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$
Recursive Permission Changes
To change the permissions of multiple files and directories with one command. Please note the warning in the chmod with sudo section and the Warning with Recursive chmod section.
Recursive chmod with -R and sudo
To change all the permissions of each file and folder under a specified directory at once, use sudo chmod with -R
user@host:/home/user$ sudo chmod 777 -R /path/to/someDirectory
user@host:/home/user$ ls -l
total 3
-rwxrwxrwx 1 user user 0 Nov 19 20:13 file1
drwxrwxrwx 2 user user 4096 Nov 19 20:13 folder
-rwxrwxrwx 1 user user 0 Nov 19 20:13 file2
Recursive chmod using find, pipemill, and sudo
To assign reasonably secure permissions to files and folders/directories, it's common to give files a permission of 644, and directories a 755 permission, since chmod -R assigns to both. Use sudo, the find command, and a pipemill to chmod as in the following examples.
To change permission of only files under a specified directory.
user@host:/home/user$ sudo find /path/to/someDirectory -type f | while read var1; do sudo chmod 644 "$var1"; done
user@host:/home/user$ ls -l
total 3
-rw-r--r-- 1 user user 0 Nov 19 20:13 file1
drwxrwxrwx 2 user user 4096 Nov 19 20:13 folder
-rw-r--r-- 1 user user 0 Nov 19 20:13 file2
To change permission of only directories under a specified directory (including that directory):
user@host:/home/user$ sudo find /path/to/someDirectory -type d | while read var1; do sudo chmod 755 "$var1"; done
user@host:/home/user$ ls -l
total 3
-rw-r--r-- 1 user user 0 Nov 19 20:13 file1
drwxr--r-- 2 user user 4096 Nov 19 20:13 folder
-rw-r--r-- 1 user user 0 Nov 19 20:13 file2
Warning with Recursive chmod
WARNING: Although it's been said, it's worth mentioning in context of a gotcha typo. Please note, Recursively deleting or chown-ing files are extremely dangerous. You will not be the first, nor the last, person to add one too many spaces into the command. This example will hose your system:
user@host:/home/user$ sudo chmod -R / home/john/Desktop/tempfiles
Note the space between the first / and home.
You have been warned.
Changing the File Owner and Group
A file's owner can be changed using the chown command. For example, to change the foobar file's owner to tux:
user@host:/home/user$ sudo chown tux foobar
To change the foobar file's group to penguins, you could use either chgrp or chown with special syntax:
user@host:/home/user$ sudo chgrp penguins foobar
user@host:/home/user$ sudo chown :penguins foobar
Finally, to change the foobar file's owner to tux and the group to penguins with a single command, the syntax would be:
user@host:/home/user$ sudo chown tux:penguins foobar
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
The Eiciel package allows GUI access to ACLs through the Nautilus file manager.
File removal
To remove a file you cannot delete use
sudo rm -rf filename
where filename is the name and path of the file to delete.
Be careful when you do this, since the -r option makes the file removal recursive and the -f will force the removal even for files which are not writable. See the rm man page for details.
== See also ==
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man chmod
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man chown
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man chgrp
ToDo
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sticky bit
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umask (add file and directory umask section, with specific focus on security)
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* Suggestion: I often use find instead of chmod -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.
