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Saturday, May 8, 2010

File ownership in Ubuntu

File ownership in Unix

Just to give you a quick reminder, I'd like to confirm that every single file in Unix belongs to some user and some group. There simply isn't a way to create a file without assigning ownership. I've briefly touched the topic of confirming file ownership in Unix before, so today I will simply build on that and show you how to change ownership of files.

Here's a setup for today: I have created a temporary directory with a few files and made myself the owner of all the files:

ubuntu$ ls -al /home/greys/example/
total 12
drwxr-xr-x 3 greys admin 4096 Feb 9 03:55 .
drwxr-xr-x 13 greys greys 4096 Feb 9 03:54 ..
drwxr-xr-x 2 greys admin 4096 Feb 9 03:55 dir1
-rw-r--r-- 1 greys admin 0 Feb 9 03:54 file1
-rw-r--r-- 1 greys admin 0 Feb 9 03:55 file2

As you can see from this listing, the owner (third field in each line) is my username – greys. The next field is a Unix group of each file's owner – admin in my example.

Changing owner of a file in Unix

Changing file ownership means only updating the association between a Unix user and a file, and nothing else. When you're changing the owner of a file, no data contained in a file is changed.

To change the owner of a file, you need to use the chown command (easy enough to remember: CHange OWNer – chown), with the following syntax:

ubuntu$ chown nobody file1

In this command, nobody is the username of the new owner for a list of files. In my example, the only file we'd like to change ownership for is file1.

It is important to realize that you can only change file ownership as a super-user (root). Any regular Unix user cannot change the ownership of any file, and I'd like to explain why.

Indeed, some people are surprised: if I'm the owner of a given file, why can't I change the ownership for it? That's because transferring the ownership will mean some other Unix user will become the owner of the file(s) in question. So changing ownership is like making a decision not only for yourself, but for the new owner of the files.This is only something a super-user – special administrative account in Unix – can do.

The same logic applies to other people not being able to become owners of your files, even if they're willing to assume the new responsibilities of owning files. They cannot revoke your ownership, because each Unix user is only allowed to make decisions and take actions on his/her own behalf.

That's why you will probably see an error like this if you attempt to change ownership of a file as your own regular Unix user:

ubuntu$ id
uid=1000(greys) gid=113(admin) groups=33(www-data),113(admin)
ubuntu$ chown nobody file1
chown: changing ownership of `file1': Operation not permitted

But if we become root:

ubuntu$ sudo -i
[sudo] password for greys:
ubuntu#

… we'll have no problem changing owners for any files:

ubuntu# cd /home/greys/example
ubuntu# chown nobody file1
ubuntu# ls -l file1
-rw-r--r-- 1 nobody admin 0 Feb 9 03:54 file1

Changing owner for multiple files

If you're going to change owner of a few files, this can easily be done using either a full list of files or a mask.

First, here's an example of updating ownership for a specified list of files (and as you can see, directories as well):

ubuntu# chown nobody file2 dir1
ubuntu# ls -al
total 12
drwxr-xr-x 3 greys admin 4096 Feb 9 03:55 .
drwxr-xr-x 13 greys greys 4096 Feb 9 03:54 ..
drwxr-xr-x 2 nobody admin 4096 Feb 9 03:55 dir1
-rw-r--r-- 1 nobody admin 0 Feb 9 03:54 file1
-rw-r--r-- 1 nobody admin 0 Feb 9 03:55 file2

IMPORTANT: here's one thing which is often forgotten: when you're changing an owner of a directory, this DOES NOT automatically change owner of all the files which already exist in this directory. So, if we check the file3 in dir1 after the example above, we can see that even though dir1 now belongs to user nobody, file3 in it still belongs to me:

ubuntu# ls -l dir1/file3
-rw-r--r-- 1 greys admin 0 Feb 9 03:55 dir1/file3

If your intention is to change ownership of all the files and directories of a certain location in your filesystem, you need to use a -R option of the chown command, which means recursive ownership change:

ubuntu# chown -R nobody dir1
ubuntu# ls -l dir1/file3
-rw-r--r-- 1 nobody admin 0 Feb 9 03:55 dir1/file3

And just to further demonstrate this, I'm going to change owner of all the files and directories in /home/greys/example directory back to my own username, greys:

ubuntu# chown -R greys /home/greys/example/
ubuntu# ls -l /home/greys/example/
total 4
drwxr-xr-x 2 greys admin 4096 Feb 9 03:55 dir1
-rw-r--r-- 1 greys admin 0 Feb 9 03:54 file1
-rw-r--r-- 1 greys admin 0 Feb 9 03:55 file2



This posting is originally found here.

1 comment:

Kappo said...

Thanks; the information helped s lot.

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