How to trim white space for every element in JQ?

If the input is an array of JSON objects and if you're looking for a one-liner, then the following seems to meet your requirements:

jq '.[]|(.k1,.k2)|if type=="string" then gsub("^\\s+|\\s+$";"") else . end' | paste -d "," - -

With your input, this produces

"http://url",null

If you are flexible about how null is presented in the output, you might also consider an all-jq solution (i.e. without paste). For example:

jq -r '.[]|[.k1,.k2]|map(if type=="string" then gsub("^\\s+|\\s+$";"") else . end)|@csv'

With your input, this produces:

"http://url",

The below will remove leading and trailing spaces in strings anywhere inside an arbitrarily complex JSON structure:

#!/usr/bin/env jq -cf# Define walk function (standard library only for jq 1.6 and newer, currently unreleased)def walk(f):  . as $in  | if type == "object" then      reduce keys_unsorted[] as $key        ( {}; . + { ($key):  ($in[$key] | walk(f)) } ) | f  elif type == "array" then map( walk(f) ) | f  else f  end;walk(  if type == "string" then    (sub("^[[:space:]]+"; "") | sub("[[:space:]]+$"; ""))  else . end)

If one saves the above (e.g. in trim-json), and marks it executable (chmod +x trim-json), then ./trim-json <<<"$json" with your given input emits:

[{"k1":"v1","k2":"v2","k3":"v3","k4":"v4"}]

Similarly, with the updated input:

$ json='[{"k1":" http://url", "k2":null, "k3":" v3", "k4":" v4"}]'$ ./trim-json <<<"$json"[{"k1":"http://url","k2":null,"k3":"v3","k4":"v4"}]

The example has a space in front of every value. For the case where you can simply remove the first (or N) characters of a string, then a simple map over range slicing works:

$ jq -c '.[] | map_values(.[1:]) | .k1,.k2' <<< "$json" | paste -d "," - -"http://url",null

This approach has the benefit of being unpretentious and simple to understand.