I can read from /dev/null; how to fix it?

/dev/null is a file. A special file. A device file like /dev/sda or /dev/tty that talks to a piece of hardware on your system.

The only difference with /dev/null is that no hardware is linked to it. Any data you send to it is silently discarded. Like the following command:

echo "Hello World" > /dev/null

which won't print anything on your terminal because you send the output of echo to null, to the void, a black hole thus.

But when you did mv test_file /dev/null you've replaced the special file /dev/null by a normal text file, holding a copy of the content of your test_file. In other words, you've lost your /dev/null.

Now, what you have to do is (to reconstruct it):

sudo rm /dev/null
sudo mknod -m 0666 /dev/null c 1 3

You should reconstruct it because a lot of scripts by default send output to /dev/null. If /dev/null is no more a black hole but a regular text file, it may grow, grow and fill your file-system up. And I'm sure you want to avoid this.

And much more dangerous, a lot of scripts assume that reading from /dev/null will read nothing; breaking this assumption can lead to random garbage written in files all around your system... practically impossible to fix up.

And remember that Linux is multi-tasking: while you are playing with /dev/null, a lot of processes are running and can wreak havoc even during a few seconds "window of opportunity".

If you want to play with /dev/null you can create a copy and experiment with it:

sudo mknod -m 0666 /tmp/null c 1 3 

Will create a /tmp/null file that works in the exactly same way of /dev/null but that you can manipulate and test without any risk for your system.

There is a big difference between overwriting a file and writing to a file.

When you write something to /dev/null, e.g.,

$ echo Hello > /dev/null

...it gets silently discarded. For this you need write permissions to /dev/null, which everyone has:

$ ls -l /dev/null 
crw-rw-rw- 1 root root 1, 3 Mar 18 13:17 /dev/null

When you overwrite /dev/null, as you did with the mv command, you replace the special file /dev/null with whatever you moved there. Don't do this! The reason you needed root privileges to do that is because to overwrite a file, you need write permissions to the directory that contains the file, in this case /dev:

$ ls -ld /dev
drwxr-xr-x 16 root root 4640 Mar 18 13:17 /dev

To restore /dev/null, issue the commands

$ sudo rm /dev/null
$ sudo mknod -m 0666 /dev/null c 1 3

(Also see U&L StackExchange: How to create /dev/null)

When you run the command

$ sudo mv test_file /dev/null

you have replaced the special file /dev/null with your text file. Subsequent attempts to read from /dev/null return the contents of your text file, and programs that attempt to use /dev/null in the normal way will probably break.

Replacing or deleting device files in /dev/ requires superuser privileges, which is why your non-sudo attempt failed with an error.

See Benoit's answer for information on how to restore /dev/null manually, but since most (if not all) of the content of /dev/ is managed dynamically by udev, I suspect a simple reboot will probably fix it too.