On NixOS, there is no central catalog file, we rely entirely on XML_CATALOG_FILES variable which is populated by dependencies inside the package scope. For example, if a package depends on XHTML 1.0 and SVG 1.1, the variable will be set as follows:

$ echo $XML_CATALOG_FILES
/nix/store/gpmc5rrkcb5w4624kijqinva2iaqpd3y-xhtml1-20020801/xml/dtd/xhtml1/catalog.xml /nix/store/hcgy8rw0wa20pwrmybqn7lvmzyz1li4s-svg-1.1/share/xml/svg-1.1/catalog.xml

Now if we want to check if a schema is available, we need to run the command for every item listed in the variable – we cannot just omit the catalog path:

$ xmlcatalog "-//W3C//DTD XHTML 1.0 Strict//EN"
Unknown option -//W3C//DTD XHTML 1.0 Strict//EN
$ xmlcatalog -- "-//W3C//DTD XHTML 1.0 Strict//EN"
Unknown option --
$ xmlcatalog "" "-//W3C//DTD XHTML 1.0 Strict//EN"
No entry for PUBLIC -//W3C//DTD XHTML 1.0 Strict//EN

It works correctly in the shell, but that is hardly usable from scripts:

$ xmlcatalog --shell
> public "-//W3C//DTD XHTML 1.0 Strict//EN"
/nix/store/gpmc5rrkcb5w4624kijqinva2iaqpd3y-xhtml1-20020801/xml/dtd/xhtml1/xhtml1-strict.dtd


Optimally, the command would allow to omit the CATALOGFILE argument but since the entities parameter is variadic, we need to introduce a placeholder or a new option. I propose a --lookup option, that would skip the CATALOGFILE requirement.