Copy value with jq

I use jq heavily in my day to day work. It's a powerful tool but not always easy, so I have piles of notes about how to do things with it.

I had to do a few iterations of copying a value from one JSON file to another the other day, and the files are large, and copying and pasting and launching editors was getting old, so I reached for jq. And after digging for a while and reading Stack Overflow answers interpreting the manual page for me, here's how to do it, in fish:

jq --argjson js (jq .path.field a.json) '.path.field = $js' b.json | sponge b.json

The subshell invoked by () reads the value from a.json, --argjson sets the value the variable js without extra quotes or escapes, as it's already a valid JSON value, and then the assignment just sets the value to the document read from b.json.

It also requires sponge because the jq developers don't want it to have an -i option. Oh well.