diff --git a/env-var.py b/env-var.py
deleted file mode 100644
index 011a93a..0000000
--- a/env-var.py
+++ /dev/null
@@ -1,80 +0,0 @@
-from enum import Enum
-from typing import Optional
-
-class EnvVarOp (Enum):
-    PREPEND=1
-    APPEND=2
-    SET=3
-
-class EnvVarKind (Enum):
-    SCALAR=2
-    LIST=2
-
-def is_env_value_list(v):
-    return isinstance(v, list) and all(isinstance(item, str) for item in v)
-
-class ListEnvVarUpdate():
-    def __init__(self, value: list[str], op: EnvVarOp):
-        self._value = value
-        self._op = op
-
-    @property
-    def op(self):
-        return self._op
-
-    @property
-    def value(self):
-        return self._value
-
-class EnvVar:
-    def __init__(self, name: str):
-        self._name = name
-
-    @property
-    def name(self):
-        return self._name
-
-class ListEnvVar(EnvVar):
-    def __init__(self, name: str, value: list[str], op: EnvVarOp):
-        super().__init__(name)
-
-        self._updates = [ListEnvVarUpdate(value, op)]
-
-    def update(self, value: list[str], op:EnvVarOp):
-        self._updates.append(ListEnvVarUpdate(value, op))
-
-class ScalarEnvVar(EnvVar):
-    def __init__(self, name: str, value: Optional[str]):
-        super().__init__(name)
-        self._value = value
-
-    @property
-    def value(self):
-        return self._value
-
-    @property
-    def is_null(self):
-        return self.value is None
-
-    def update(self, value: Optional[str]):
-        self._value = value
-
-class Env:
-    def __init__(self):
-        self._vars = {}
-
-    def apply(self, var: EnvVar):
-        self._vars[var.name] = var
-
-    def print(self):
-        for name, var in self._vars.items():
-            print(f"{name}: {var.value}")
-
-env = Env()
-
-s = ScalarEnvVar("CUDA_HOME", "/opt/nvidia/cuda12")
-#l = ListEnvVar("PATH", ["/usr/bin", "/opt/nvidia/cuda12/bin"], EnvVarOp.APPEND)
-
-env.apply(s)
-#env.apply(l)
-env.print()
diff --git a/lib/env.py b/lib/env.py
new file mode 100644
index 0000000..19fb237
--- /dev/null
+++ b/lib/env.py
@@ -0,0 +1,169 @@
+from enum import Enum
+from typing import Optional, List
+
+class EnvVarOp (Enum):
+    PREPEND=1
+    APPEND=2
+    SET=3
+
+class EnvVarKind (Enum):
+    SCALAR=2
+    LIST=2
+
+list_variables = {
+        "ACLOCAL_PATH",
+        "CMAKE_PREFIX_PATH",
+        "CPATH",
+        "LD_LIBRARY_PATH",
+        "LIBRARY_PATH",
+        "MANPATH",
+        "PATH",
+        "PKG_CONFIG_PATH",
+        }
+
+class EnvVarError(Exception):
+    """Exception raised when there is an error with environment variable manipulation."""
+
+    def __init__(self, message):
+        self.message = message
+        super().__init__(self.message)
+
+    def __str__(self):
+        return self.message
+
+def is_env_value_list(v):
+    return isinstance(v, list) and all(isinstance(item, str) for item in v)
+
+class ListEnvVarUpdate():
+    def __init__(self, value: List[str], op: EnvVarOp):
+        # strip white space from each entry
+        self._value = [v.strip() for v in value]
+        self._op = op
+
+    @property
+    def op(self):
+        return self._op
+
+    @property
+    def value(self):
+        return self._value
+
+    def __repr__(self):
+        return f"env.ListEnvVarUpdate({self.value}, {self.op})"
+
+    def __str__(self):
+        return f"({self.value}, {self.op})"
+
+class EnvVar:
+    def __init__(self, name: str):
+        self._name = name
+
+    @property
+    def name(self):
+        return self._name
+
+class ListEnvVar(EnvVar):
+    def __init__(self, name: str, value: List[str], op: EnvVarOp):
+        super().__init__(name)
+
+        self._updates = [ListEnvVarUpdate(value, op)]
+
+    def update(self, value: List[str], op:EnvVarOp):
+        self._updates.append(ListEnvVarUpdate(value, op))
+
+    @property
+    def updates(self):
+        return self._updates
+
+    def concat(self, other: 'ListEnvVar'):
+        self._updates += other.updates
+
+    # Given the current value, return the value that should be set
+    def get_value(self, current: Optional[str]):
+        v = current
+
+        # if the variable is currently not set, first initialise it as empty.
+        if v is None:
+            if len(self._updates)==0:
+                return None
+            v = ""
+
+        for update in self._updates:
+            joined = ":".join(update.value)
+            if v == "" or update.op==EnvVarOp.SET:
+                v = joined
+            elif update.op==EnvVarOp.APPEND:
+                v = ":".join([v, joined])
+            elif update.op==EnvVarOp.PREPEND:
+                v = ":".join([joined, v])
+            else:
+                raise EnvVarError(f"Internal error: implement the operation {update.op}");
+            # strip any leading/trailing ":"
+            v = v.strip(':')
+
+        return v
+
+    def __repr__(self):
+        return f"env.ListEnvVar(\"{self.name}\", {self._updates})"
+
+    def __str__(self):
+        return f"(\"{self.name}\": [{','.join([str(u) for u in self._updates])}])"
+
+
+class ScalarEnvVar(EnvVar):
+    def __init__(self, name: str, value: Optional[str]):
+        super().__init__(name)
+        self._value = value
+
+    @property
+    def value(self):
+        return self._value
+
+    @property
+    def is_null(self):
+        return self.value is None
+
+    def update(self, value: Optional[str]):
+        self._value = value
+
+    def __repr__(self):
+        return f"env.ScalarEnvVar(\"{self.name}\", \"{self.value}\")"
+
+    def __str__(self):
+        return f"(\"{self.name}\": \"{self.value}\")"
+
+class Env:
+    def __init__(self):
+        self._vars = {}
+
+    def apply(self, var: EnvVar):
+        self._vars[var.name] = var
+
+# returns true if the environment variable with name is a list variable,
+# e.g. PATH, LD_LIBRARY_PATH, PKG_CONFIG_PATH, etc.
+def is_list_var(name: str) -> bool:
+    return name in list_variables
+
+class Env:
+
+    def __init__(self):
+        self._lists = {}
+        self._scalars = {}
+
+    @property
+    def lists(self):
+        return self._listself._lists
+
+    @property
+    def scalars(self):
+        return self._scalars
+
+    def set_scalar(self, var: ScalarEnvVar):
+        self._scalars[var.name] = var
+
+    def set_list(self, var: ListEnvVar):
+        if var.name in self._lists.keys():
+            old = self._lists[var.name]
+            self._lists[var.name] = old.concat(var)
+        else:
+            self._lists[var.name] = var
diff --git a/test/unit/test_env.py b/test/unit/test_env.py
new file mode 100644
index 0000000..df1e129
--- /dev/null
+++ b/test/unit/test_env.py
@@ -0,0 +1,90 @@
+import shutil
+import unittest
+
+import env
+
+class TestListEnvVars(unittest.TestCase):
+
+    def test_env_var_name(self):
+        # test that the name is set correctly
+        self.assertEqual("PATH", env.ListEnvVar("PATH", ["/foo/bin"], env.EnvVarOp.SET).name)
+
+    def test_env_var_list_shor(self):
+        self.assertEqual(
+            "/foo/bin:/bar/bin",
+            env.ListEnvVar("PATH", ["/foo/bin", "/bar/bin"], env.EnvVarOp.SET).get_value(None))
+        self.assertEqual(
+            "/foo/bin:/bar/bin",
+            env.ListEnvVar("PATH", ["/foo/bin", "/bar/bin"], env.EnvVarOp.SET).get_value(""))
+        self.assertEqual(
+            "/foo/bin:/bar/bin",
+            env.ListEnvVar("PATH", ["/foo/bin", "/bar/bin"], env.EnvVarOp.SET).get_value("/wombat"))
+        self.assertEqual(
+            "/foo/bin:/bar/bin",
+            env.ListEnvVar("PATH", ["/foo/bin", "/bar/bin"], env.EnvVarOp.PREPEND).get_value(None))
+        self.assertEqual(
+            "/foo/bin:/bar/bin",
+            env.ListEnvVar("PATH", ["/foo/bin", "/bar/bin"], env.EnvVarOp.PREPEND).get_value(""))
+        self.assertEqual(
+            "/foo/bin:/bar/bin:/wombat",
+            env.ListEnvVar("PATH", ["/foo/bin", "/bar/bin"], env.EnvVarOp.PREPEND).get_value("/wombat"))
+        self.assertEqual(
+            "/foo/bin:/bar/bin",
+            env.ListEnvVar("PATH", ["/foo/bin", "/bar/bin"], env.EnvVarOp.APPEND).get_value(None))
+        self.assertEqual(
+            "/foo/bin:/bar/bin",
+            env.ListEnvVar("PATH", ["/foo/bin", "/bar/bin"], env.EnvVarOp.APPEND).get_value(""))
+        self.assertEqual(
+            "/wombat:/foo/bin:/bar/bin",
+            env.ListEnvVar("PATH", ["/foo/bin", "/bar/bin"], env.EnvVarOp.APPEND).get_value("/wombat"))
+
+    def test_env_var_list_long(self):
+        v = env.ListEnvVar("PATH", ["c"], env.EnvVarOp.PREPEND)
+        v.update(["a","b"], env.EnvVarOp.PREPEND)
+        v.update(["e","f"], env.EnvVarOp.APPEND)
+        self.assertEqual("a:b:c:d:e:f", v.get_value("d"))
+        self.assertEqual("a:b:c:e:f", v.get_value(None))
+
+        v = env.ListEnvVar("PATH", ["c"], env.EnvVarOp.SET)
+        v.update(["a","b"], env.EnvVarOp.PREPEND)
+        v.update(["e","f"], env.EnvVarOp.APPEND)
+        self.assertEqual("a:b:c:e:f", v.get_value("d"))
+
+        self.assertEqual("a:b:c:e:f", v.get_value(None))
+
+    def test_env_var_list_concat(self):
+        v  = env.ListEnvVar("PATH", ["a"], env.EnvVarOp.PREPEND)
+        x = env.ListEnvVar("PATH", ["c"], env.EnvVarOp.APPEND)
+        v.concat(x)
+        self.assertEqual("a:b:c", v.get_value("b"))
+
+    def test_env_var_scalars(self):
+        v = env.ScalarEnvVar("HOME", "/users/bob")
+        self.assertEqual("HOME", v.name)
+        self.assertEqual("/users/bob", v.value)
+
+        v = env.ScalarEnvVar("HOME", None)
+        self.assertEqual("HOME", v.name)
+        self.assertEqual(None, v.value)
+
+class TestEnvVarCategories(unittest.TestCase):
+
+    def test_env_var_categories(self):
+        self.assertTrue(env.is_list_var("LD_LIBRARY_PATH"))
+        self.assertTrue(env.is_list_var("PKG_CONFIG_PATH"))
+        self.assertFalse(env.is_list_var("HOME"))
+
+class TestEnv(unittest.TestCase):
+
+    def test_env(self):
+        e = env.Env()
+
+        e.set_scalar(env.ScalarEnvVar("HOME", "/users/bob"))
+        e.set_scalar(env.ScalarEnvVar("VISIBLE", ""))
+
+        print()
+        print(e.scalars)
+        print()
+
+        #for name, value in e.scalars:
+            #print(f"{name}: {value}")