Why does bash try to execute strings in a string substitution?

Question

My script is supposed to take input from a pipe and replace newline characters with commas, using the bash's string substitution:

#! /bin/bash

read -d -r input 
echo $input 
$input=${input//\n/,}
echo $input

However, instead of replacing the newline character, bash tries to execute the first matching pattern:

echo -e "this\nis\na\ntest\n" | test.sh 

will give the following output:

test.sh: line 5: this: command not found 

and the variable $input is not changed. Double or single quotes did not help either. I'm using bash version 4.3.11 on Linux Mint.

Answer 1

Remove the $ from the assignment line inside the loop. You're trying to use the value as the name of the variable you're assigning to, which is causing Very Strange Behavior.