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: 
You probably want to use something similar to this:

```
find dir -type f -exec shred -fuz {} +
rm -rf dir

```

First command finds only files and passes them to shred (as many at once as possible - no need to start a new shred process for every file like \; does). Finally, remove the directories too.
When I need to shred multiple files or an entire directory I simply use `shred -vzn 20 ./shredme/*.*` for example, which overwrites all files with any file extension in the "shredme" folder. Then you can use the standard rm -rf ./shredme command to remove the folder itself (or just right click and delete the folder) as all the data has been overwritten 20 times for this example.

I did a quick example of this with a bunch of duplicate images as an example. 

![Terminal screenshot](https://i.stack.imgur.com/s9ULy.png)