Answer by mook765 for What does this if-then statement mean?

The script /usr/bin/startxfce4 seems to be generated by a parser, we can see that when we look at the source code, let's take a look at the corresponding snippet:

if test "x$XDG_DATA_DIRS" = "x"
then
  if test "x@_datadir_@" = "x/usr/local/share" -o "x@_datadir_@" = "x/usr/share"; then
    XDG_DATA_DIRS="/usr/local/share:/usr/share"
  else
    XDG_DATA_DIRS="@_datadir_@:/usr/local/share:/usr/share"
  fi
else
  XDG_DATA_DIRS="$XDG_DATA_DIRS:@_datadir_@"
fi
export XDG_DATA_DIRS

Here we can see the meaning of this if-block, the developers give package-maitainers the opportunity to add a custom path to XDG_DATA_DIRS by parsing the desired path to the script and substituting the string @_datadir_@ with this path.

This will work perfectly, if a path is parsed which is not contained in XDG_DATA_DIRS at the time the script runs, but it will result in the same path appearing twice in XDG_DATA_DIRS if we parse a path which already exists in XDG_DATA_DIRS at the time the script runs.

This could be avoided by not parsing the standard XDG-folders (/usr/share, /usr/local/share) to the script, I don't know if that is possible though.

Another solution is to change the source code to

if test "x$XDG_DATA_DIRS" = "x"
then
  if test "x@_datadir_@" = "x/usr/local/share" -o "x@_datadir_@" = "x/usr/share"; then
    XDG_DATA_DIRS="/usr/local/share:/usr/share"
  else
    XDG_DATA_DIRS="@_datadir_@:/usr/local/share:/usr/share"
  fi
else
  if test "x@_datadir_@" != "x/usr/local/share" -a "x@_datadir_@" != "x/usr/share"; then
    XDG_DATA_DIRS="$XDG_DATA_DIRS:@_datadir_@"
  fi
fi
export XDG_DATA_DIRS

That would cover only the directories /usr/share and /usr/local/share though and one should indeed check if @_datadir_@ already exists in XDG_DATA_DIRS or not, but I don't know who to do that, my knowledge ends here.

In addition:

The same applies to this part of the script:

if test "x$XDG_CONFIG_DIRS" = "x"
then
  if test "x@_sysconfdir_@" = "x/etc"; then
    XDG_CONFIG_DIRS="/etc/xdg"
  else
    XDG_CONFIG_DIRS="/etc/xdg:@_sysconfdir_@/xdg"
  fi
else
  XDG_CONFIG_DIRS="$XDG_CONFIG_DIRS:@_sysconfdir_@/xdg"
fi
export XDG_CONFIG_DIRS

When we parse /etc/xdg/ to substitute @_sysconfdir_@ it leeds to a duplicated path (/etc/xdg:/etc/xdg in XDG_CONFIG_DIRS.

Greetings

I'm an end user, far from being a developer, so I'm unable to solve the problem completely. Thanks to dessert and melebius who pushed me in the right direction, at least I understand this if-then-statement now.

I believe that there is no harm in having a path twice in this environment variables, so I'll refrain from reporting a bug. Let the developers do more valuable things.

My personal solution

I changed the lines 67-89 in /usr/bin/startxfce4

if test "x$XDG_DATA_DIRS" = "x"
then
  if test "x/usr/share" = "x/usr/local/share" -o "x/usr/share" = "x/usr/share"; then
    XDG_DATA_DIRS="/usr/local/share:/usr/share"
  else
    XDG_DATA_DIRS="/usr/share:/usr/local/share:/usr/share"
  fi
else
  XDG_DATA_DIRS="$XDG_DATA_DIRS:/usr/share"
fi
export XDG_DATA_DIRS

if test "x$XDG_CONFIG_DIRS" = "x"
then
  if test "x/etc" = "x/etc"; then
    XDG_CONFIG_DIRS="/etc/xdg"
      else
   XDG_CONFIG_DIRS="/etc/xdg:/etc/xdg"
  fi
else
  XDG_CONFIG_DIRS="$XDG_CONFIG_DIRS:/etc/xdg"
fi
export XDG_CONFIG_DIRS

to

if test "x$XDG_DATA_DIRS" = "x"
then
  XDG_DATA_DIRS="/usr/local/share:/usr/share"
fi
export XDG_DATA_DIRS

if test "x$XDG_CONFIG_DIRS" = "x"
then
  XDG_CONFIG_DIRS="/etc/xdg"
fi
export XDG_CONFIG_DIRS

Of course, I backed up the original file first.