Bash script to change the Gnome 3 GDM background

gdmbackground.sh

#!/bin/sh -e

#------------------------------------------------------------------------------------
# Change the GDM background.
# based on https://wiki.archlinux.org/index.php/GDM
# by https://twitter.com/jeffprod
#
# If you have dual screens, you should copy your current settings to gdm
# so that the image is not reversed right-left :
# sudo cp ~/.config/monitors.xml ~gdm/.config
#
# If you have dual screen, the image should be for example : 3840x1080
#
# USAGE
#  chmod +x gdmbackground.sh
#  ./gdmbackground.sh <path-to-image-filename>
#
# EXEMPLE
#  ./gdmbackground.sh ~/wallpaper.jpg
#------------------------------------------------------------------------------------

if [ -z "$1" ]; then
    echo "USAGE:"
	echo "./gdmbackground.sh <path-to-image-filename>"
	exit 0
fi

extractDir="$(pwd)/extract"
echo "Extracting /usr/share/gnome-shell/gnome-shell-theme.gresource to ${extractDir}"
gst=/usr/share/gnome-shell/gnome-shell-theme.gresource
for r in `gresource list $gst`; do
	r=${r#\/org\/gnome\/shell/}
	if [ ! -d $extractDir/${r%/*} ]; then
	  mkdir -p $extractDir/${r%/*}
	fi
done
for r in `gresource list $gst`; do
    gresource extract $gst $r >$extractDir/${r#\/org\/gnome\/shell/}
done

echo "Copy $1 ${extractDir}/theme"
cp "$1" "${extractDir}/theme"

filename=$(basename $1)
cat > "${extractDir}/theme/gnome-shell-theme.gresource.xml" << EOL
<?xml version="1.0" encoding="UTF-8"?>
<gresources>
  <gresource prefix="/org/gnome/shell/theme">
    <file>calendar-today.svg</file>
    <file>checkbox-focused.svg</file>
    <file>checkbox-off-focused.svg</file>
    <file>checkbox-off.svg</file>
    <file>checkbox.svg</file>
    <file>dash-placeholder.svg</file>
    <file>gnome-shell.css</file>
    <file>gnome-shell-high-contrast.css</file>
    <file>icons/message-indicator-symbolic.svg</file>
    <file>icons/pointer-double-click-symbolic.svg</file>
    <file>icons/pointer-drag-symbolic.svg</file>
    <file>icons/pointer-primary-click-symbolic.svg</file>
    <file>icons/pointer-secondary-click-symbolic.svg</file>
    <file>key-enter.svg</file>
    <file>key-hide.svg</file>
    <file>key-layout.svg</file>
    <file>key-shift-latched-uppercase.svg</file>
    <file>key-shift.svg</file>
    <file>key-shift-uppercase.svg</file>
    <file>no-events.svg</file>
    <file>noise-texture.png</file>
    <file>${filename}</file>
    <file>no-notifications.svg</file>
    <file>pad-osd.css</file>
    <file>process-working.svg</file>
    <file>toggle-off-dark.svg</file>
    <file>toggle-off-hc.svg</file>
    <file>toggle-off.svg</file>
    <file>toggle-on-dark.svg</file>
    <file>toggle-on-hc.svg</file>
    <file>toggle-on.svg</file>
  </gresource>
</gresources>
EOL

echo "Modifying ${extractDir}/theme/gnome-shell.css"
sed -i -e "/#lockDialogGroup *{/,/}/c#lockDialogGroup { background: url($filename); background-repeat: no-repeat; }" "${extractDir}/theme/gnome-shell.css"

echo "Compiling"
cd "${extractDir}/theme"
glib-compile-resources gnome-shell-theme.gresource.xml

echo "Copy gnome-shell-theme.gresource to /usr/share/gnome-shell"
sudo cp gnome-shell-theme.gresource /usr/share/gnome-shell

echo "Clean up"
/bin/rm -rf ${extractDir}

echo "You may now reboot, or restart GDM (this will log you out!) :"
echo "sudo systemctl restart gdm.service"

Thanks for you feedback.
I don't force to restart gdm, the command is printed with echo.
I use /bin/rm because i made an alias on my system : alias rm=trash in the bashrc so that files go to the trash instead of beeing removed definitly.
You are right, currentDir was useless, i removed it.

Oups yeah for some reason my brain skipped the "echo" thanks.
Ah interesting concept for rm.