jq create object with property name from variable

Question

$ n="foo"; echo "{}" | jq --arg n "$n" '. += { "$n": $n }'
{
  "$n": "foo"
}

My reading of https://jqlang.github.io/jq/manual/#types-and-values suggests that this should work -- but clearly it doesn't. What's going on?

Answer 1

You can accomplish this by wrapping $n in parenthesis to tell jq to evaluate the expression:

n="foo"; echo "{}" | jq --arg n "$n" '. += { ($n): $n }'

Or probably better suited for this task would be jo(1):

$ jo "$n"="$n"
{"foo":"foo"}

I'm not sure if the documentation is just wrong or if my ability to comprehend is not at a high enough level but it does seem to say that your example should work:

Key expressions other than constant literals, identifiers, or variable references, need to be parenthesized, e.g., {("a"+"b"):59}.

And one might assume that is referring to a jq native variable rather than one injected by the shell as they are slightly different, but alas both need to be parenthesized:

$ echo '{}' | jq 'def myvar: "foo"; {myvar: myvar}'
{
  "myvar": "foo"
}
$ echo '{}' | jq 'def myvar: "foo"; {(myvar): myvar}'
{
  "foo": "foo"
}

Answer 2

You can use:

$ n=foo
$ jq -cn --arg n "$n" '{$n:$n}'
{"foo":"foo"}

That is use the variable $n as the key, not the "$n" string. jq -n is like echo null | jq.

$ jq -cn --arg n "$n" '.[$n]=$n'
{"foo":"foo"}

Not using the {...} object constructor but assigning a value for a given key.

$ jq -cn --arg n "$n" '{"key\($n)":$n}'
{"keyfoo":"foo"}

Showing how to dereference a jq variable inside a string literal (in \(expr), expr can be any jq expression, not just a variable).

$ jq -cn --arg n "$n" '{("key"+$n):$n}'
{"keyfoo":"foo"}

In an object constructor, when keys are to be the result of a jq expression, they must be inside (...).