Merge pull request #709 from claui/fix-osx-subshell-prompt

Preserve `PROMPT_COMMAND` exportedness for subshells

plugins/available/osx.plugin.bash

@@ -5,7 +5,10 @@ about-plugin 'osx-specific functions'
 if [ $(uname) = "Darwin" ]; then
   if type update_terminal_cwd > /dev/null 2>&1 ; then
     if ! [[ $PROMPT_COMMAND =~ (^|;)update_terminal_cwd($|;) ]] ; then
-      export PROMPT_COMMAND="$PROMPT_COMMAND;update_terminal_cwd"
+      PROMPT_COMMAND="$PROMPT_COMMAND;update_terminal_cwd"
+      declared="$(declare -p PROMPT_COMMAND)"
+      [[ "$declared" =~ \ -[aAilrtu]*x[aAilrtu]*\  ]] 2>/dev/null
+      [[ $? -eq 0 ]] && export PROMPT_COMMAND
     fi
   fi
 fi