/usr/bin/lightdm-gtk-greeter-settings-pkexec: 7: /usr/bin/lightdm-gtk-greeter-settings-pkexec: [[: not found

# Context

Running xfce4-settings-manager as user from a terminal to set gtk-greeter parameters.

## Expected

No output in terminal.

## Observed

This output in terminal:

/usr/bin/lightdm-gtk-greeter-settings-pkexec: 7: /usr/bin/lightdm-gtk-greeter-settings-pkexec: [[: not found
/usr/bin/lightdm-gtk-greeter-settings-pkexec: 7: /usr/bin/lightdm-gtk-greeter-settings-pkexec: [[: not found

/usr/bin/lightdm-gtk-greeter-settings-pkexec
uses bash-specific syntax but calls /bin/sh which enables sh-only mode.

In practice it means that special handling of "-s=" and "--socket-id=" arguments is not performed.

I don't know if this may be the sign of more bugs.

The script itself is very short, here it is:

#!/bin/sh

APP_PATH="/usr/bin/lightdm-gtk-greeter-settings"

# Check for "socket" argument
for i in "$@"; do
  if [[ "$i" == "-s="* ]] || [[ "$i" == "--socket-id="* ]]; then
    pkexec "${APP_PATH}" "$@" || "${APP_PATH}" "$@"
    exit $?
  fi
done

pkexec "${APP_PATH}" "$@"

Thank you for your attention.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: lightdm-gtk-greeter-settings 1.2.1-0ubuntu1
ProcVersionSignature: Ubuntu 4.4.0-34.53-generic 4.4.15
Uname: Linux 4.4.0-34-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: XFCE
Date: Wed Aug 17 08:15:22 2016
InstallationDate: Installed on 2016-07-05 (42 days ago)
InstallationMedia: Xubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
PackageArchitecture: all
SourcePackage: lightdm-gtk-greeter-settings
UpgradeStatus: No upgrade log present (probably fresh install)