jq - Update value in json with variable

I use jq for reading and filtering json on the command line.

Given the json below, I’d like to update the value of b to be 30. jq offers the update assignment, |=, for this.

{
  "a": {
    "b": 10
   },
  "c": 20
}

Try out the snippet at https://jqplay.org/s/t39OXPDrll-

Doing the same with that data saved to file, say myData.json and PowerShell and passing the new value as a jq variable would look like this:

{
  "a": {
    "b": 10
   },
  "c": 20
}

In the command line, run the following:

# retrieve b's value
C:\opt\jq-win64.exe '.a.b' myData.json

# set 30 to a powershell variable
$newValue = 30

# assign newValue to jq variable 'jqVarToUse' and pass it to the assigment operator
C:\opt\jq-win64.exe --arg jqVarToUse $newValue '.a.b |= $jqVarToUse' myData.json

jq gives the following output, and does not modify the original file.

{
  "a": {
    "b": "30"
   },
  "c": 20
}

To let jq make the change and directly modify the file, enclose the update in cat. In bash, it looks like this:

new_value=40
cat <<< "$(jq --arg jqVarToUse $newnew_valueValue '.a.b |= $jqVarToUse' myData.json)" > myData.json

What to watch out for: if there’s an error anywhere, the modified file (myData.json) will be empty.

Beware!

if there’s an error anywhere, the modified file (myData.json) will be empty.