Bash: Check json response and write to a file if string exists - json

Question

I curl an endpoint for a json response and write the response to a file.
So far I've got a script that:
1). does the curl if the file does not exist and
2). else sets a variable
#!/bin/bash
instance="server1"
curl=$(curl -sk https://my-app-api.com | python -m json.tool)
json_response_file="/tmp/file"
if [ ! -f ${json_response_file} ] ; then
${curl} > ${json_response_file}
instance_info=$(cat ${json_response_file})
else
instance_info=$(cat ${json_response_file})
fi
The problem is, the file may exist with a bad response or is empty.
Possibly using bash until, I'd like to
(1). check (using JQ) that a field in the curl response contains $instance and only then write the file.
(2). retry the curl XX number of times until the response contains $instance
(3). write the file once the response contains $instance
(4). set the variable instance_info=$(cat ${json_response_file}) when the above is done correctly.
I started like this... then got stuck...
until [[ $(/usr/bin/jq --raw-output '.server' <<< ${curl}) = $instance ]]
do

Answer 1

One sane implementation might look something like this:
retries=10
instance=server1
response_file=filename
# define a function, since you want to run this code multiple times
# the old version only ran curl once and reused that result
fetch() { curl -sk https://my-app-api.com; }
instance_info=
for (( retries_left=retries; retries_left > 0; retries_left-- )); do
content=$(fetch)
server=$(jq --raw-output '.server' <<<"$content")
if [[ $server = "$instance" ]]; then
# Writing isn't atomic, but renaming is; doing it this way makes sure that no
# incomplete response will ever exist in response_file. If working in a directory
# like /tmp where others users may have write, use $(mktemp) to create a tempfile with
# a random name to avoid security risk.
printf '%s\n' "$content" >"$response_file.tmp" \
&& mv "$response_file.tmp" "$response_file"
instance_info=$content
break
fi
done
[[ $instance_info ]] || { echo "ERROR: Giving up after $retries retries" >&2; }