The file permissions, or attributes, are a very important aspect of any Linux system & basically define what is allowed to be carried out
& by who! If you don't have the necessary permissions required to carry out the task then you can forget it. EVERY single file
& directory on a Linux machine has 3 critical "attributes" assigned to it.
1. A set of permissions (read/write/execute).
2. An owner.
3. A group.
Only if you meet the above criteria will Linux allow you access to that particular file! Take a look at the ls listings below to
see how these attributes are assigned.
drwxr-xr-x 27 root root 4096 May 3 15:05 etc
-rw-r--r-- 1 root root 20025 May 2 09:18 rc.config
drwxr-xr-x 2 unclebob ftp_users 4096 May 3 11:35 auto.scripts
Like I said above, the attributes are separated into 3 different categories, the group of letters (drwx--..) define the file/folder permissions
(d=directory, r=read, w=write, x=execute), the first name (root, unclebob) defines who owns the file, & the second name
(root, ftp_users) defines to which group the file belongs. You can naturally change these permissions to whatever you wish,
providing that is that you have "permission" to do so to that file. With the command chmod you can change the read / write/ execute permissions
on the file/folder, & with chown you can change the owner & group of the file/folder.
We will have a look at the chmod command first. There are 2 ways of changing the r/w/x file permissions, both are constructed in a similar
way using the owners/group/others + r/w/x categories to define the new permissions. The simplest method of changing permissions is using the "octal
approach" as set out in the table below. Have a look at the example below of the rc.config file.
-rw-r--r-- 1 root root 20025 May 2 09:18 rc.config
Each file has the same layout for these attributes, being a 10 position "code" (drwxrwxrwx)for every file or directory.
The very first position "d" defines a directory, then comes 3 identical blocks of "rwx". The first block of "rwx" defines
the permissions for the owner of the file (in this case "root"), the second "rwx" defines the permissions for the group to
which the file belongs (in this case "root") (NB: not necessarily the same group to which the owner of the file belongs), & the third "rwx"
defines the permissions for all "other" users (which are not necessarily registered "users" of the system, for example a visitor to
your website).
owner
group
others
r
w
x
r
w
x
r
w
x
400
200
100
40
20
10
4
2
1
Using this method of changing the attributes is as simple as building a number with the appropriate permissions. For example, what octal number does
the file rc.config already have? The "owner" has read & write permissions (400+200), the group has read permission (40), & others has
read permission (4), i.e. 400+200+40+4 = 644. Simple huh?! Lets change /home/user/test.file to owner rwx, group r, others
nothing. What octal number do you need? Owner 400+200+100 (700), group 40, others 0, i.e. 700+40+0=740. To then change the file to this
new set of permissions simply type chmod 740 /home/user/test.file & it will be changed accordingly. It's a good idea getting used to this
type of permission changing as less mistakes can occur in this way.
The file permissions are absolutely critical for very many applications, take the web server daemon Apache for example. Apache sends, upon request, pages &
files from your website to a visitor anywhere in the world, BUT only if apache is allowed to!!! That is, if the requested file has
the correct permissions for the person that has requested it. For example if the .htm & .html files (also jpeg, gif,.......)
are not "world readable" (readable to the "others" category) then apache will send the person who requested the file the response that access
is denied. The same applies for directories that are not "world executable", a directory must be "executable" to allow apache (amongst others)
to enter the directory. This concept is very important when it comes to access & security of any machine that is connected to the
internet.
The chown command allows us to change the ownership of a file or a folder into any given "system registered" user or group, providing you
have the necessary permission to change this. Take the following example.
drwxr-xr-x 2 unclebob ftp_users 4096 May 3 11:35 auto.scripts
We wish to change the owner of the directory, & also the directory's group, including all the files & folders contained in the auto.scripts folder
to root owner & root group. Naturally we MUST be logged in as "root" to carry out this ownership change. Simply type:
chown -R root:root auto.scripts
The "root:root" part defines the user "root" as the new owner & separated by the : the group "root" as the new group that the directory
belongs to. The -R defines a recursive change of ownership which takes in all the files & folders contained within auto.scripts.
