UNIX file system permissions tutorial

Checking and Understanding Permissions

Access permissions of every file and folder in a UNIX system is controlled by a system based on two identification numbers: user-identification number (UID) and group identification number (UID). Every file or folder should belong to a UID and GID. In practice is it hard to remember these numbers, so the accepted way is to map a user-name and a group-name to each number. In a typical stand-aloone, UNIX computer, all UIDs along with these mappings are listed in the UNIX file /etc/passwd. A typical entry of passwd file is like the following:

alex:x:503:100:Dr. Alexander the Great:/home/alex:/bin/bash

The fields of this entry (seperated by colons, :), relevant to this discussion are:

1. alex: username (a short name to identify the user, which is normally used in place of UID). Typically a lowercase, single word.
2. 503 : UID
3. 100 : GID of the primary group the user belongs to.
4. Dr. Alexander the Great : A descriptive name

The primary administrative user -- root, present in every UNIX system has UID 0.

root:x:0:0:root:/root:/bin/bash

An example of the output produced by 'ls -l' is shown below.

 drwx------ 2 richard staff  2048 Jan  2 1997  private
 drwxrws--- 2 richard staff  2048 Jan  2 1997  admin
 -rw-rw---- 2 richard staff 12040 Aug 20 1996  admin/userinfo
 drwxr-xr-x 3 richard user   2048 May 13 09:27 public

Understanding how to read this output is useful to all unix users, but especially people using group access permissions.

Field 1

a set of ten permission flags.

Field 2

link count (don't worry about this)

Field 3

owner of the file

Field 4

associated group for the file

Field 5

size in bytes

Field 6-8

date of last modification (format varies, but always 3 fields)

Field 9

name of file (possibly with path, depending on how ls was called)

The permission flags are read as follows (left to right)

position	Meaning
1	directory flag, 'd' if a directory, '-' if a normal file, something else occasionally may appear here for special devices.
2,3,4	read, write, execute permission for User (Owner) of file
5,6,7	read, write, execute permission for Group
8,9,10	read, write, execute permission for Other
value	Meaning
-	in any position means that flag is not set
r	file is readable by owner, group or other
w	file is writeable. On a directory, write access means you can add or delete files
x	file is executable (only for programs and shell scripts - not useful for data files). Execute permission on a directory means you can list the files in that directory
s	in the place where 'x' would normally go is called the set-UID or set-groupID flag.

On an executable program with set-UID or set-groupID, that program runs with the effective permissions of its owner or group.

For a directory, the set-groupID flag means that all files created inside that directory will inherit the group of the directory. Without this flag, a file takes on the primary group of the user creating the file. This property is important to people trying to maintain a directory as group accessible. The subdirectories also inherit the set-groupID property.

Setting default file permissions

When a user creates a file or a directory, the initial permissoins of those are determined by the 'uname' value which is set by

1. uname command
2. users initialization file
3. system-wide initialization file

Typically the default configuration is equivalent to typing 'umask 22' which produces permissions of:

 -rw-r--r-- for regular files, or
 drwxr-xr-x for directories.

In other words, user has full access, everyone else (group and other) has read access to files, lookup access to directories.

When working with group-access files and directories, it is common to use 'umask 2' which produces permissions of:

 -rw-rw-r-- for regular files, or
 drwxrwxr-x for directories.

For private work, use 'umask 77' which produces permissions:

 -rw------- for regular files, or
 drwx------ for directories.

umask Unmasked

umask is a ocatal number between 000 and 777, directly affecting the resulting file and directory permissions. The three numbers from left, governs the permissions for user, group and others respectively. Each number relates to the respective chmod value as follows.

	umask	File	Directory
	0	6	7
	1	6	6
	2	4	5
	3	4	4
	4	2	3
	5	2	2
	6	0	1
	7	0	0

Some common umask settings in a more understandable way.

The results shown in the table can be obtained from this bash script.

  umask   file   dirs          files           dirs
    022    759    648     -rw-r--r--     drwxr-xr-x
    027    754    643     -rw-r-----     drwxr-x---
    002    775    664     -rw-rw-r--     drwxrwxr-x
    006    771    660     -rw-rw----     drwxrwx--x
    007    770    659     -rw-rw----     drwxrwx---
    077    714    603     -rw-------     drwx------

Checking umask

The umask command can be used to check the current umask values. There are two ways to use this. Either

umask -p 

to get the numeric umask value or

umask -S

to get a more human readable output. The results of these commands are like,

umask 0022

and

u=rwx,g=rx,o=rx

, respectively.

A note on source

This article has used sections from several outside sources.

1. Computing at Dartmouth College.
2. linuxforms.

I have assumed that I am not violating a copyright by doing so. Please contact me if this is not the case.

Please use this e-mail address 3tv-assela@pathirana.net to contact me, if needed.

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