Cannot use variable inside json –data

I have a working API (linode) to update a domain. So, below is a normal API call that work just fine to update data:

  #!/bin/bash

  # hardcoded data for testing
  LINODE_API_KEY=1234
  domain_id=1931316
  domain_name="abx.com"
  domain_type="master"
  domain_email="[email protected]"
  domain_ttl=30

  
  # Update the domain
  curl -H "Content-Type: application/json" 
  -H "Authorization: Bearer ${LINODE_API_KEY}" 
  -X PUT -d "{

  "domain": "${domain_name}", "type": "${domain_type}", "soa_email": "${domain_email}", "ttl_sec": ${domain_ttl}
  
  }" "https://api.linode.com/v4/domains/${domain_id}"

When I executed the above update API, it’s working fine and I get the json response as:

{"id": 1931316, "type": "master", "domain": "abx.com", "tags": [], "group": "", "status": "active", "errors": "", "description": "", "soa_email": "[email protected]", "retry_sec": 0, "master_ips": [], "axfr_ips": [], "expire_sec": 0, "refresh_sec": 0, "ttl_sec": 30, "created": "2022-12-13T09:01:01", "updated": "2022-12-14T03:26:27"}

But the problem is I want to use variable inside the data. So I change the working code above into this (with extra data that I want to pass):

  #!/bin/bash
  # hardcoded data for testing
  LINODE_API_KEY=1234
  domain_id=1931316
  domain_name="abx.com"
  domain_type="master"
  domain_email="[email protected]"
  domain_ttl=30
  
  # This value must be provided first
  if [ -z "${domain_name}" ]; then
     echo "You must provide domain name"
     exit 1
  else
     change_domain_name="\"domain\": \"${domain_name}\""
  fi      

  if [ -n "${domain_type}" ]; then
     change_domain_type=", \"type\": \"${domain_type}\""
  else
     change_domain_type=""
  fi

  if [ -n "${soa_email}" ]; then
     change_domain_email=", \"soa_email\": \"${domain_email}\""
  else
     change_domain_email=""
  fi

  if [ -n "${domain_ttl}" ]; then
     change_domain_ttl=", \"ttl_sec\": ${domain_ttl}"
  else
     change_domain_ttl=""
  fi
  update_string="${change_domain_name}${change_domain_type}${change_domain_email}${change_domain_ttl}"

  # Update the domain
  curl -H "Content-Type: application/json" 
  -H "Authorization: Bearer ${LINODE_API_KEY}" 
  -X PUT -d "{

  ${update_string} # THE PROBLEM IS HERE WHEN USING THIS VARIABLE
  
  }" "https://api.linode.com/v4/domains/${domain_id}"

The API will complain about Invalid JSON

{"errors": [{"reason": "Invalid JSON"}]}
  

But when I use echo ${update_string} variable, I will get the same data syntax like I use so why does it complain this is invalid?

I can even copy paste back the echo result above inside the -d data and it’s working fine.

I rolled back my question to the original as to explain why I use the above method instead of creating jq --arg as suggested @Arnaud Valmary.

So for example here I only want to pass value domain_name and domain_ttl. So the value of the update string would be like this:

update_string="${change_domain_name}${change_domain_ttl}"

where the others were empty. So, I’m not sure how to achieve this using jq --arg

In brief, if domain_type="" or domain_type is empty, I don’t want this variable to be in the –arg options, so user have a choice not to update this value.