Bash/*NIX: split a file into multiple files on a substring

Question

Variants of this question have been asked and answered before, but I find that my sed/grep/awk skills are far too rudimentary to work from those to a custom solution since I hardly ever work in shell scripts.

I have a rather large (100K+ lines) text file in which each line defines a GeoJSON object, each such object including a property called "county" (there are, all told, 100 different counties). Here's a snippet:

{"type": "Feature", "properties": {"county":"ALAMANCE", "vBLA": 0, "vWHI": 4, "vDEM": 0, "vREP": 2, "vUNA": 2, "vTOT": 4}, "geometry": {"type":"Polygon","coordinates":[[[-79.537429,35.843303],[-79.542428,35.843303],[-79.542428,35.848302],[-79.537429,35.848302],[-79.537429,35.843303]]]}},
{"type": "Feature", "properties": {"county":"NEW HANOVER", "vBLA": 0, "vWHI": 0, "vDEM": 0, "vREP": 0, "vUNA": 0, "vTOT": 0}, "geometry": {"type":"Polygon","coordinates":[[[-79.532429,35.843303],[-79.537428,35.843303],[-79.537428,35.848302],[-79.532429,35.848302],[-79.532429,35.843303]]]}},
{"type": "Feature", "properties": {"county":"ALAMANCE", "vBLA": 0, "vWHI": 0, "vDEM": 0, "vREP": 0, "vUNA": 0, "vTOT": 0}, "geometry": {"type":"Polygon","coordinates":[[[-79.527429,35.843303],[-79.532428,35.843303],[-79.532428,35.848302],[-79.527429,35.848302],[-79.527429,35.843303]]]}},

I need to split this into 100 separate files, each containing one county's GeoJSONs, and each named xxxx_bins_2016.json (where xxxx is the county's name). I'd also like the final character (comma) at the end of each such file to go away.

I'm doing this in Mac OSX, if that matters. I hope to learn a lot by studying any solutions you could suggest, so if you feel like taking the time to explain the 'why' as well as the 'what' that would be fantastic. Thanks!

EDITED to make clear that there are different county names, some of them two-word names.

Answer 1

jq can kind of do this; it can group the input and output one line of text per group. The shell then takes care of writing each line to an appropriately named file. jq itself doesn't really have the ability to open files for writing that would allow you to do this in a single process.

jq -Rn -c '[inputs[:-1]|fromjson] | group_by(.properties.county)[]' tmp.json |
  while IFS= read -r line; do
    county=$(jq -r '.[0].properties.county' <<< $line)
    jq -r '.[]' <<< "$line" > "$county.txt"
done

[inputs[:-1]|fromjson] reads each line of your file as a string, strips the trailing comma, then parses the line as JSON and wraps the lines into a single array. The resulting array is sorted and grouped by county name, then written to standard output, one group per line.

The shell loop reads each line, extracts the county name from the first element of the group with a call to jq, then uses jq again to write each element of the group to the appropriate file, again one element per line.

(A quick look at https://github.com/stedolan/jq/issues doesn't appear to show any requests yet for an output function that would let you open and write to a file from inside a jq filter. I'm thinking of something like

jq -Rn '... | group_by(.properties.county) | output("\(.properties.county).txt")' tmp.json

without the need for the shell loop.)