Digimer: Created page with '{{fs_header}} NOTE: Currently, this document covers only UNIX-style permissions. Permissions are used to restrict access to data. In POSIX systems, the most common form…'

2009-10-11T09:04:41Z

Created page with '{{fs_header}} NOTE: Currently, this document covers only UNIX-style permissions. Permissions are used to restrict access to data. In POSIX systems, the most common form…'

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NOTE: Currently, this document covers only [[UNIX]]-style permissions.

Permissions are used to restrict access to data. In [[POSIX]] systems, the most common form of permissions (sometimes called a file's 'mode'), is by specifying who the owning user and group is, and then defining what access the owning user, group and "other" are. This is specified in a 9-byte string which can be described using a 3-digit number.

To note; this string will appear to be 10-bytes, but the first byte describes the file type ('-' for a file, 'd' for a directory, 's' for a [[symlink]], etc). The 3-byte digit may also be expressed as four or five-digits as well, which describes certain special characteristics of the file. For now though, we'll keep it simple.

So, using this as an example (simplified string):
<permission> <user> <group> <file name>
-rw-rw-r-- www-data digimer README

* This shows a file owned by the user 'www-data' and group 'digimer'.
* The file name is 'README'
* This file's permission string is '-rw-rw-r--'.

The permission string is made up of four parts;
* 1) first byte is the file type (as mentioned above), we will ignore this for now.
* 2) 3-bytes showing the ''''r'''ead', ''''w'''rite and 'e'''x'''ecute' writes of the owning user.
* 3) 3-bytes showing the ''''r'''ead', ''''w'''rite and 'e'''x'''ecute' writes of the owning group.
* 4) 3-bytes showing the ''''r'''ead', ''''w'''rite and 'e'''x'''ecute' writes of everyone else.

When a value is not present (not allowed), it is shown as a hyphen (-). So then:

-rw-rw-r--
|\_/\_/\_/
| | | \--- 'Other' (everyone else), can ''''r'''ead the file, but not ''''w'''rite to the file or
| | | 'e'''x'''ecute' the file (which means to run it, if it was a program).
| | \------ 'Group', users who are a members of this group are allowed to 'read' and 'write' the file.
| \--------- 'User', The owning user is allowed to 'read' and 'write' to this file.
\----------- File type; being a hyphen indicates that this is a normal file. Ignore for now.

Because writing out that string is tedious to humans, a short-form was developed to describe the permissions. Following the example of [[binary]], each section of three bytes was assigned a value, from right to left.

rwx
||\- 1
|\-- 2
\--- 4

By adding up the values of the "true" or set bytes, the permissions can be describe. So to use our example above, the permission could have been listed as:

664

Where:

Note: A 'hyphen' indicated that the permission is not granted (not 'true' or 'set'), so it is not added.

rw-rw-r-- = (rwx)(rwx)(rwx) = (4+2+0)(4+2+0)(4+0+0) = (6)(6)(4) = 644
||||||||\- 0 \
|||||||\-- 0 |-> Permissions for "everyone else".
||||||\--- 4 /
|||||\---- 0 \
||||\----- 2 |-> Permissions for people in the owning group.
|||\------ 4 /
||\------- 0 \
|\-------- 2 |-> Permissions for the owner directly.
\--------- 4 /

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Digimer: Created page with '{{fs_header}} NOTE: Currently, this document covers only UNIX-style permissions. Permissions are used to restrict access to data. In POSIX systems, the most common form…'