Only colorize warning messages if stderr is a tty

This is to avoid outputting ANSI escape codes to scripts and log files.

hooks/post-up

@@ -10,9 +10,15 @@ vim -u $HOME/.vimrc.bundles +PlugInstall +PlugClean! +qa
 
 # detect old OS X broken /etc/zshenv and suggest rename
 if grep -qw path_helper /etc/zshenv 2>/dev/null; then
-  fg_red=$'\e[31m'
-  fg_bold_white=$'\e[1;37m'
-  reset_color=$'\e[m'
+  if [ -t 2 ]; then
+    fg_red=$'\e[31m'
+    fg_bold_white=$'\e[1;37m'
+    reset_color=$'\e[m'
+  else
+    fg_red=""
+    fg_bold_white=""
+    reset_color=""
+  fi
 
   # resolve BASH_SOURCE to absolute path
   bash_source="$BASH_SOURCE"

zshenv

@@ -6,9 +6,15 @@ local _old_path="$PATH"
 if [[ $PATH != $_old_path ]]; then
   # `colors` isn't initialized yet, so define a few manually
   typeset -AHg fg fg_bold
-  fg[red]=$'\e[31m'
-  fg_bold[white]=$'\e[1;37m'
-  reset_color=$'\e[m'
+  if [ -t 2 ]; then
+    fg[red]=$'\e[31m'
+    fg_bold[white]=$'\e[1;37m'
+    reset_color=$'\e[m'
+  else
+    fg[red]=""
+    fg_bold[white]=""
+    reset_color=""
+  fi
 
   cat <<MSG >&2
 ${fg[red]}Warning:${reset_color} your \`~/.zshenv.local' configuration seems to edit PATH entries.