Unix/Linux permissions can be a confusing thing. I have seen a large number of posts asking about permisisons, so I hope that this
might go some way to assisting folks to understand what they are and how they work.

The basic unix permissions come in three flavors;

Owner Permissions : These are permissions that you have on your own files or directories.
Group Permissions : These are permissions that you and anyone in your group have on the file files or directories.
Other Permissions : These are permissions that other people have on your files or directories if not you, or in your group


So, in Unix, when permissions are configured, the server allows you to define different permissions for each these three different categories of users. In a Web Serving environment these permissions are used to control which website owners can access which directories and files.

What do Unix permissions look like?
When viewing your files through an FTP client or from the servers command line;

filename.php username usergroup rwx r-x r-x

The first entry is the name of the file, the next entry is your username on the server, the second entry is the group that you are a member of and the last entry is the permissions assigned to that this file (or directory).

If you notice, I have intentionally spaced out the permissions section, I have grouped the 9 characters into 3 sets of 3. This separation is key to how the permissions system works.

The first set of 3 permissions (rwx) relate to the username seen above, the second set of 3 permissions (r-x) relate to the usergroup seen above and the final set of 3 permissions (r-x) relate to anyone else who is not associated with the username or groupname.


Owner (User) relates to username
The Owner (User) is normally you, these permissions will be enforced on your hosting account name.

Group relates to usergroup
The Group permissions will be enforced on other people that are in the same group as you, within a hosting environment, there is very rarely other people in the same group as you. This protects your files and directories from being made available to anybody else who may also have a hosting account on the same server as you.

Other relates to everyone else
The Other permissions, these will be enforced on anybody else on the server that is either not you or not in your group. So in a Web Serving environment, remembering that no-one else is normally in your group, then this is everybody else accessing the server except for you.

Each of the three sets of permissions are defined in the following manner;

r = Read permissions
w = Write permissions
x = Execute permissions



Owner Group Other
r w x r w x r w x

As many of you already know, permissions are normally expressed as a numeric value, something like 755 or 644. so, how does this relate to what we have discussed above?

Each character of the permissions are assigned a numeric value, this is assigned in each set of three, so we only need to use three values and reuse them for each set.

Owner Group Other
r w x r w x r w x
4 2 1 4 2 1 4 2 1

Now that we have a value that represents each permission, we can express them in numeric terms. The values are simply added together in the respective sets of 3, which will in turn give us just three numbers that will tell us what permissions are being set.

So, if we are told that a file has the permissions of 777, this would mean that the following was true.


Owner Group Other
r w x r w x r w x
4 2 1 4 2 1 4 2 1

Thus...
4+2+1 4+2+1 4+2+1
= 7 = 7 = 7

The Owner of the file would have full Read, Write and Execute permissions, the group would also have full Read, Write and Execute permissions, and the rest of the world can also Read, Write and Execute the file.

This article the work of RussW who has given permission for it's use here.