UbuntuHelp:FilePermissions
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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 referred 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 referred to as bits.
To change or edit files that are owned by root, sudo must be used - please see RootSudo for details.
If the owner read & execute bit are on, then the permissions are:
-r-x------
There are three types of access restrictions:
| Permission | Action | chmod option |
| read | (view) | r or 4 |
| write | (edit) | w or 2 |
| execute | (execute) | x or 1 |
There are also three types of user restrictions:
| User | ls output |
| owner | -rwx------
|
| group | ----rwx---
|
| other | -------rwx
|
Directories have 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 or 4 |
| write | (create or remove files from dir) | w or 2 |
| execute | (cd into directory) | x or 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
Usage: chmod {options} filename
| Options | Definition |
| u | owner |
| g | group |
| o | other |
| x | execute |
| w | write |
| r | read |
| + | add permission |
| - | remove permission |
| = | set permission |
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
Usage: chmod {options} filename
| Options | Definition |
#-- |
owner |
-#- |
group |
--# |
other |
| 1 | execute |
| 2 | write |
| 4 | read |
Owner, Group and Other is represented by three numbers. To get the value for the options determine the type of access needed for the file then add.
For example if you want a file that has -rw-rw-rwx permissions you will use the following:
| Owner | Group | Other |
| read & write | read & write | read, write & execute |
| 4+2=6 | 4+2=6 | 4+2+1=7 |
user@host:/home/user$ chmod 667 filename
Another example if you want a file that has --w-r-x--x permissions you will use the following:
| Owner | Group | Other |
| write | read & execute | execute |
| 2 | 4+1=5 | 1 |
user@host:/home/user$ chmod 251 filename
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 ownership of: (Note that changing permissions the wrong way on the wrong files can quickly mess up your system a great deal! Please be careful when using sudo!)
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
- man chmod
- man chown
- man chgrp
Changing Permissions For Volumes With umask
This section has been moved: VolumePermissions
ACLs
Posix ACLs are a way of achieving a finer granularity of permissions than is possible with the standard Unix file permissions.
To enable Posix ACLs, install the acl package
sudo apt-get install acl
Documentation can then be found in the online man pages:
- man acl
- man setfacl
- man getfacl
ToDo
- sticky bit
- umask (add file and directory umask section, with specific focus on security)
- Recursive chmod with -R
- * Suggestion: I often use
findinstead ofchmod -R, because it's easier to differentiate between files and directories that way. Yes, I know about the 'X' permission, but I don't trust it.
