diff --git a/src/testdir/unix.vim b/src/testdir/unix.vim
index 36dd4e1..2290ec9 100644
--- a/src/testdir/unix.vim
+++ b/src/testdir/unix.vim
@@ -2,4 +2,9 @@
 " Always use "sh", don't use the value of "$SHELL".
 set shell=sh
 
+" While some tests overwrite $HOME to prevent them from polluting user files,
+" we need to remember the original value so that we can tell external systems
+" where to ask about their own user settings.
+let g:tester_HOME = $HOME
+
 source setup.vim