared_ptr<State> m_state;
+    std::shared_ptr<Loader> m_loader;
+    std::shared_ptr<Executor> m_executor;
+
     std::unordered_map<std::filesystem::path,
                        std::vector<std::filesystem::path>> m_dependencies;
 
diff --git a/src/Loader.cpp b/src/Loader.cpp
new file mode 100644
index 0000000..94df384
--- /dev/null
+++ b/src/Loader.cpp
@@ -0,0 +1,46 @@
+#include "Loader.hpp"
+#include "Lexer.hpp"
+#include "Parser.hpp"
+
+namespace sn
+{
+  /*explicit*/ Loader::Loader()
+  {
+  }
+
+  /*virtual*/ Loader::~Loader()
+  {
+  }
+
+  std::filesystem::path Loader::find_snakefile()
+  {
+    auto path = std::filesystem::current_path() / "Snakefile";
+
+    if (!std::filesystem::exists(path))
+      {
+        throw load_error {"Snakefile not found"};
+      }
+
+    return path;
+  }
+
+  std::string Loader::load_snakefile()
+  {
+    std::ifstream file {find_snakefile()};
+    assert(file);
+    std::stringstream ss;
+    ss << file.rdbuf();
+    return ss.str();
+  }
+
+  std::shared_ptr<Node> Loader::parse_snakefile()
+  {
+    Lexer lexer;
+    lexer.scan(load_snakefile());
+
+    Parser parser;
+    auto node = parser.parse(lexer.all());
+
+    return node;
+  }
+}
diff --git a/src/Loader.hpp b/src/Loader.hpp
new file mode 100644
index 0000000..eab91f5
--- /dev/null
+++ b/src/Loader.hpp
@@ -0,0 +1,30 @@
+#ifndef sn_LOADER_HPP
+#define sn_LOADER_HPP
+
+#include "commons.hpp"
+#include "Node.hpp"
+
+namespace sn
+{
+  SN_ERROR(load_error);
+
+  /**
+   * Find and compile snakefiles.
+   * @see Lexer
+   * @see Parser
+   **/
+  class Loader
+  {
+  public:
+    explicit Loader();
+    virtual ~Loader();
+
+    virtual std::filesystem::path find_snakefile();
+    virtual std::string load_snakefile();
+    virtual std::shared_ptr<Node> parse_snakefile();
+
+  private:
+  };
+}
+
+#endif
diff --git a/src/State.hpp b/src/State.hpp
index 264b000..9e7c466 100644
--- a/src/State.hpp
+++ b/src/State.hpp
@@ -22,10 +22,10 @@ namespace sn
     explicit State();
     virtual ~State();
 
-    void init(std::vector<std::filesystem::path> const& files);
-    void update(std::filesystem::path path);
+    virtual void init(std::vector<std::filesystem::path> const& files);
+    virtual void update(std::filesystem::path path);
 
-    std::vector<std::filesystem::path>
+    virtual std::vector<std::filesystem::path>
     get_modified_files(std::vector<std::filesystem::path> const& files);
 
     std::filesystem::path format_path(std::filesystem::path path) const;
diff --git a/src/main.cpp b/src/main.cpp
index 1500ad8..5d2a7d9 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -3,10 +3,15 @@
 #include "snake.hpp"
 #include "Interpreter.hpp"
 #include "State.hpp"
+#include "Executor.hpp"
 
 int main(int, char**)
 {
-  sn::Interpreter interpreter;
+  auto state = std::make_shared<sn::State>();
+  auto loader = std::make_shared<sn::Loader>();
+  auto executor = std::make_shared<sn::Executor>();
+
+  sn::Interpreter interpreter {state, loader, executor};
   interpreter.run();
 
   return 0;
diff --git a/tests/Interpreter.cpp b/tests/Interpreter.cpp
new file mode 100644
index 0000000..50c24e6
--- /dev/null
+++ b/tests/Interpreter.cpp
@@ -0,0 +1,103 @@
+#include <catch2/catch.hpp>
+#include "../src/Interpreter.hpp"
+
+#define TEST_RUN(SRC, ...) \
+  test_run(SRC, {__VA_ARGS__})
+
+using namespace sn;
+
+struct StateMock: public State {
+  std::vector<std::filesystem::path> modified_files;
+
+  StateMock(std::vector<std::filesystem::path> const& p_modified_files)
+    : modified_files { p_modified_files }
+  {
+  }
+
+  virtual void init(std::vector<std::filesystem::path> const&) override
+  {
+  }
+
+  virtual void update(std::filesystem::path) override
+  {
+  }
+
+  virtual std::vector<std::filesystem::path>
+  get_modified_files(std::vector<std::filesystem::path> const&) override
+  {
+    return modified_files;
+  }
+};
+
+struct LoaderMock: public Loader {
+  std::string snakefile;
+
+  LoaderMock(std::string const& p_snakefile)
+    : snakefile { p_snakefile }
+  {
+  }
+
+  virtual std::filesystem::path find_snakefile() override { return ""; }
+
+  virtual std::string load_snakefile() override
+  {
+    return snakefile;
+  }
+};
+
+struct ExecutorMock: public Executor {
+  std::vector<std::string> history;
+
+  virtual std::string execute(std::string const& command) override
+  {
+    history.push_back(command);
+    return "";
+  }
+};
+
+class InterpreterTest
+{
+public:
+  explicit InterpreterTest() {}
+  virtual ~InterpreterTest() {}
+
+  std::vector<std::string>
+  test_run(std::string const& snakefile,
+           std::vector<std::filesystem::path> const& modified)
+  {
+    auto state = std::make_shared<StateMock>(modified);
+
+    auto loader = std::make_shared<LoaderMock>(snakefile);
+    auto executor = std::make_shared<ExecutorMock>();
+
+    Interpreter interpreter {state, loader, executor};
+    interpreter.run();
+
+    return executor->history;
+  }
+
+  void test_contains(std::string const& str, std::vector<std::string> vec)
+  {
+    INFO("'" << str << "' not found");
+    REQUIRE(std::find(std::begin(vec), std::end(vec), str)
+            != std::end(vec));
+  }
+
+protected:
+};
+
+TEST_CASE_METHOD(InterpreterTest, "Interpreter_simple_var")
+{
+  auto modified = std::vector<std::filesystem::path> {
+    std::filesystem::path("a"),
+  };
+
+  std::stringstream ss;
+  ss << "$X = a" << std::endl;
+  ss << "b -> $X {" << std::endl;
+  ss << "executed" << std::endl;
+  ss << "}" << std::endl;
+
+  auto res = TEST_RUN(ss.str(), std::filesystem::path("a"));
+  test_contains("executed", res);
+}