jq 1.8 Manual

A jq program is a "filter": it takes an input, and produces an output. There are a lot of builtin filters for extracting a particular field of an object, or converting a number to a string, or various other standard tasks.

Filters can be combined in various ways - you can pipe the output of one filter into another filter, or collect the output of a filter into an array.

Some filters produce multiple results, for instance there's one that produces all the elements of its input array. Piping that filter into a second runs the second filter for each element of the array. Generally, things that would be done with loops and iteration in other languages are just done by gluing filters together in jq.

It's important to remember that every filter has an input and an output. Even literals like "hello" or 42 are filters - they take an input but always produce the same literal as output. Operations that combine two filters, like addition, generally feed the same input to both and combine the results. So, you can implement an averaging filter as add / length - feeding the input array both to the add filter and the length filter and then performing the division.

But that's getting ahead of ourselves. :) Let's start with something simpler: