I have been looking for a page like this. I have been trying to set permissions on files so my Windows machine and rest of the family could access directories and files on my Linux machine and kept failing permissions and couldn't figure out why. This is great.

Hi this is rally nice. I havent been to the Answers page ina while and so I saw this recently. Nicely done.

Cheers and thanks.

I agree that article explains file permissions in a way a noob can understand........................When I first read the article (quite a few months ago), the light suddenly went on in my head about the way permissions work in the unix world..............Before that, I was perceiving it as being harder than it actually is.......

I think bulliver has done a great job, and since chattr is
ext2/3 specific it doesn't really belong into an article on
Linux permissions at all ...



Cheers,
Tink

Thanks for the positive feedback everyone.

I have to agree with Tinkster. I wanted it too be very bare-bones and to the point, after all it's the "Quick and Dirty Guide to Linux File Permissions" not the "Long and Drawn Out Guide to Linux File Permissions"

Maybe someone could write another article about the chattr, extended attributes, Access Control Listss. and any other more specialized and exotic methods used for permissions as a compliment to this article, if they're so inclined..........I don't have any experience with any of them using reiserfs, or I might do it.........

Warning!!! Noob here!

Great article!! Finaly someone have put together everything in one file! I'v googled and found them all separatly. So this really helps! (bookmarking it now)

One thing I dont understand is how do you know its 755? Why 7? and why 5? Where does these numbers come from?

Maybe I should read it 2 more times...

Kanon:

Have a look at a permission string:

-rwxr-xr-x

You can see there are 10 slots. The first one (generally) describes the file. It is a simple dash for plain files.

Then we have three groups of three permissions in the order:
owner group other

each of these groups has three permissions:
read write execute

read permission => 4
write permission => 2
execute permission => 1
NO permissions (ie '-') => 0

It's just basic math. Have a look at our permission string again:

-rwxr-xr-x

The first slot is a dash: '-', so it is a regular file.
The next three slots describe the file owners permissions: 'rwx'
4 + 2 + 1 = [ 7 ]
The next three slots describe the file's group permissions: 'r-x'
4 + 0 + 1 = [ 5 ]
The next three slots describe everyone else's permissions: 'r-x'
4 + 0 + 1 = [ 5 ]

So our final permissions for the file are 755

Hope this clears it up....

AH! Yes, it's crystal now!

All I have to do now is: remember.
But I did bookmark it!

Tnx for the input!

come on guys Iam a little bit advance newbie
but I really found the article very helpful, since it was plain simple informative and the author did stick to the point of permissions or modes and did not talk about this and that like many other how tos

With regards ALInux

Don't forget to include lcap as well. With lcap, not even root can restore, let's say, /var/log/messages apend only feature, unless root reboot the machine and disable the append only feature of a file.


Raxxal

What I didn't understand/see was anything about changing permissions for directory's and their contents.

I just did a reinstall/upgrade and from disks reloaded all the files I needed.

They are all now read-only by user.
I can go in a change each one individually but is there a way to change an entire directory?
Jay

That is a good point. I neglected to say that permissions on directories work a bit different...

r permissions on a directory lets you list the directory
w permissions on a directory lets you create and remove files in the directory
x permissions on a directory lets you traverse a directory (ie: cd into it)

However, because the permissions mean different things when talking about directories as opposed to regular files, running a recursive chmod: 'chmod -R 755 /directory' to modify the permissions of the files inside the directory may not work as you anticipated, because it will adjust the directory (and subdirectories) as well. In this case your best bet is to use shell-globbing:
$ chmod 755 /directory/*

First : thank you for this article.
I also have a question. I found a file on my system which is this :

-rwsr-xr-x 1 root root 481196 Jul 21 2004 /usr/bin/smbumount2

I just don't understant what the "s" mean in place of the "x" position.
Is anybody knows ?

Thanks,
Matthieu

It means that the file is set SUID to run as root. Some files, such as /etc/passwd need to run SUID so that you can change your own passwd (ie: a regular user cannot edit the file).

In your case, the smbumount2 needs root permissions to run.