Account for defined but empty shell variables

Fixes #407, Thanks to #723

Author: Ivoz

docs/changes.rst

@@ -14,6 +14,8 @@ Release History
 
 * Account for a ``site.pyo`` correctly in some python implementations
 
+* Properly restore an empty PS1
+
 13.1.2 (2015-08-23)
 -------------------
 

virtualenv.py

@@ -2033,21 +2033,20 @@ def convert(s):
 
 ##file activate.sh
 ACTIVATE_SH = convert("""
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 """)
 
 ##file activate.fish

virtualenv_embedded/activate.sh

@@ -5,12 +5,12 @@ deactivate () {
     unset pydoc
 
     # reset old environment variables
-    if [ -n "${_OLD_VIRTUAL_PATH-}" ] ; then
+    if ! [ -z "${_OLD_VIRTUAL_PATH+_}" ] ; then
         PATH="$_OLD_VIRTUAL_PATH"
         export PATH
         unset _OLD_VIRTUAL_PATH
     fi
-    if [ -n "${_OLD_VIRTUAL_PYTHONHOME-}" ] ; then
+    if ! [ -z "${_OLD_VIRTUAL_PYTHONHOME+_}" ] ; then
         PYTHONHOME="$_OLD_VIRTUAL_PYTHONHOME"
         export PYTHONHOME
         unset _OLD_VIRTUAL_PYTHONHOME
@@ -23,7 +23,7 @@ deactivate () {
         hash -r 2>/dev/null
     fi
 
-    if [ -n "${_OLD_VIRTUAL_PS1-}" ] ; then
+    if ! [ -z "${_OLD_VIRTUAL_PS1+_}" ] ; then
         PS1="$_OLD_VIRTUAL_PS1"
         export PS1
         unset _OLD_VIRTUAL_PS1
@@ -47,9 +47,7 @@ PATH="$VIRTUAL_ENV/__BIN_NAME__:$PATH"
 export PATH
 
 # unset PYTHONHOME if set
-# this will fail if PYTHONHOME is set to the empty string (which is bad anyway)
-# could use `if (set -u; : $PYTHONHOME) ;` in bash
-if [ -n "${PYTHONHOME-}" ] ; then
+if ! [ -z "${PYTHONHOME+x}" ] ; then
     _OLD_VIRTUAL_PYTHONHOME="$PYTHONHOME"
     unset PYTHONHOME
 fi