CHMOD UBUNTU

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

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.

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

Folders (directories) must have 'execute' permissions set (x or 1), or folders (directories) will NOT FUNCTION as folders (directories) and WILL DISAPPEAR from view in the file browser (Nautilus).

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
a	all (same as ugo)
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 -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.

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

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 (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

1. Install the acl package:

sudo apt-get install acl

2. Edit /etc/fstab and add option acl to partition(s) on which you want to enable ACL. For example:

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

3. Remount partition(s) on which you want to enable ACL. For example:

sudo mount -o remount /home

4. Verify acl is enabled on the partition(s):

mount | grep acl

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

Example Usage

This is a simple example for use with a Samba share to ensure that any files or sub-directories created could also be modified by any Samba user.

1. Create a directory with full permission:

mkdir shared_dir
chmod 777 shared_dir

2. 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

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

Useful ACL Resources

• http://brunogirin.blogspot.com/2010/03/shared-folders-in-ubuntu-with-setgid.html
• http://wiki.kaspersandberg.com/doku.php?id=howtos:acl
• man acl
• man setfacl
• man getfacl

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.

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:

chmod +t <directory>
chmod -t <directory>

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

• man chmod
• man chown
• man chgrp
• FindingFiles
• User Private Groups

---

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 

--------------

Hak akses folder + linux

Pada Sistem Operasi Linux semua file memiliki hak aksesnya masing-masing. Hak akses tersebut terdiri atas tiga bagian:

r untuk read (membaca)
w untuk write (menulis)
x untuk execute (menjalankan)
Untuk melakukan check terhadap hak akses suatu file bisa dilakukan dengan menjalankan perintah ls -la pada sebuah shell atau konsol. Berikut salah satu contoh pada saat saya menjalankan perintah ini di dalam folder /home/

# ls -l
drwxrwxrwx 3 root users 4096 1996-02-02 10:59 allusers
drwxr-xr-x 14 gagoyiku gagoyiku 4096 1996-02-02 08:36 gagoyiku
drwxrwxr-x 2 root users 4096 1996-02-02 08:37 windowshare

Berikut adalah penjelasan kolom-kolom yang saya anggap paling penting untuk diketahui artinya.

Kolom pertama pada hasil diatas adalah yang menggambarkan perijinannya, terdiri atas 10 karakter.
Karakter pertama akan menunjukkan apakah objek tersebut adalah sebuah direktori (d), file (-), atau sebuah link ( l ) yang merujuk kepada direktory atau file lainnya
3 karakter selanjutnya akan memperlihatkan ijin untuk membaca, menulis dan menjalankan objek dimaksud, bagi si pemilik objek
3 karakter selanjutnya akan memperlihatkan ijin untuk group pengguna yang mengatur objek
3 karakter selanjutnya akan memperlihatkan ijin untuk pengguna yang lain
Kolom ke tiga akan menunjukkan pemilik objek
Kolom ke empat akan menunjukkan group pengguna pemilik objek tersebut
Kolom terakhir menunjukkan nama dari objek di system
Dari hasil jalannya perintah sebelumnya, bisa kita baca bahwa pemilik dari direktori windowshare adalah root dan group pengguna yang memiliki direktori tersebut adalah group users yang memiliki hak untuk baca, menulis dan menjalankan berbagai macam operasi di folder tersebut; sedangkan pengguna lainnya yang tidak termasuk root dan anggota group users hanya bisa membaca dan menjalankan file (read only).

Namun yang harus kita ingat bahwa user root memiliki kemampuan untuk melakukan apa saja terhadap hak akses tersebut.

Sebagai sebuah persetujuan awal, apabila saya mempergunakan kata file, maka ini bisa merujuk pada file data atau folder.

Merubah Hak Akses Suatu File
Perintah chmod “Numeric Mode”
Perintah ini akan merubah perijinan suatu file/direktori menggunakan kode akses berupa 3 digit nomor tertentu, yang merupakan perwujudan dari hak akses suatu file di Linux. Masing-masing kode tersebut adalah 4 untuk membaca (read), 2 untuk menulis, dan yang terakhir adalah 1 untuk menjalankan sebuah file.

Sebagai contoh, kita ingin sebuah file hanya bisa untuk di baca (4) dan di tulis (2) tapi tidak untuk di jalankan, maka kita bisa mempergunakan perintah 4+2 = 6. Menggunakan cara yang sama apabila kita ingin memberikan hak akses hanya untuk membaca (4), dan memberikan semua hak akses yang ada (7 = 1+2+4).

Lalu kode akses tersebut di kombinasikan berdasarkan urutan ~ hak akses untuk pemilik, group pemilik dan pengguna lain ~ hak kepemilikan sebuah file, dengan sintak perintahnya adalah:

chmod

Sebagai contoh berdasarkan perintah ls -l sebelumnya, kita akan melakukan setting agar folder windowshare bisa di pergunakan oleh semua pengguna agar bisa menulis, membaca, dan menjalankan file di folder tersebut, maka kita mempergunakan perintah:

# chmod 777 /home/windowshare

Sehingga bila kita perlihatkan lagi hak akses menggunakan perintah ls -l, akan kita dapatkan hasil seperti berikut:

# ls -l
<>
drwxrwxrwx 2 root users 4096 1996-02-02 08:37 windowshare
<>

Perhatikan sekarang kode akses yang menjadi drwxrwxrwx, dari yang semulanya drwxrwxr-x.
Namun apabila kita menginginkan hanya si pemilik file saja yang memiliki hak akses dan yang lainnya (bahkan group pemiliknya) hanya memiliki akses membaca saja (read only), kita bisa menggunakan perintah:

# chmod 744 /home/windowshare
# ls -l
<>
drwxr-xr-x 2 root users 4096 1996-02-02 08:37 windowshare
<>

Atau kalau si pemilik saja yang memiliki hak akses, maka kita bisa menjalankan perintah:

# chmod 700 /home/windowshare
# ls -l
<>
drwx—— 2 root users 4096 1996-02-02 08:37 windowshare
<>

Apabila kita ingin mengubah hak akses di folder beserta semua isinya, maka dibutuhkan tambahan perintah berupa tanda -R (recursive). Sehingga bila kita ingin mengubah hak akses di folder /home/windowshare beserta isinya, kita tinggal menjalankan perintah:

# chmod -R 700 /home/windowshare

Perintah chmod “Symbolic Mode”
Kalau pada Numeric Mode menggunakan angka-angka, maka pada symbolic mode mempergunakan huruf yang bisa dikombinasikan. Alhasil perintahnya lebih mudah untuk dimengerti. Berikut sintak penulisannya:

chmod [flags] [u/g/o/a] [+/-/=] [r/w/x]

Kombinasi [u/g/o/a] digunakan untuk mengatur hak akses pengguna, yaitu u (pengguna yang memilikinya), g (group yang memilikinya), o (other/pengguna lain yang bukan termasuk dalam group pemiliknya), atau a (all – semua pengguna). Operator untuk + (melakukan setting/menambah), – (mengurangi hak akses) dan = (set hak akses) harus dikombinasikan dengan perintah pilihan selanjutnya yaitu r (read – membaca), w (write – menulis) dan x (execute – menjalankan) sebuah file.

Sebagai contoh kita dasarkan pada contoh sebelumnya. Misalkan kita ingin agar folder windowshare hanya bisa dipergunakan oleh pemiliknya saja:

#chmod u+rwx,og-rwx /home/windowshare

Atau kita ingin agar semua orang hanya memiliki hak akses untuk membaca saja (read only)

#chmod a+rx-w /home/windowshare

Kita juga bisa memberikan setting hak akses sekaligus untuk isi folder tersebut (recursive)

#chmod -R a+rx-w /home/windowshare

Mengubah Kepemilikan File
Untuk mengubah kepemilikan sebuah file kita bisa mempergunakan perintah chown yang memiliki format yang sama dengan perintah chmod. Bedanya yang kita ubah adalah kepemilikan sebuah file. Sintak yang digunakan adalah:

chown

Misalnya kita ingin mengubah kepemilikan folder windowshare diatas, dari root kepada user dengan login linuz, maka kita tinggal melakukan perintah:

# chown linuz /home/windowshare

Mengubah Group Pemilik File
Untuk mengubah group pemilik sebuah file kita bisa mempergunakan perintah chgrp yang juga memiliki format yang sama dengan perintah chown. Bedanya yang kita ubah adalah group pemiliknya. Misalkan kita ingin mengubah group pemilik folder windowshare diatas, dari users kepada group linuzgroup, kita tinggal melakukan perintah:

#chgrp linuzgroup /home/windowshare

http://ulunrapuydebian.wordpress.com/2008/07/05/hak-aksesperijinan-file-di-linux/