diff --git a/cibuildwheel/linux.py b/cibuildwheel/linux.py
index 3497d4ff..e3cd9fa5 100644
--- a/cibuildwheel/linux.py
+++ b/cibuildwheel/linux.py
@@ -63,17 +63,25 @@ def build(project_dir, output_dir, test_command, test_requires, test_extras, bef
             mkdir /output
             cd /project
 
-            {environment_exports}
+            for PYBIN in {pybin_paths}; do (
+                # Temporary hack/workaround, putting loop body in subshell; fixed in PR #256
+
+                export PATH="$PYBIN:$PATH"
+                {environment_exports}
+
+                # check the active python and pip are in PYBIN
+                # if `test` returns false, the script will exit due to errexit
+                test "$(which pip)" = "$PYBIN/pip"
+                test "$(which python)" = "$PYBIN/python"
 
-            for PYBIN in {pybin_paths}; do
                 if [ ! -z {before_build} ]; then
-                    PATH="$PYBIN:$PATH" sh -c {before_build}
+                    sh -c {before_build}
                 fi
 
                 # Build the wheel
                 rm -rf /tmp/built_wheel
                 mkdir /tmp/built_wheel
-                PATH="$PYBIN:$PATH" "$PYBIN/pip" wheel . -w /tmp/built_wheel --no-deps {build_verbosity_flag}
+                pip wheel . -w /tmp/built_wheel --no-deps {build_verbosity_flag}
                 built_wheel=(/tmp/built_wheel/*.whl)
 
                 # repair the wheel
@@ -92,9 +100,9 @@ def build(project_dir, output_dir, test_command, test_requires, test_extras, bef
                 if [ ! -z {test_command} ]; then
                     # Set up a virtual environment to install and test from, to make sure
                     # there are no dependencies that were pulled in at build time.
-                    "$PYBIN/pip" install virtualenv
+                    pip install virtualenv
                     venv_dir=`mktemp -d`/venv
-                    "$PYBIN/python" -m virtualenv "$venv_dir"
+                    python -m virtualenv "$venv_dir"
 
                     # run the tests in a subshell to keep that `activate`
                     # script from polluting the env
@@ -123,7 +131,7 @@ def build(project_dir, output_dir, test_command, test_requires, test_extras, bef
                     )
                     # exit if tests failed (needed for older bash versions)
                     if [ $? -ne 0 ]; then
-                      exit 1;
+                        exit 1;
                     fi
 
                     # clean up
@@ -133,7 +141,7 @@ def build(project_dir, output_dir, test_command, test_requires, test_extras, bef
                 # we're all done here; move it to output
                 mv "${{repaired_wheels[@]}}" /output
                 for repaired_wheel in "${{repaired_wheels[@]}}"; do chown {uid}:{gid} "/output/$(basename "$repaired_wheel")"; done
-            done
+            ) done
         '''.format(
             pybin_paths=' '.join(c.path + '/bin' for c in platform_configs),
             test_requires=' '.join(test_requires),
diff --git a/cibuildwheel/macos.py b/cibuildwheel/macos.py
index fa56a66c..b0bbc18d 100644
--- a/cibuildwheel/macos.py
+++ b/cibuildwheel/macos.py
@@ -137,11 +137,15 @@ def build(project_dir, output_dir, test_command, test_requires, test_extras, bef
         # check what version we're on
         call(['which', 'python'], env=env)
         call(['python', '--version'], env=env)
+        # TODO Cleanup/merge with above `call` once we have `subprocess.run` after dropping Python 2 support?
+        assert subprocess.check_output(['which', 'python'], env=env, universal_newlines=True).strip() == '/tmp/cibw_bin/python'
 
         # install pip & wheel
         call(['python', get_pip_script], env=env, cwd="/tmp")
         assert os.path.exists(os.path.join(installation_bin_path, 'pip'))
         call(['pip', '--version'], env=env)
+        # TODO Cleanup/merge with above `call` once we have `subprocess.run` after dropping Python 2 support?
+        assert subprocess.check_output(['which', 'pip'], env=env, universal_newlines=True).strip() == '/tmp/cibw_bin/pip'
         call(['pip', 'install', '--upgrade', 'setuptools', 'wheel', 'delocate'], env=env)
 
         # setup target platform, only required for python 3.5
diff --git a/cibuildwheel/windows.py b/cibuildwheel/windows.py
index 4263d759..8a8aff97 100644
--- a/cibuildwheel/windows.py
+++ b/cibuildwheel/windows.py
@@ -141,11 +141,15 @@ def build(project_dir, output_dir, test_command, test_requires, test_extras, bef
         simple_shell(['where', 'python'], env=env)
         simple_shell(['python', '--version'], env=env)
         simple_shell(['python', '-c', '"import struct; print(struct.calcsize(\'P\') * 8)"'], env=env)
+        # TODO Cleanup/merge with above `simple_shell` once we have `subprocess.run` after dropping Python 2 support?
+        assert subprocess.check_output(['where', 'python'], env=env, universal_newlines=True).splitlines()[0] == os.path.join(installation_path, 'python.exe')
 
         # make sure pip is installed
         if not os.path.exists(os.path.join(installation_path, 'Scripts', 'pip.exe')):
             simple_shell(['python', get_pip_script], env=env, cwd="C:\\cibw")
         assert os.path.exists(os.path.join(installation_path, 'Scripts', 'pip.exe'))
+        # TODO Cleanup/merge with above `simple_shell` once we have `subprocess.run` after dropping Python 2 support?
+        assert subprocess.check_output(['where', 'pip'], env=env, universal_newlines=True).splitlines()[0] == os.path.join(installation_path, 'Scripts', 'pip.exe')
 
         # prepare the Python environment
         simple_shell(['python', '-m', 'pip', 'install', '--upgrade', 'pip'], env=env)