FilePermissions

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||<tablestyle="float:right; font-size: 0.9em; width:40%; background:#F1F1ED; margin: 0 0 1em 1em;" style="padding:0.5em;">'''Contents'''[[BR]][[TableOfContents]]||


== Understanding and Using File Permissions ==

In Linux and Unix everything is a file. Directories are files, files are files and devices are files. Devices are usually refered to as a node, however, they are still files. All of the files on a system have permissions that allow or prevent others from viewing, modifying or executing. If the file is of type Directory then it restricts different actions than files and device nodes. The super user "root" has the ability to access any file on the system. Each file has access restrictions with permissions, user restrictions with owner/group association. Permissions are refered to as bit's.



If the owner read & execute bit are on, then the permissions are:
{{{
-r-x------
}}}

Three types of access restrictions.

|| '''Permission'''|| '''Action'''|| '''chmod option'''||
|| read|| (view) ||r = 4||
|| write|| (edit) ||w = 2||
|| execute|| (execute) ||x = 1||


Three types of user restrictions.

|| '''User'''|| '''''ls'' output'''||
|| owner||-rwx--``--``--||
|| group||--``--rwx---||
|| other||--``--``---rwx||


Directories have whats called directory permissions. The directory permissions restrict different actions than with files or device nodes.

|| '''Permission'''|| '''Action'''|| chmod option'''||
|| read|| (view contents, i.e. ls command) ||r = 4||
|| write|| (create or remove files from dir) ||w = 2||
|| execute|| (cd into directory) ||x = 1||


    * read restricts or allows viewing the directories contents, i.e. ''ls'' command
    * write restricts or allows creating new files or deleting files in the directory. (Caution: write access for a directory allows deleting of files in the directory even if the user does not have write permissions for the file!)
    * execute restricts or allows changing into the directory, i.e. ''cd'' command



== Permissions in Action ==

{{{
user@host:/home/user$ ls -l /etc/hosts
-rw-r--r-- 1 root root 288 2005-11-13 19:24 /etc/hosts
user@host:/home/user$
}}}

Using the example above we have the file "/etc/hosts" which is owned by the user root and belongs to the root group.

What are the permissions from the above /etc/hosts ls output?

{{{
-rw-r--r--

owner = Read & Write (rw-)
group = Read (r--)
other = Read (r--)
}}}


=== Changing Permissions ===

The command to use when modifying permissions is chmod. There are two ways to modify permissions, with numbers or with letters. Using letters is easier to understand for most people. When modifying permissions be careful not to create security problems. Some files are configured to have very restrictive permissions to prevent unauthorized access. For example, the /etc/shadow file (file that stores all local user passwords) does not have permissions for regular users to read or otherwise access.

{{{
user@host:/home/user# ls -l /etc/shadow
-rw-r----- 1 root shadow 869 2005-11-08 13:16 /etc/shadow
user@host:/home/user#

Permissions:
owner = Read & Write (rw-)
group = Read (r--)
other = None (---)

Ownership:
owner = root
group = shadow
}}}


==== chmod with Letters ====

Here are a few examples of chmod usage with letters (try these out on your system):

''First create some empty files:''
{{{
user@host:/home/user$ touch file1 file2 file3 file4
user@host:/home/user$ ls -l
total 0
-rw-r--r-- 1 user user 0 Nov 19 20:13 file1
-rw-r--r-- 1 user user 0 Nov 19 20:13 file2
-rw-r--r-- 1 user user 0 Nov 19 20:13 file3
-rw-r--r-- 1 user user 0 Nov 19 20:13 file4
}}}
''Add owner execute bit:''
{{{
user@host:/home/user$ chmod u+x file1
user@host:/home/user$ ls -l file1
-rwxr--r-- 1 user user 0 Nov 19 20:13 file1
}}}
''Add other write & execute bit:''
{{{
user@host:/home/user$ chmod o+wx file2
user@host:/home/user$ ls -l file2
-rw-r--rwx 1 user user 0 Nov 19 20:13 file2
}}}
''Remove group read bit:''
{{{
user@host:/home/user$ chmod g-r file3
user@host:/home/user$ ls -l file3
-rw----r-- 1 user user 0 Nov 19 20:13 file3
}}}
''Add read, write and execute to everyone:''
{{{
user@host:/home/user$ chmod ugo+rwx file4
user@host:/home/user$ ls -l file4
-rwxrwxrwx 1 user user 0 Nov 19 20:13 file4
user@host:/home/user$
}}}


==== chmod with Numbers ====

Here are a few examples of chmod usage with numbers (try these out on your system):

''First create some empty files:''
{{{
user@host:/home/user$ touch file1 file2 file3 file4
user@host:/home/user$ ls -l
total 0
-rw-r--r-- 1 user user 0 Nov 19 20:13 file1
-rw-r--r-- 1 user user 0 Nov 19 20:13 file2
-rw-r--r-- 1 user user 0 Nov 19 20:13 file3
-rw-r--r-- 1 user user 0 Nov 19 20:13 file4
}}}
''Add owner execute bit:''
{{{
user@host:/home/user$ chmod 744 file1
user@host:/home/user$ ls -l file1
-rwxr--r-- 1 user user 0 Nov 19 20:13 file1
}}}
''Add other write & execute bit:''
{{{
user@host:/home/user$ chmod 647 file2
user@host:/home/user$ ls -l file2
-rw-r--rwx 1 user user 0 Nov 19 20:13 file2
}}}
''Remove group read bit:''
{{{
user@host:/home/user$ chmod 604 file3
user@host:/home/user$ ls -l file3
-rw----r-- 1 user user 0 Nov 19 20:13 file3
}}}
''Add read, write and execute to everyone:''
{{{
user@host:/home/user$ chmod 777 file4
user@host:/home/user$ ls -l file4
-rwxrwxrwx 1 user user 0 Nov 19 20:13 file4
user@host:/home/user$
}}}

==== chmod with sudo ====

Changing permissions on files that you do not have permission to access:
{{{
user@host:/home/user$ ls -l /usr/local/bin/somefile
-rw-r--r-- 1 root root 550 2005-11-13 19:45 /usr/local/bin/somefile
user@host:/home/user$

user@host:/home/user$ sudo chmod o+x /usr/local/bin/somefile

user@host:/home/user$ ls -l /usr/local/bin/somefile
-rw-r--r-x 1 root root 550 2005-11-13 19:45 /usr/local/bin/somefile
user@host:/home/user$
}}}

For more information see:
 * ''man chmod''
 * ''man chown''
 * ''man chgrp''

== ToDo ==

 * sticky bit
 * umask
 * Recursive chmod with -R
#REFRESH 0 http://help.ubuntu.com/community/FilePermissions

FilePermissions (last edited 2008-08-06 16:27:42 by localhost)