Add environment code satisfying unit test

cibuildwheel/bashlex_eval.py

@@ -0,0 +1,39 @@
+import subprocess, shlex
+
+
+def evaluate_node(node, environment):
+    if node.kind == 'word':
+        return evaluate_word_node(node, environment=environment)
+    elif node.kind == 'commandsubstitution':
+        return evaluate_command_node(node.command, environment=environment)
+    elif node.kind == 'parameter':
+        return evaluate_parameter_node(node, environment=environment)
+    else:
+        raise ValueError('Unsupported bash construct: "%s"' % node.word)
+
+
+def evaluate_word_node(node, environment):
+    letters = list(node.word)
+
+    for part in node.parts:
+        part_start = part.pos[0]
+        part_end = part.pos[1]
+
+        # Set all the characters in the part to None
+        for i in range(part_start, part_end):
+            letters[i] = None
+
+        letters[part_start] = evaluate_node(part, environment=environment)
+
+    # remove the None letters and concat
+    return ''.join(l for l in letters if l is not None)
+
+
+def evaluate_command_node(node, environment):
+    words = [evaluate_node(part, environment=environment) for part in node.parts]
+    command = ' '.join(words)
+    return subprocess.check_output(shlex.split(command), env=environment)
+
+
+def evaluate_parameter_node(node, environment):
+    return environment.get(node.value, '')