bash – Crontab: script running Ubuntu (noob)

I am new to crontab and would like to run the following script from / etc / crontab:

0 15 * * * root bash-c for i in / home / dell / Downloads / *. {pdf, docx, png, jpg, PDF, DOCX}; shred -zvu "$ i" -n20; finished & # 39;

I have tried with and without bash-c option, but the script does not run.

My goal is:

  1. Run this script from Crontab
  2. Run this script at startup

Help is appreciated.