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When trying to move a test_dir directory to /dev/null, I get the message

mv: cannot overwrite non-directory ‘/dev/null’ with directory ‘test_dir/’

Then why do people say "Don't run the command sudo mv ~ /dev/null, it will move your home directory to a hole?"

Link

But /home is also a directory.

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4 Answers 4

up vote 22 down vote accepted

Because people assume. I was one of those people until I tested it. It's easy to understand why people assume... It looks dangerous...

... but you can't actually move things to /dev/null — It's a special file that just absorbs redirects (and sends them into nothingness). If you try to move a directory to it, the filesystem will verbosely explode in your face and if you try to move a file to it, you will probably end up replacing it.

The first link will deal with directories, but here's a separate test just for overwriting it with a file. As Rmano points out in the comments, this is probably something you shouldn't do without adult supervision. There is risk involved.

$ echo "this is my file" > test
$ cat test
this is my file

$ sudo mv test /dev/null
$ cat /dev/null
this is my file

# Fix this!
$ sudo rm /dev/null
$ sudo mknod -m 0666 /dev/null c 1 3
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2  
No, moving a file to /dev/null will throw out your null device and replace it with that file, not get rid of the file's contents. –  psusi Mar 18 at 14:05
2  
The answer is a bit ambiguous. Moving a file to /dev/null will not result in that data being moved into nothingness. It will result in the data being moved alright, but /dev/null will be overwritten, and so you can move it back yet :) Also see askubuntu.com/questions/435887/… –  Malte Skoruppa Mar 18 at 14:05
1  
Yep, because sudo mv file /dev/null will succeed, and can easily lead to a completely unusable system. See my comments on the question linked in the comment of @MalteSkoruppa. This is another clear example of "prepend sudo only if you thrice checked you know what you're doing" ;-) –  Rmano Mar 18 at 14:25
4  
You can do experiment creating your own copy of /dev/null: sudo mknod -m 0666 /tmp/mynull c 1 3 --- and then play away with it ;-) –  Rmano Mar 18 at 14:45
7  
This is where the best questions come from. "Dear Ask Ubuntu, I recently moved my homework to /dev/null by mistake and that was sucked into 90% of my system configuration files. I now see Shakespeare essays every time I boot. And my trackpad doesn't work. Halp!" –  Oli Mar 18 at 15:06

/dev/null is just a file, it's a "special character" file but it's non the less still bound by the rules that files must follow. That being said you could never run this command:

$ mv ~ /dev/null

The mv command won't allow this since you're moving a directory to a file, that just doesn't make sense contextually and mv knows this.

Example

$ mkdir dir
$ touch afile
$ mv dir afile
mv: cannot overwrite non-directory ‘afile’ with directory ‘dir’

You can't copy onto /dev/null either, given it's a character file, if you try to copy a regular file onto it.

$ cp ~/bzip2_1.0.6-4_amd64.deb /dev/null
$ ls -l |grep null
crw-rw-rw-  1 root root        1,   3 Mar 16 14:25 null

About the only thing you can do to this file is copy mv over it another file or delete it.

$ mv /path/to/afile /dev/null

After this command, /dev/null is a regular file. The most dangerous effect of this change is that /dev/null is supposed to never output any data, so a number of shell script will assume that

`... < /dev/null` 

is equivalent to say "nothing". Having this assumption broken can lead to random data (well, the data the last process wrote to `/dev/null') inserted in system files all around the system --- which could lead to an utterly broken and unrecoverable system.

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I hope you don't mind the edit, feel free to correct my language. That operation is so dangerous that I felt I need to point it out... –  Rmano Mar 18 at 14:39
    
@Rmano - no problem, edit away 8-) –  slm Mar 18 at 15:52
    
It is just a file, not a directory, which is why you cannot move a directory to it. –  Thorbjørn Ravn Andersen Mar 19 at 8:59

You can write files or other input streams to /dev/null but not directories. If you try moving a directory to /dev/null it would report an error since /dev/null is not a directory but a file.

However, since you want to experiment with /dev/null, you are first suggested to know the consequences to moving a file to overwrite /dev/null and how to recover from that situation:

As suggested by @Rmano in this answer to that question, in order to experiment with /dev/null we should rather create a copy of it and then do our experimentation. So, let's create /tmp/null and use it for our experimentation purposes:

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

Now onwards, /tmp/null is our /dev/null for all purposes:

Let us create a test_file and a test_dir inside a directory called ask_ubuntu.

$ mkdir ask_ubuntu
$ cd ask_ubuntu
$ touch test_file
$ mkdir test_dir
$ echo "Let us test if we can recover our test_file." > test_file

The following shows the contents of ask_ubuntu directory:

$ ls -la
total 12
drwxr-xr-x 3 aditya aditya 4096 Mar 18 17:10 .
drwxr-xr-x 4 aditya aditya 4096 Mar 18 17:10 ..
drwxr-xr-x 2 aditya aditya 4096 Mar 18 17:10 test_dir
-rw-r--r-- 1 aditya aditya    0 Mar 18 17:10 test_file

Now try to move our test_file to /tmp/null and see the contents of ask_ubuntu:

$ sudo mv test_file /tmp/null   # This succeeds
$ ls -la
total 12
drwxr-xr-x 3 aditya aditya 4096 Mar 18 17:12 .
drwxr-xr-x 4 aditya aditya 4096 Mar 18 17:10 ..
drwxr-xr-x 2 aditya aditya 4096 Mar 18 17:10 test_dir

The command succeeds and test_file is no longer available. Now try to move test_dir to /tmp/null which doesn't succeed:

$ sudo mv test_dir/ /tmp/null 
mv: cannot overwrite non-directory ‘/tmp/null’ with directory ‘test_dir/’

test_dir is still present inside ask_ubuntu:

$ ls -la
total 12
drwxr-xr-x 3 aditya aditya 4096 Mar 18 17:12 .
drwxr-xr-x 4 aditya aditya 4096 Mar 18 17:10 ..
drwxr-xr-x 2 aditya aditya 4096 Mar 18 17:10 test_dir

Now, let us figure if we can recover our test_file from /tmp/null:

$ cat /tmp/null
Let us test if we can recover our test_file.

So, it is still there and /tmp/null which was a special file has been overwritten and it has become like any other normal file. We can recover our file by copying /tmp/null just like any other file:

$ cp /tmp/null our_test_file
$ cat our_test_file
Let us test if we can recover our test_file.

File recovered.

Note:

If you didn't create /tmp/null and tried those commands directly using /dev/null; make sure you recover the file (if you need to) by running cp /dev/null our_test_file; and restore /dev/null for the purposes it exists on our system by running the following commands as given in the linked question as soon as possible:

$ sudo rm /dev/null
$ sudo mknod /dev/null c 1 3
$ sudo chmod 666 /dev/null

Conclusion:

  • So, it is impossible to move a directory to /dev/null and hence there is no question of recovering the directory from there.

  • As far as files are concerned, if you directly move files to /dev/null, you can still recover it as demonstrated above. However, there are two exceptions:

    1. During the period you run sudo mv test_file /dev/null and cp /dev/null our_test_file, if any root script in the system overwrites it by running echo "Whatever text the root script wants to send to /dev/null" > /dev/null (or other similar commands). Then we do not have any easy way to recover our file.

    2. If you reboot the system between running those two commands. /dev/null gets re-created at boot, so our file gets lost when we shut down the computer.

  • But if you want to recover input streams like echo "Stream this line to /dev/null" > /dev/null, you cannot recover that since /dev/null is a special file to dispose off unwanted files and input streams and as the Wikipedia article mentions, it doesn't provide any data to a process that reads from it.


Reference: Wikipedia Article on /dev/null

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4  
sudo mv test_file /dev/null replaces /dev/null with test_file. So after that /dev/null is just a normal file and you can read from it anything you wrote into it. DON'T DO THAT –  Florian Diesch Mar 18 at 12:03
    
"So, it's not possible to recover any file that you have moved to /dev/null" strictly speaking not true. If you were to shut down the system, mount it and run data recovery on it, you might very well be able to recover the file if it hadn't been overwritten. –  bigbadonk420 Mar 18 at 13:28
    
@bigbadonk420: I have edited the answer :) –  Aditya Mar 18 at 13:33
    
@FlorianDiesch: Updated the answer :) –  Aditya Mar 18 at 13:33

Everything sent to /dev/null is silently discarded. If you type :

$ echo "Hello World"

you get Hello World on the screen. If you type :

$ echo "Hello World" >/dev/null

you don't get anything on the screen.

But in the case of the move command, the command mv try to replace the file /dev/null by the directory, what is not possible. Because everything is a file in Linux, /dev/null is a file. A special one of course (a device file), a special file allowing accessing piece of hardware (like disks, partitions, sound cards, serial ports, ...). In the case of /dev/null, this is not linked to any piece of hardware so the data sent to it is silently discarded. This is why "they" may have called it a blackhole.

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directories are not files. –  Thorbjørn Ravn Andersen Mar 19 at 9:00
    
@ThorbjørnRavnAndersen In Linux world, directories are files! –  HabeebPerwad Mar 19 at 9:17
    
@HabeebPerwad only inside the kernel. –  Thorbjørn Ravn Andersen Mar 19 at 9:56
    
@ThorbjørnRavnAndersen Sorry, I didn't get into kernel, so no idea :) –  HabeebPerwad Mar 19 at 10:19

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