Ampersand to run process in background causes invalid parameters in Bash 5 - bash

Question

I have a bash script that runs perfectly well on Bash 3.2. The script contains an ampersand to run a process in the background. However, when I run it in Bash 5.x, it doesn't pass the variables correctly (I get a "SyntaxError: Unexpected end of JSON input"). When I take off the ampersand at the end (of the mgeneratejs line), it executes normally in Bash 5.
#!/bin/bash
#Works on Bash 3.2 on MacOS
#Doesn't work in bash-5.0/5.1
##!/usr/bin/env bash
NUM_ROWS_PER_RUN=5
NUM_RUNS=2
TEMPLATE_STRING='{
name: "$name"
}'
for i in $(seq 1 "$NUM_RUNS")
do
echo "Starting run ${i}"
#If you dont have it, then run "npm install -g mgeneratejs"
mgeneratejs -n "$NUM_ROWS_PER_RUN" "${TEMPLATE_STRING//[$'\r\n ']}" &
done
echo "Waiting"
wait
echo "Finished"
How can I get the process (mgeneratejs) to run in the background when using Bash 5.x?

Answer 1

Bash may or may not be in fault here, but be sure that the problema is in mgeneratejs.
Taking a look mgeneratejs's source code I found this:
if (process.stdin.isTTY) {
var str = argv._[0];
template = _.startsWith(str, '{') ? parseTemplate(str) : parseTemplate(read(str, 'utf8'));
generate();
} else {
template = '';
process.stdin.setEncoding('utf-8');
process.stdin.on('readable', function() {
var chunk = process.stdin.read();
if (chunk !== null) {
template += chunk;
}
});
process.stdin.on('end', function() {
template = JSON.parse(template);
generate();
});
}
If stdin is not a TTY then mgeneratejs assumes that stdin is a pipe, and tries to read from it. This is wrong, they should at least check if the template has been given in the command line args.
I would't recommend that you fix mgeneratejs, but I can recommend you to do this:
function do_run() {
echo "${TEMPLATE_STRING//[$'\r\n ']}" | mgeneratejs -n "$NUM_ROWS_PER_RUN"
}
for i in $(seq 1 "$NUM_RUNS")
do
echo "Starting run ${i}"
#If you dont have it, then run "npm install -g mgeneratejs"
do_run &
done