/usr/lib/firebird/2.1/bin/isql is in firebird2.1-server-common 2.1.4.18393-0.ds2-6build1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/firebird/2.1/bin/../../../../bin/isql-fb
/usr/lib/firebird/2.1/bin/../../../../bin/isql-fb