Rights and Permissions

Permissions

Unix is a multi-users system. So some users have some rights to do something and some other don't.
The system allows to protect your data from an other user.
The system allows some users to work in groups.

All this features are managed by the permissions (or right access)

3 classes of permissions

$ ls -l
total 1176
drwxr-xr-x 4 bneron CIB   4096 Mar  4  2014 Alignment
-rw-r--r-- 1 bneron CIB 220196 Mar  4  2014 CIP-55-138_A.b_A06_037.ab1
drwxr-xr-x 3 bneron bneron   4096 Jun  4 14:51 HMM

U: user
G: group
O: others
r: read      (4)
w: write     (2)
x: execute   (1)

rwx what does it really mean?

		file	directory
r	4	read	allow to list
w	2	write	allow to create and delete file and directories
x	1	the file is executable	allow to enter and pass through the directory

who am I?

Permisions are based on your identity and the groups to which you belong.

whoami => give you the login you logged with

id <login> => give the id and the groups to which the login belongs (login is optional)

bneron@bic-t2a:~$id
uid=2896(bneron) gid=3044(CIB) groups=3044(CIB),87(biok),110(sis),547(mobyle),990(gensoft),3160(vpn-ssl-users),20000(bioweb)

uid is my login
gid is my primary group
groups is the list of all groups to which I belong (max 16 groups).

change permissions

chmod [ugo] [+-] [rwx] <target>

chmod [ugo] [+-] [rwx] <target>

drwxr-xr-x 4 bneron CIB   4096 Mar  4  2014 Alignment
chmod g+w Alignment
drwxrwxr-x 4 bneron CIB   4096 Mar  4  2014 Alignment
chmod o-rx  Alignment
drwxrwx--- 4 bneron CIB   4096 Mar  4  2014 Alignment
chmod g-wo+x Alignment
chmod: invalid mode: 'g-wo+x'
chmod go +rx Alignment
drwxrwxr-x 4 bneron CIB   4096 Mar  4  2014 Alignment

you can use -R option to apply permission to a directory and recursively to its contents.

change permissions syntax 2

chmod num_value <target>

num value is composed of 3 digits respectively for
user, group, other
each value is the sum of the octal symbols
for instance for w => 4, read rw => 4+2 = 6 ...

-rw-r--r-- 1 bneron sis 0 Oct 16 13:59 file1
chmod 755 file1
-rwxr-xr-x 1 bneron sis 0 Oct 16 13:59 file1
chmod 640 file1
-rw-r----- 1 bneron sis 0 Oct 16 13:59 file1

octal notation

Symbolic Notation	Octal Notation	English
----------	000	no permissions
---x--x--x	111	execute
--w--w--w-	222	write
--wx-wx-wx	333	write & execute
-r--r--r--	444	read
-r-xr-xr-x	555	read & execute
-rw-rw-rw-	666	read & write
-rwxrwxrwx	777	read, write & execute

exercises:

create files and directories,

change their permissions.

permissions by default

To control the file/dir's permissions at their creation.

umask display the mask in octal notation: 0022
umask -S display the mask in symbolic notation: u=rwx,g=rx,o=rx

umask is the complement to 7 to the permissions applied

umask = 0022

directories: permissions are 777
7-0=7, 7-0=7, 7-2=5, 7-2=5
directories will be created with 755 rights (rwxr-xr-x)

files is same as dir except, never x by default
7-0=7, 7-0=7, 7-2=5, 7-2=5 then remove x if necessary
files will be created with 644 rights (rw-r--r--)

octal codes meaning

Octal digit in umask command	Allows on directory	allows on file
0	read, write,execute	read, write
1	read and write	read, write
2	read and execute	read
3	read only	read
4	write and execute	write
5	write only	write
6	execute only	no permissions
7	no permissions	no permissions

setting the umask

umask octal_value
umask -S symbolic value

umask 001
umask -S
u=rwx,g=rx,o=rx
umask -S o+w
u=rwx,g=rx,o=rwx

exercise:

create dir and files (use touch) with different umask

check their permissions with ls -l

To set x on file you need to use chmod.

change owner/group

chown command (for change ow ner)

chown owner file/dir
chown owner.group file/dir

the ownership of a file may only be altered by a super-user

chgrp group file/dir

Warning

Only a member of a group can change a file's group
And only to one of which they are a member.

-R option recurse through subdirectories.