This is useful when one has a database (in JSON form) to query using jq
input data.
% echo '{"a":1, "c":5}' > db.json
% echo '"c"'|./jq --argfile f /tmp/a '$f[.]'
5
% echo '"a"'|./jq --argfile f /tmp/a '$f[.]'
1
% echo '"b"'|./jq --argfile f /tmp/a '$f[.]'
null
%