How to make zenity "transient parent" warning disappear permanently

Automatically add 2>/dev/null every time zenity is called

Edit the file ~/.bashrc and search for these lines:

# some more ls aliases
alias ll='ls -alF'
alias la='ls -A'
alias l='ls -CF'

Add the following lines after:

# Add zenity alias to make the annoying terminal error message disappear forever:
# "Gtk-Message: GtkDialog mapped without a transient parent. This is discouraged."
alias zenity="zenity 2>/dev/null"

Save the file and open a new terminal window to test:

zenity --info --text "Hello Zenity-Silly-Error-Free World"

Voila! All your old code is fixed and future code doesn't need to have 2>/dev/null appended to it like all the other answers instruct.

Update 2019-05-17: OP has found another resolution along the same method presented here via global redirection by use of exec command. Please refer to linked duplicate post for more info

Basically, there's no other way except redirecting stderr to /dev/null. The main reason is because Gtk requires dialog windows ( which is what zenity windows actually are ) to have a parent application window. This same thing happens if you build GUI dialog from scratch in c or python.

Thus, the only "real" way is for Gtk developers to actually allow dialog windows to stand on their own, or zenity developers to silence those warning from within their source code. Otherwise, your only option is to constantly add 2>/dev/null to any zenity command. Of course you can always make an alias or wrapper function that will do it for you, something along the lines of this:

zenity(){
    /usr/bin/zenity "$@" 2>/dev/null
}

This, however, may have issues with quoting and passing arguments, but for the most part should work. If you work in Python or another language that can actually handle Gtk, you can brew your own popup dialogs with hidden parent windows, which comes at the cost of complexity and learning how to create those things. But again, the real way would be for zenity developers actually fix this or Gtk developers to acknowledge the need for standalone dialog windows.

I prefer to edit ~/.bashrc with:

alias zenity="zenity 2> >(grep -v 'GtkDialog' >&2)"

This is more restrictive, inhibits only the warning messages that have the GtkDialog string, leaving the other error or warning messages to be displayed