Index: pym/portage/__init__.py
===================================================================
--- pym/portage/__init__.py	(revision 11803)
+++ pym/portage/__init__.py	(revision 11804)
@@ -987,6 +987,11 @@
 		"INFOPATH", "MANPATH",
 	]
 
+	# variables that break bash
+	_environ_filter += [
+		"POSIXLY_CORRECT",
+	]
+
 	# portage config variables and variables set directly by portage
 	_environ_filter += [
 		"ACCEPT_KEYWORDS", "AUTOCLEAN",
Index: bin/ebuild.sh
===================================================================
--- bin/ebuild.sh	(revision 11803)
+++ bin/ebuild.sh	(revision 11804)
@@ -1595,7 +1595,7 @@
 		SANDBOX_DEBUG_LOG SANDBOX_DISABLED SANDBOX_LIB
 		SANDBOX_LOG SANDBOX_ON"
 	filtered_vars="${readonly_bash_vars} ${READONLY_PORTAGE_VARS}
-		BASH_.* PATH"
+		BASH_.* PATH POSIXLY_CORRECT"
 	if hasq --filter-sandbox $* ; then
 		filtered_vars="${filtered_vars} SANDBOX_.*"
 	else