tail must to read the whole file. Calling sort with a -r (aka --reverse option) you can use head instead.

touch takes multiple arguments. You may

  touch $DIRNAME/first.txt $DIRNAME/second.txt $DIRNAME/third.json

and further use brace expansion:

  touch $DIRNAME/{first.txt,second.txt,third.json}

I would consider here-documenting the json files instead of echoing them, along the lines of:

  cat < EOF >> $DIRNAME/test.json
  {
       "enabled": true
  }
  EOF

tail must to read the whole file. Calling sort with a -r (aka --reverse option) you can use head instead.

touch takes multiple arguments. You may

  touch $DIRNAME/first.txt $DIRNAME/second.txt $DIRNAME/third.json

and further use brace expansion:

  touch $DIRNAME/{first.txt,second.txt,third.json}

I would consider here-documenting the json files instead of echoing them, along the lines of:

  cat << EOF >> $DIRNAME/test.json
  {
       "enabled": true
  }
  EOF