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: 
For files not directories, here's a more simple way instead of `-exec shred -u {} \;` type of way:

```
cd to your directory.

```

then

```
find . -type f -print0 | xargs -0 shred -fuzv -n 48

```

this does 48 passes recursively to the current directory you `cd`'ed into.

Hope this helps some.
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).