/etc/sgml/docbook.cat is in docbook 4.5-5.1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | CATALOG /usr/share/sgml/docbook/dtd/catalog
CATALOG /usr/share/sgml/docbook/dtd/2.4.1/catalog
CATALOG /usr/share/sgml/docbook/dtd/3.0/catalog
CATALOG /usr/share/sgml/docbook/dtd/3.1/catalog
CATALOG /usr/share/sgml/docbook/dtd/4.0/catalog
CATALOG /usr/share/sgml/docbook/dtd/4.1/catalog
CATALOG /usr/share/sgml/docbook/dtd/4.2/catalog
CATALOG /usr/share/sgml/docbook/dtd/4.3/catalog
CATALOG /usr/share/sgml/docbook/dtd/4.4/catalog
CATALOG /usr/share/sgml/docbook/dtd/4.5/catalog
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