Question: I want a command to shred completely the contents of a folder/directory (which may be inside folders/directories).
Also please explain the command.

Answer: 
`sudo apt install wipe`

`$ wipe -rfi dir/*`

where the flags used:
`-r – tells wipe to recurse into subdirectories
-f – enables forced deletion and disable confirmation query
-i – shows progress of deletion process`
If you want to do this from Nautilus (aka 'Files' app), then you could use the [nautilus-wipe](https://launchpad.net/ubuntu/+source/nautilus-wipe) package.

* `sudo apt-get install nautilus-wipe`

Once installed, there will be two new options when you right-click on a folder: `Wipe` and `Wipe available disk space`. Choosing `Wipe` on the folder will give further options (e.g. number of passes, fast mode, last pass with zeros).