Bash script leaving nothing but numbers, asterisks and hashes in the file?

So, I'm working on a project where I need to read the output from a serial port and save it as a JSON file. This has already been done:

#!/bin/bash
while :
do
    echo '{"content": "' > /home/pi/bticino/assets/bash/lastCommands.json
    cat /home/pi/bticino/assets/bash/serialContent.json >> ./lastCommands.json
    echo '"}' >> /home/pi/bticino/assets/bash/lastCommands.json
    cat /home/pi/bticino/assets/bash/lastCommands.json |tr -d "\n" > /home/pi/bticino/assets/bash/serialF.json
    sleep 5
done

      

        
        
        
      

    

The problem occurs when sometimes (accidentally), strange characters start to appear in my output files (serialContent.json and serialF.json). And the JSON parser fails when it does.

In this file, I only need to store 0-9 digits, * and #. Is there a way to achieve this with a regular expression?

This is the content I need to save to the file:

* # 1 ### 1 ### 1 ### 1 ### 1 ### 1 ### 1 ### * 13 * 198564874 * 1 * ## * # 1 ### 1 # ## 1 ### 1 ### 1 ### * 1 ##

Thanks for your answer @William Pursell, I had to change my script a bit so it doesn't truncate my json syntax (which I forgot to mention), ending like this:

#!/bin/bash

dir=/home/pi/bticino/assets/bash
while :
do
    echo '{"content": "' > $dir/lastCommands.json
    < $dir/serialContent.json tr -dc '[:digit:]*#' >> $dir/lastCommands.json
    echo '"}' >> $dir/lastCommands.json
    cat $dir/lastCommands.json |tr -d "\n" > $dir/serialF.json
    sleep 5
done