Unix Create a File Only Sudo Can Read
Contents
- Understanding and Using File Permissions
- Binder/Directory Permissions
- Permissions in Activity
- Changing Permissions
- chmod with Letters
- chmod with Numbers
- chmod with sudo
- Recursive Permission Changes
- Recursive chmod with -R and sudo
- Recursive chmod using find, pipemill, and sudo
- Warning with Recursive chmod
- Changing the File Owner and Group
- Volume Permissions with umask
- ACL (Access Control List)
- Setting up ACL
- Instance Usage
- GUI ACL Editor
- Useful ACL Resources
- File removal
- Gluey Flake
- See likewise
- ToDo
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 normally referred to as a node; withal, they are still files. All of the files on a system take 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 organisation. Each file has admission restrictions with permissions, user restrictions with owner/group association. Permissions are referred to as $.25.
To modify or edit files that are endemic by root, sudo must be used - please run across RootSudo for details.
If the owner read & execute bit are on, then the permissions are:
-r-ten------
There are three types of access restrictions:
| Permission | Action | chmod option |
| read | (view) | r or four |
| write | (edit) | w or 2 |
| execute | (execute) | x or 1 |
There are also three types of user restrictions:
| User | ls output |
| owner | -rwx------ |
| group | ----rwx--- |
| other | -------rwx |
Notation: The restriction type scope is non inheritable: the file owner will be unaffected by restrictions set for his group or everybody else.
Binder/Directory Permissions
Directories have directory permissions. The directory permissions restrict different actions than with files or device nodes.
| Permission | Action | chmod option |
| read | (view contents, i.east. ls command) | r or 4 |
| write | (create or remove files from dir) | westward or 2 |
| execute | (cd into directory) | x or one |
-
read restricts or allows viewing the directories contents, i.e. ls command
-
write restricts or allows creating new files or deleting files in the directory. (Caution: write access for a directory allows deleting of files in the directory even if the user does non have write permissions for the file!)
-
execute restricts or allows changing into the directory, i.e. cd control
Folders (directories) must have 'execute' permissions fix (x or 1), or folders (directories) will NOT FUNCTION equally folders (directories) and WILL DISAPPEAR from view in the file browser (Nautilus).
Permissions in Action
user@host:/home/user$ ls -50 /etc/hosts -rw-r--r-- 1 root root 288 2005-xi-thirteen 19:24 /etc/hosts user@host:/habitation/user$
Using the example above nosotros accept 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-- possessor = Read & Write (rw-) grouping = Read (r--) other = Read (r--)
Changing Permissions
The command to use when modifying permissions is chmod. There are ii ways to modify permissions, with numbers or with letters. Using letters is easier to empathize for most people. When modifying permissions be careful not to create security bug. 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 non accept permissions for regular users to read or otherwise access.
user@host:/habitation/user# ls -fifty /etc/shadow -rw-r----- one root shadow 869 2005-11-08 xiii:sixteen /etc/shadow user@host:/home/user# Permissions: owner = Read & Write (rw-) grouping = Read (r--) other = None (---) Ownership: owner = root group = shadow
chmod with Letters
Usage: chmod {options} filename | Options | Definition |
| u | owner |
| g | group |
| o | other |
| a | all (same equally ugo) |
| x | execute |
| w | write |
| r | read |
| + | add permission |
| - | remove permission |
| = | ready permission |
Here are a few examples of chmod usage with letters (try these out on your system).
Starting time create some empty files:
user@host:/home/user$ bear upon 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:xiii file2 -rw-r--r-- 1 user user 0 Nov 19 20:thirteen file3 -rw-r--r-- 1 user user 0 November 19 20:13 file4
Add owner execute scrap:
user@host:/home/user$ chmod u+x file1 user@host:/habitation/user$ ls -l file1 -rwxr--r-- 1 user user 0 Nov nineteen 20:thirteen file1
Add other write & execute fleck:
user@host:/home/user$ chmod o+wx file2 user@host:/home/user$ ls -fifty file2 -rw-r--rwx one user user 0 Nov 19 xx:13 file2
Remove grouping read chip:
user@host:/home/user$ chmod yard-r file3 user@host:/abode/user$ ls -l file3 -rw----r-- 1 user user 0 November nineteen 20:13 file3
Add together read, write and execute to everyone:
user@host:/domicile/user$ chmod ugo+rwx file4 user@host:/home/user$ ls -50 file4 -rwxrwxrwx ane user user 0 Nov 19 20:thirteen file4 user@host:/dwelling house/user$
chmod with Numbers
Usage: chmod {options} filename | Options | Definition |
| #-- | owner |
| -#- | grouping |
| --# | other |
| 1 | execute |
| two | write |
| iv | read |
Owner, Group and Other is represented by iii numbers. To go the value for the options determine the type of access needed for the file then add.
For case if y'all want a file that has -rw-rw-rwx permissions you volition employ the following:
| Possessor | Group | Other |
| read & write | read & write | read, write & execute |
| 4+2=vi | 4+2=6 | 4+2+1=7 |
user@host:/dwelling house/user$ chmod 667 filename
Another case if you want a file that has --westward-r-x--x permissions you will use the following:
| Owner | Group | Other |
| write | read & execute | execute |
| two | four+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 arrangement).
First create some empty files:
user@host:/home/user$ touch file1 file2 file3 file4 user@host:/dwelling house/user$ ls -50 total 0 -rw-r--r-- 1 user user 0 Nov 19 twenty:13 file1 -rw-r--r-- 1 user user 0 November 19 20:13 file2 -rw-r--r-- ane user user 0 Nov xix 20:13 file3 -rw-r--r-- one user user 0 November xix 20:13 file4
Add owner execute bit:
user@host:/dwelling house/user$ chmod 744 file1 user@host:/home/user$ ls -l file1 -rwxr--r-- 1 user user 0 Nov xix xx:13 file1
Add other write & execute bit:
user@host:/abode/user$ chmod 647 file2 user@host:/home/user$ ls -l file2 -rw-r--rwx i user user 0 Nov 19 twenty:13 file2
Remove group read bit:
user@host:/home/user$ chmod 604 file3 user@host:/home/user$ ls -fifty file3 -rw----r-- 1 user user 0 Nov nineteen twenty:13 file3
Add read, write and execute to everyone:
user@host:/dwelling house/user$ chmod 777 file4 user@host:/domicile/user$ ls -l file4 -rwxrwxrwx 1 user user 0 Nov nineteen 20:xiii file4 user@host:/home/user$
chmod with sudo
Changing permissions on files that you lot do non take ownership of: (Note that changing permissions the wrong way on the wrong files can quickly mess up your organisation a great deal! Please be conscientious when using sudo!)
user@host:/habitation/user$ ls -l /usr/local/bin/somefile -rw-r--r-- one root root 550 2005-eleven-13 19:45 /usr/local/bin/somefile user@host:/home/user$ user@host:/dwelling/user$ sudo chmod o+ten /usr/local/bin/somefile user@host:/dwelling/user$ ls -50 /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 modify the permissions of multiple files and directories with 1 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 binder nether a specified directory at once, apply sudo chmod with -R
user@host:/home/user$ sudo chmod 777 -R /path/to/someDirectory user@host:/home/user$ ls -l total three -rwxrwxrwx ane user user 0 November nineteen 20:thirteen file1 drwxrwxrwx 2 user user 4096 Nov xix 20:13 folder -rwxrwxrwx 1 user user 0 Nov 19 20:13 file2
Recursive chmod using notice, pipemill, and sudo
To assign reasonably secure permissions to files and folders/directories, it'south common to give files a permission of 644, and directories a 755 permission, since chmod -R assigns to both. Use sudo, the find control, and a pipemill to chmod equally in the following examples.
To alter permission of only files under a specified directory.
user@host:/dwelling house/user$ sudo discover /path/to/someDirectory -type f -print0 | xargs -0 sudo chmod 644 user@host:/home/user$ ls -l full 3 -rw-r--r-- 1 user user 0 Nov 19 20:xiii file1 drwxrwxrwx 2 user user 4096 Nov nineteen twenty:13 folder -rw-r--r-- ane user user 0 November 19 20:13 file2
To modify permission of just directories nether a specified directory (including that directory):
user@host:/domicile/user$ sudo find /path/to/someDirectory -type d -print0 | xargs -0 sudo chmod 755 user@host:/abode/user$ ls -l total 3 -rw-r--r-- 1 user user 0 Nov 19 20:thirteen file1 drwxr-xr-x 2 user user 4096 Nov xix xx:thirteen binder -rw-r--r-- one user user 0 November 19 20:13 file2
Warning with Recursive chmod
Alert: Although it's been said, it'southward worth mentioning in context of a gotcha typo. Please note, Recursively deleting or chown-ing files are extremely dangerous. You lot volition not be the outset, nor the last, person to add together 1 too many spaces into the command. This example will hose your system:
user@host:/home/user$ sudo chmod -R / home/john/Desktop/tempfiles
Notation the space between the kickoff / and abode.
Yous accept been warned.
Changing the File Possessor 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 modify the foobar file'southward group to penguins, you lot could utilize either chgrp or chown with special syntax:
user@host:/habitation/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 exist:
user@host:/home/user$ sudo chown tux:penguins foobar
Note that, by default, you must use sudo to change a file's owner or group.
Volume Permissions with umask
This section has been moved to: Fstab#Options
ACL (Admission Command List)
Posix ACLs are a way of achieving a effectively granularity of permissions than is possible with the standard Unix file permissions. Encounter the full page on ACLs FilePermissionsACLs
Setting upwards ACL
- Install the acl package:
sudo apt-get install acl
-
Edit /etc/fstab and add option acl to partition(s) on which y'all want to enable ACL. For instance:
... UUID=d027a8eb-e234-1c9f-aef1-43a7dd9a2345 /home ext4 defaults,acl 0 2 ...
- Remount partition(due south) on which y'all want to enable ACL. For example:
sudo mount -o remount /home
-
Verify acl is enabled on the partition(s):
mountain | grep acl
The commands, setfacl and getfacl, set and read ACLs on files and directories.
Instance Usage
This is a unproblematic example for employ with a Samba share to ensure that any files or sub-directories created could besides be modified past any Samba user.
- Create a directory with full permission:
mkdir shared_dir chmod 777 shared_dir
-
Set the default ACL with '-d' and modify with '-thou' the permissions for samba nobody user nogroup group which volition apply to all newly created file/directories.
setfacl -d -one thousand u:nobody:rwx,g:nogroup:rwx,o::r-x shared_dir
GUI ACL Editor
The Eiciel
Useful ACL Resource
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http://brunogirin.blogspot.com/2010/03/shared-folders-in-ubuntu-with-setgid.html
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http://wiki.kaspersandberg.com/doku.php?id=howtos:acl
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man acl
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homo setfacl
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man getfacl
File removal
To remove a file you lot cannot delete employ
sudo rm -rf filename
where filename is the name and path of the file to delete.
Nota bene: Exist very careful when using the command rm with the -rf option since -r makes the file removal recursive (meaning it volition remove files inside of folders) and -f volition force the removal even for files which aren't writable. To play information technology safe, please consider typing in the absolute path to the file
sudo rm -rf /path/to/file/filename
to forbid any mishaps that can/volition occur. It takes longer to type only you can't put a price on peace of mind. Run across the rm man folio for details.
Viscid Fleck
The sticky bit applies but to directories, and is typically used on publicly-writeable directories. Within a directory upon which the sticky flake 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:
chmod +t <directory> chmod -t <directory>
The status of the sticky scrap 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 scrap is set, annihilation else indicates information technology is non.
Making a public directory:
user@host:/home/user$ mkdir folder user@host:/home/user$ chmod 777 folder user@host:/abode/user$ ls -l total 3 drwxrwxrwx 2 user user 4096 November 19 20:thirteen folder
Adding the viscous bit (note the "t" in the other execute field):
user@host:/home/user$ chmod +t binder user@host:/habitation/user$ ls -l total iii drwxrwxrwt 2 user user 4096 Nov 19 twenty:13 binder
Come across too
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man chmod
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man chown
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human chgrp
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FindingFiles
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User Individual Groups
ToDo
- umask (add together file and directory umask section, with specific focus on security)
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The User Private Group scheme. In other words, this page does the basics and bolts ok, only we demand to describe what the permissions should be. The default Ubuntu set upwards is not doubter: Every user has their default private group. Directories for collaboration demand to have special grouping and permission set for right functioning.
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* Suggestion: I oftentimes use find instead of chmod -R, because it's easier to differentiate between files and directories that manner. Yes, I know most the 'X' permission, but I don't trust it.
- The gluey bit. It's needed for "other" in shared directories similar /tmp. Information technology's needed for "group" in shared directories where write permission is given to a group, like /var/www
Source: https://help.ubuntu.com/community/FilePermissions
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