Why does curl see several parameters? [duplicate] - bash

Question

This question already has answers here:
When to wrap quotes around a shell variable?
(5 answers)
Closed last year.
I want to send a message to a slack app in a script:
...
declare -a models=("TR10"
"TR100"
"TR1000")
for i in "${models[#]}"
do
#Send Message
if [ $use_slack_app ]
then
message="Starting model $i ($c of $lenght)!"
data=''\''{"text":"'"$message"'"}'''\'
echo $data
curl -X POST -H 'Content-type: application/json' --data $data $slack_app_url
echo "curl -X POST -H 'Content-type: application/json' --data $data $slack_app_url"
fi
#Counter
((c=c+1))
done
...
return of echo $data:
'{"text":"Starting model TR10 (1 of 3)!"}'
curl message:
curl: (3) unmatched close brace/bracket in URL position 5:
3)!"}'
^
curl: (6) Could not resolve host: model
curl: (6) Could not resolve host: TR10
curl: (6) Could not resolve host: (1
curl: (6) Could not resolve host: of
invalid_payload
It looks like it sees the string as several parameters, but I don't understand why. If I echo the line out, and run it in the terminal it works. I don't have much experience with bash scripting, so I don't understand what the problem here is.

Answer 1

Quote the variable expansions to ensure values with spaces are sent as one word: "$var" vs. $var.
Get rid of the single quotes (''\') surrounding $data.
data='{"text":"'"$message"'"}'
curl -X POST -H 'Content-Type: application/json' --data "$data" "$slack_app_url"