What does this if-then statement mean?

You are right, the command

test "x/usr/share" = "x/usr/local/share" -o "x/usr/share" = "x/usr/share"

returns true (0) always.

It looks like if the file in question was generated from a more generic version but the way of generation was not optimal. The script should either ask for actual paths each time or keep just the relevant branch in the generated file.

However, this particular file comes from a package – not being generated on your machine. This is likely something to be fixed/improved… You can file a bug on Xfce Bugzilla (if it’s not present there already) or fix it yourself. You can clone the Git repository for Xfce4 session, you can also get in touch with Xfce4 developers using their mailing list. Good luck with improving the code!

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.