Get all unique JSON key names with JQ

Of course.

$ jq -n '[inputs | keys[]] | unique | sort' input.json
[
  "a",
  "b",
  "c",
  "d"
]

Here's another option that may perform better as it doesn't require collecting the keys into an array.

$ jq -n 'reduce (inputs | keys[]) as $k ({}; .[$k] = null) | keys' input.json

Or perhaps, even better:

$ jq -n 'foreach (inputs | keys[]) as $k ({}; .[$k]+=1; if .[$k]==1 then $k else empty end)' input.json

And for larger files, you will want to stream them in anyway so use this variation:

$ jq --stream -n 'foreach inputs[0][-1] as $k ({}; .[$k]+=1; if .[$k]==1 then $k else empty end)' input.json

Two points:

The original solution invoking jq and then sort is efficient, especially with respect to memory usage. (The solution involving the -s option effectively forces the entire file to be read into memory).
jq's unique implies sort. That is, unique|sort should be simplified to unique to avoid sorting twice.

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