Virtual Private Server User Guide

Internet and UNIX Basics

In this example, the file is owned by the root user and belongs to the group named 

mailadmin. The permissions of the file are displayed as a series or letters and 

dashes in the first column of the display. This set of characters tells us four things 

about the file: what type of file it is, what the owner can do to the file, what the 

group can do to the file, and what everyone else (e.g. "the world") can do to the 

file. There are three basic types of permissions that are represented as follows:

r    Read the file or directory   

w    Write to the file or directory

x    Execute the file or search the directory   

The set of permissions on the aliases file in the example above looks like:

 rw rw r  

Here is an explanation of how to interpret these characters in order to understand 

the permissions:

   The first character, from left to right, is a special character that tells if this is 

a regular file ( ), a directory (d), or something special such as a symbolic 

link (represented by an l). In the above example, the beginning dash ( ) 

denotes that this is a regular file.

   The next three characters, designated in this example as rw , indicate what 

the user who owns the file can do with it. In this example, the owner is 

allowed to read and write to the file.

   The next three characters, rw , give the permissions for the group (g) that 

the file belongs to. The group that owns this sample file can read and write 

to the file. 

   The final three characters, r  , give the permissions for the rest of the world 

(o). In this example, the "world" may read the file, but not write to it.

To summarize, the permissions on this sample file are set so the owner and group 

can read and write to the file, but the rest of the world can only read the file. 

Note that directories are also treated as files. They have read, write, and execute 

permissions. The executable bit for a directory has a slightly different meaning 

than it does for a file. When a directory is marked executable, it means it can be 

searched into; for example, a directory listing can be done in that directory. Taking 

the execute permissions off of a directory prevents users from viewing its 

contents.

The Permissions Numbering System

As previously mentioned, directory listings display permissions as three sets of 

characters one for the owner of the file, one for the group that the file belongs to, 

and one for everyone else, as shown below:

user (u)    group (g)  others (o)

rwx           rwx          rwx

Technical Training Department

August 2004

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