Why can't jq's slurp handle a combination of here-strings and other files? - bash

Question

I want to merge some json in a file with some json generated at runtime. jq seems to have no difficulty if all the files passed to it are here-strings, or files in the system. But if I try to mix the file types it seems the here-strings are ignored, see snippet below:
Two normal files:
bash-4.2# echo '{"key":0}' > zero
bash-4.2# echo '{"key":1}' > one
bash-4.2# jq --slurp add zero one
{
"key": 1
}
Normal file and here-string (only normal file appears in result!):
bash-4.2# jq --slurp add zero <<< '{"key":1}'
{
"key": 0
}
Here-string first, then normal file (only normal file appears in result!):
bash-4.2# jq --slurp add <<< '{"key":0,"anotherkey":2}' one
{
"key": 1
}
Single here-string (works fine):
bash-4.2# jq --slurp add <<< '{"key":0}'
{
"key": 0
}
Two here-strings (works fine): EDIT: Output is misleading, something else is going on here.
bash-4.2# jq --slurp add <<< '{"key":0}' <<< '{"key":1}'
{
"key": 1
}
My suspicion is that jq works just fine and I am ignorant of how bash resolves the here-strings. But, how would I debug this to improve my understanding?
Note: A very easy workaround would be to evaluate my runtime json and produce a file, then merge the two files as above. I really want to know why the bold examples above don't produce what I would expect.

Answer 1

After reading through the comments this is my understanding:
<<< is evaluated by the shell first and redirects stdin. If jq receives no positional arguments after the filter, it reads from stdin. Therefore all these statements are equivalent:
echo "{}" | jq --slurp add
<<< {} jq --slurp add
jq <<< {} --slurp add
jq --slurp <<< {} add
jq --slurp add <<< {}
If jq does receive positional arguments after the filter, it interprets them as filenames. It adheres to the convention of treating - as stdin.
bash-4.2# echo '{"one":1,"two":1}' > first
bash-4.2# echo '{"three":3}' > third
bash-4.2# jq --slurp add first - third <<< '{"two":2}'
{
"one": 1,
"two": 2,
"three": 3
}

Answer 2

The here-string construct simply redirects standard input. You will separately need to tell jq to read standard input if you call it in a way where it receives file name arguments. The de facto standard way to do that is to specify - as the input (pseudo-) filename.
I believe one of your test cases didn't actually work, and just looked like it did because the input data was constructed so as to be a no-op.

Answer 3

One idea would be to use process substitution which, in essence, provides jq with a (temp) file descriptor it can work with.
Using awk to demonstrate the file descriptor idea:
$ awk '{print FILENAME}' <(echo 'abc')
/dev/fd/63
Demonstrating with a few of your examples:
$ jq --slurp add zero <(echo '{"key":1}')
{
"key": 1
}
$ jq --slurp add zero <(echo '{"keyx":1}')
{
"key": 0,
"keyx": 1
}
$ jq --slurp add <(echo '{"key":0,"anotherkey":2}') one
{
"key": 1,
"anotherkey": 2
}
$ jq --slurp add <(echo '{"key":0}') <(echo '{"key":1}')
{
"key": 1
}
$ jq --slurp add <(echo '{"key":0}') <(echo '{"keyx":1}')
{
"key": 0,
"keyx": 1
}