eval printf works from command line but not in script - shell

Question

When I run the following command in a terminal it works, but not from a script:
eval $(printf "ssh foo -f -N "; \
for port in $(cat ~/bar.json | grep '_port' | grep -o '[0-9]\+'); do \
printf "-L $port:127.0.0.1:$port ";\
done)
The error I get tells me that printf usage is wrong, as if the -L argument within quotes would've been an argument to printf itself.
I was wondering why that is the case. Am I missing something obvious?
__
Context (in case my issue is an XY problem): I want to start and connect to a jupyter kernel running on a remote computer. To do so I wrote a small script that
sends a command per ssh for the remote to start the kernel
copies via scp a configuration file that I can use to connect to the kernel from my local computer
reads the configuration file and opens appropriate ssh tunnels between local and remote
For those not familiar with jupyter, a configuration file (bar.json) looks more or less like the following:
{
"shell_port": 35932,
"iopub_port": 37145,
"stdin_port": 42704,
"control_port": 39329,
"hb_port": 39253,
"ip": "127.0.0.1",
"key": "4cd3e12f-321bcb113c204eca3a0723d9",
"transport": "tcp",
"signature_scheme": "hmac-sha256",
"kernel_name": ""
}
And so, in my command above, the printf statement creates an ssh command with all the 5 -L port forwarding for my local computer to connect to the remote, and eval should run that command. Here's the full script:
#!/usr/bin/env bash
# Tell remote to start a jupyter kernel.
ssh foo -t 'python -m ipykernel_launcher -f ~/bar.json' &
# Wait a bit for the remote kernel to launch and write conf. file
sleep 5
# Copy the conf. file from remote to local.
scp foo:~/bar.json ~/bar.json
# Parse the conf. file and open ssh tunnels.
eval $(printf "ssh foo -f -N "; \
for port in $(cat ~/bar.json | grep '_port' | grep -o '[0-9]\+'); do \
printf "-L $port:127.0.0.1:$port ";\
done)
Finally, jupyter console --existing ~/foo.json connects to remote.

Answer 1

As #that other guy says, bash's printf builtin barfs on printf "-L ...". It thinks you're passing it a -L option. You can fix it by adding --:
printf -- "-L $port:127.0.0.1:$port "
Let's make that:
printf -- '-L %s:127.0.0.1:%s ' "$port" "$port"
But since we're here, we can do a lot better. First, let's not process JSON with basic shell tools. We don't want to rely on it being formatting a certain way. We can use jq, a lightweight and flexible command-line JSON processor.
$ jq -r 'to_entries | map(select(.key | test(".*_port"))) | .[].value' bar.json
35932
37145
42704
39329
39253
Here we use to_entries to convert each field to a key-value pair. Then we select entries where the .key matches the regex .*_port. Finally we extract the corresponding .values.
We can get rid of eval by constructing the ssh command in an array. It's always good to avoid eval when possible.
#!/bin/bash
readarray -t ports < <(jq -r 'to_entries | map(select(.key | test(".*_port"))) | .[].value' bar.json)
ssh=(ssh foo -f -N)
for port in "${ports[#]}"; do ssh+=(-L "$port:127.0.0.1:$port"); done
"${ssh[#]}"