Interpolating Strings Using jq

Question

I'm using jq to parse JSON data and pull out a value based on a dynamic key value given as a command line argument. I noticed that this works, but I'm skeptical about how it's interpolating $v within the single quotes. Is it possible to rewrite the expression '.["\($v)"]' within double quotes? What am I doing wrong?

Trying the following doesn't work for whatever reason:

recipe_url=$(cat ./*.json | jq -r --arg v "$recipe_key" ".[\"\\($v)\"]")

However, this does:

recipe_key='Wilted Greens' # example key to interpolate

recipe_url=$(cat ./*.json | jq -r --arg v "$recipe_key" '.["\($v)"]') # works!

Answer 1

You have not escaped the $v, so Bash thinks it is a Bash variable, probably an empty one. Try this

".[\"\\(\$v)\"]"
#       ^
#       |
#       --- notice

In my opinion, it is better in this situation to use use --arg with single quotes. --arg was implemented similar to awk -v, to fix exactly this type of situation with quoting headaches.

Answer 2

I noticed that this works, but I'm skeptical about how it's interpolating $v within the single quotes.

Single quotes don't interpolate variables or evaluate anything else.

Nothing is magical within single quotes. You can't even escape single quotes within single quotes.

The quoting/escaping problem you are having exists only because you are attempting to use double quotes instead of single quotes here.