nix quoting weirdness

aaa-README

so the problem was: the unquoted string was being pathname expanded and nullglob was set by $stdenv/setup

nix.sh

$ cat default.nix
{ pkgs ? import <nixpkgs> {} }: with pkgs;
let
  x = builtins.toXML [ { foo = 1; bar = "  baz  "; } ];
  y = runCommand "foo" { inherit x; } ''echo $x > $out'';
  z = runCommand "foo" { inherit x; } ''echo "$x" > $out'';
in { inherit y z; }

$ nix-build
/nix/store/zflgjbm8jhrfrqjz2bxi14d5i8r36f9w-foo
/nix/store/6077l132s2mfcihk3f7g86v70yyliqlq-foo

# where did the `<?xml` go?
$ head /nix/store/zflgjbm8jhrfrqjz2bxi14d5i8r36f9w-foo /nix/store/6077l132s2mfcihk3f7g86v70yyliqlq-foo
==> /nix/store/zflgjbm8jhrfrqjz2bxi14d5i8r36f9w-foo <==
version='1.0' <expr> <list> [...]

==> /nix/store/6077l132s2mfcihk3f7g86v70yyliqlq-foo <==
<?xml version='1.0' encoding='utf-8'?>
[...]

# trying to reproduce by hand (and failing)
$ nix-shell -A y 
nix-shell$ echo $x
<?xml version='1.0' encoding='utf-8'?> <expr> <list> [...]
nix-shell$ echo $shell
/nix/store/.../bin/bash

$ NIX_BUILD_SHELL=/nix/store/.../bin/bash nix-shell -A y --command "out=$PWD/_out; genericBuild"
$ cat _out
<?xml version='1.0' encoding='utf-8'?> <expr> <list> [...]

nix2.sh

$ head default.nix *.sh 
==> default.nix <==
{ pkgs ? import <nixpkgs> {} }: with pkgs;
let
  snippet = "<? <! <foo> !> ?>";
in {
  t1 = stdenv.mkDerivation { inherit snippet; name = "foo"; builder = ./x.sh; };
  t2 = stdenv.mkDerivation { inherit snippet; name = "foo"; builder = ./y.sh; };
}

==> x.sh <==
source $stdenv/setup
# snippet='<? <! <foo> !> ?> <?bar baz?>qux'
echo $snippet >&2
echo $snippet > $out

==> y.sh <==
# source $stdenv/setup
# snippet='<? <! <foo> !> ?> <?bar baz?>qux'
echo $snippet >&2
echo $snippet > $out

$ nix-build -j1
these derivations will be built:
  /nix/store/hq5v6n70ddv3avj5lskwz08j4gfa7zfw-foo.drv
  /nix/store/zc7rgy8jv39ap9vynnphrh0r7fm2nnkw-foo.drv
building '/nix/store/hq5v6n70ddv3avj5lskwz08j4gfa7zfw-foo.drv'...
<! <foo> !>
building '/nix/store/zc7rgy8jv39ap9vynnphrh0r7fm2nnkw-foo.drv'...
<? <! <foo> !> ?>
/nix/store/2998f6pbdm28j20ppvhx3cp8qmxni3xf-foo
/nix/store/44g4lnzqm31kaxwg1z5g0a3x7xxpgdqz-foo