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FilePermissions

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:

There are three types of access restrictions:

There are also three types of user restrictions:

Note: The restriction type scope is not inheritable: the file owner will be unaffected by restrictions set for his group or everybody else.

Folder/Directory Permissions

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

  • 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

Info <! data-lazy-src=

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
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
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

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:

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:

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
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
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.

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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 -print0 | xargs -0 sudo chmod 644 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 -print0 | xargs -0 sudo chmod 755 user@host:/home/user$ ls -l total 3 -rw-r--r-- 1 user user 0 Nov 19 20:13 file1 drwxr-xr-x 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.

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

Volume Permissions with umask

This section has been moved to: Fstab#Options

ACL (Access Control List)

Posix ACLs are a way of achieving a finer granularity of permissions than is possible with the standard Unix file permissions. See the full page on ACLs FilePermissionsACLs

Setting up ACL

. UUID=d027a8eb-e234-1c9f-aef1-43a7dd9a2345 /home ext4 defaults,acl 0 2 .
sudo mount -o remount /home

The commands, setfacl and getfacl, set and read ACLs on files and directories.

Example Usage

mkdir shared_dir chmod 777 shared_dir
  1. Set the default ACL with ‘-d’ and modify with ‘-m’ the permissions for samba nobody user nogroup group which will apply to all newly created file/directories.
setfacl -d -m u:nobody:rwx,g:nogroup:rwx,o::r-x shared_dir

GUI ACL Editor

http://apt.ubuntu.com/p/eiciel

The Eiciel package allows GUI access to ACLs through the Nautilus file manager.

Читайте также:  Linux dhcp client config

Useful ACL Resources

File removal

To remove a file you cannot delete use

where filename is the name and path of the file to delete.

Nota bene: Be very careful when using the command rm with the -rf option since -r makes the file removal recursive (meaning it will remove files inside of folders) and -f will force the removal even for files which aren’t writable. To play it safe, please consider typing in the absolute path to the file

sudo rm -rf /path/to/file/filename

to prevent any mishaps that can/will occur. It takes longer to type but you can’t put a price on peace of mind. See the rm man page for details.

Sticky Bit

The sticky bit applies only to directories, and is typically used on publicly-writeable directories. Within a directory upon which the sticky bit is applied, users are prevented from deleting or renaming any files that they do not personally own.

To add or remove the sticky bit, use chmod with the «t» flag:

The status of the sticky bit is shown in the other execute field, when viewing the long output of ls. «t» or «T» in the other execute field indicates the sticky bit is set, anything else indicates it is not.

Making a public directory:

user@host:/home/user$ mkdir folder user@host:/home/user$ chmod 777 folder user@host:/home/user$ ls -l total 3 drwxrwxrwx 2 user user 4096 Nov 19 20:13 folder

Adding the sticky bit (note the «t» in the other execute field):

user@host:/home/user$ chmod +t folder user@host:/home/user$ ls -l total 3 drwxrwxrwt 2 user user 4096 Nov 19 20:13 folder

See also

ToDo

  • umask (add file and directory umask section, with specific focus on security)
  • The User Private Group scheme. In other words, this page does the nuts and bolts ok, but we need to describe what the permissions should be. The default Ubuntu set up is not agnostic: Every user has their default private group. Directories for collaboration need to have special group and permission set for correct functioning.
  • * 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.
  • The sticky bit. It’s needed for «other» in shared directories like /tmp. It’s needed for «group» in shared directories where write permission is given to a group, like /var/www

FilePermissions (последним исправлял пользователь ti-225-214-154 2013-11-10 12:51:38)

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