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Day 14 - Who has permission?

INTRO

Files on a Linux system always have associated “permissions” - controlling who has access and what sort of access. You’ll have bumped into this in various ways already - as an example, yesterday while logged in as your “ordinary” user, you could not upload files directly into /var/www or create a new folder at /.

The Linux permission system is quite simple, but it does have some quirky and subtle aspects, so today is simply an introduction to some of the basic concepts.

This time you really do need to work your way through the material in the RESOURCES section!

YOUR TASKS TODAY

  • Change the ownership of a file to root
  • Change file permissions

OWNERSHIP

First let’s look at “ownership”. All files are tagged with both the name of the user and the group that owns them, so if we type ls -l and see a file listing like this:

-rw-------  1 steve  staff      4478979  6 Feb  2011 private.txt
-rw-rw-r--  1 steve  staff      4478979  6 Feb  2011 press.txt
-rwxr-xr-x  1 steve  staff      4478979  6 Feb  2011 upload.bin

Then these files are owned by user “steve”, and the group “staff”. Anyone that is not “steve” or is not part of the group “staff” is considered “other”. Others may still have permissions to handle these files, but they do not have any ownership.

If you want to change the ownership of a file, use the chown utility. This will change the user owner of file to a new user:

sudo chown user file

You can also change user and group at the same time:

sudo chown user:group file

If you only need to change the group owner, you can use chgrp command instead:

sudo chgrp group file

Since you created new users in the previous lesson, switch logins and create a few files to their home directories for testing. See how they show with ls -l

PERMISSIONS (SYMBOLIC NOTATION)

Looking at the -rw-r--r-- at the start of a directory listing line, (ignore the first “-” for now), and see these as potentially three groups of “rwx”: the permission granted to the “user” who owns the file, the “group”, and “other people” - we like to call that UGO.

For the example list above:

  • private.txt - Steve has rw (ie Read and Write) permission, but neither the group “staff” nor “other people” have any permission at all
  • press.txt - Steve can Read and Write to this file too, but so can any member of the group “staff” and anyone, i.e. “other people”, can read it
  • upload.bin - Steve has rwx, he can read, write and execute - i.e. run this program - but the group and others can only read and execute it

You can change the permissions on any file with the chmod utility. Create a simple text file in your home directory with vim (e.g. tuesday.txt) and check that you can list its contents by typing: cat tuesday.txt or less tuesday.txt.

Now look at its permissions by doing: ls -ltr tuesday.txt

-rw-rw-r-- 1 ubuntu ubuntu   12 Nov 19 14:48 tuesday.txt

So, the file is owned by the user “ubuntu”, and group “ubuntu”, who are the only ones that can write to the file - but any other user can only read it.

CHANGING PERMISSIONS

Now let’s remove the permission of the user and “ubuntu” group to write their own file:

chmod u-w tuesday.txt

chmod g-w tuesday.txt

…and remove the permission for “others” to read the file:

chmod o-r tuesday.txt

Do a listing to check the result:

-r--r----- 1 ubuntu ubuntu   12 Nov 19 14:48 tuesday.txt

…and confirm by trying to edit the file with nano or vim. You’ll find that you appear to be able to edit it - but can’t save any changes. (In this case, as the owner, you have “permission to override permissions”, so can can write with :w!). You can of course easily give yourself back the permission to write to the file by:

chmod u+w tuesday.txt

POSTING YOUR PROGRESS

Just for fun, create a file: secret.txt in your home folder, take away all permissions from it for the user, group and others - and see what happens when you try to edit it with vim.

EXTENSION

If all of this is old news to you, you may want to look into Linux ACLs:

Also, SELinux and AppArmour:

RESOURCES

Some rights reserved. Check the license terms here