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UNIX File Permissions

UNIX allows you to set very specific access permissions for all files on the server. You can set any combination of three user types and three access types.

User & Access types

UNIX defines three types of users:

owner - the login account that created the file (probably you!)
group - a group of users on the server (ex. all web site owners)
other - everyone else on the server

and three types of file access:

read - permission to view the file
write - permission to edit the file
execute - permission to "run" the program (ex. a script)

You can define what level of access each of the three users can have for a specific file. For example, I may have a file on my server that I will let everyone read, but only I should be able edit it. I would set the following permissions:

owner (that's me!) - read & write
group - read
other - read

Viewing permissions

From a telnet window, you can view the file permissions for any file by issuing this command:

ls -l <filename>

You will get back something that looks like this:

-rw-rw-r--   1 ashley   enscript       94 May 10  1999 index.html

The first grouping spells out the permissions for the file using a special notation:

The first column is either a d (for directory) or - (for a regular file)
The next 3 columns define the permissions for the owner
The next 3 columns define the permissions for the group
The last 3 columns define the permissions for the other users

For each of the three user types, the permissions are defined this way:

An r indicates read permission
A w indicates write permission
A x indicates execute permission

Here are some examples to further illustrate this notation:

Notation owner group other

-rwxrwxr-x read, write, execute read, write, execute read, execute

-rwxr-xr-x read, write, execute read, execute read, execute

drwxrwxrwx read, write, execute read, write, execute read, write, execute

-rw-rw-rw- read, write read, write read, write

An alternate shorthand notation for the permissions is to use a hexidecimal number to represent the rwx triplet. The following table shows the translations:

--- 0 no access

--x 1 execute only

-w- 2 write only

-wx 3 write, execute

r-- 4 read only

r-x 5 read, execute

rw- 6 read, write

rwx 7 read, write, execute

So, we can add this information to the previous example:

Notation Hexidecimal
Shorthand owner group other

-rwxrwxr-x 775 read, write, execute read, write, execute read, execute

-rwxr-xr-x 755 read, write, execute read, execute read, execute

drwxrwxrwx 777 read, write, execute read, write, execute read, write, execute

-rw-rw-rw- 666 read, write read, write read, write

Setting permissions

On UNIX systems, you have two methods of setting file permissions: (1) FTP and (2) telnet.

Using your FTP program, you should be able to view and change the file permissions on any file or directory. On Fetch, for example, there is a Set Permissions option in the Remote menu that brings up a window for setting the appropriate permissions.

Fig 1. Set Permissions screen using Fetch

If using telnet, you can set the file permissions using the UNIX chmod command. You should specify the permissions using the hexidecimal shorthand for the desired access. Once you are in the directory where the file or directory resides, you can type a command similar to the following:

chmod 666 hits

Script Permissions

Most CGI scripts will tell you in the README file exactly how to set the file permissions. For example, from the Readme.txt file for Links 2.0 from Gossamer Threads:


Set permissions:
  chmod 755 (-rwxr-xr-x) on all .cgi files.
  chmod 666 (-rw-rw-rw-) on all files in the data directory.
  chmod 666 (-rw-rw-rw-) on all your template files (if using the online editor).
  chmod 777 (drwxrwxrwx) on the hits directory
  chmod 777 (drwxrwxrwx) on the ratings directory
  chmod 777 (drwxrwxrwx) on the directory where Links pages will be created.

To complete the setup, you need to look at all of the files in the Links directory and ensure they have the proper permissions.

At a bare minimum for any CGI installation, you must make sure that the script itself is executable by all users. A permissions setting of 755 is generally appropriate for all CGI scripts.

Permissions on NT