Lesson 1.10: Basics of Users, Groups & Permissions

Users

Type of Users

Type of Users	UID
Root User	0
System User	1-200
App User	201-999
Normal User	>=1000

Adding User

 
[root@client ~]# useradd ribik 
[root@client ~]# passwd ribik 
Changing password for user ribik.
New password: 
Retype new password: 
passwd: all authentication tokens updated successfully.

User's Database file : `/etc/passwd`

Format : ribik:x:1001:1001::/home/ribik:/bin/bash
< username >:< encrypted password >:< uid >:< gid >:< comment >:< home directory >:< shell >
Example: ribik:x:1001:1001::/home/ribik:/bin/bash

Groups

Type of Groups

Type of Groups	GID
Root User's Group	0
System User's Group	1-200
App User's Group	201-999
Normal User's Group	>=1000

Permissions

Conversions

r (read) = 4
w (write) = 2
x (execute) = 1

chmod 777 filename

Identity & Position

User = First or left-most
Group = Middle
Others = Last or right-most

chmod u=rwx,g=rw,o=x filename

Viewing Permissions on a file/directory

ls -l : Show Long Listing
Format : < Permissions > < hardlink > < owner name > < group name > < file size > < date & time of modification > < name >

 
[root@client ~]# ls -l 
total 4
-rw-------. 1 root root 1143 Dec  6  2023 anaconda-ks.cfg
drwxr-xr-x. 2 root root    6 Mar  7  2024 Desktop
drwxr-xr-x. 2 root root    6 Mar  7  2024 Documents
drwxr-xr-x. 2 root root    6 Mar  7  2024 Downloads
drwxr-xr-x. 2 root root    6 Mar  7  2024 Music
drwxr-xr-x. 2 root root    6 Mar  7  2024 Pictures
drwxr-xr-x. 2 root root    6 Mar  7  2024 Public
drwxr-xr-x. 2 root root    6 Mar  7  2024 Templates
drwxr-xr-x. 2 root root    6 Mar  7  2024 Videos

Permissions Field

The field is of 10 Character
< Type >< Owner >< Group >< Others >
Type ( - normal fiel ), ( d directory ), (l soft link), (b block device file), (c Character device file)

UMASK

umask is a value that determines default permission on a file/directory in the time of creation.

 
[root@client ~]# umask 
0022

Default value of permission for file is 644 (-rw-r--r--) and for folder is 755 (drwxr-xr-x).

 
[root@client class]# touch file1 && mkdir folder1 && ls -l
total 0
-rw-r--r--. 1 root root 0 Sep 21 09:37 file1
drwxr-xr-x. 2 root root 6 Sep 21 09:37 folder1

Formula to Calculate default permission of file

Maximum allowed permission at the time of file creation (666)
Substract : Max allowed Permission (666) - umask value (022) : 644 (Default value)

Formula to Calculate default permission of directory

Maximum allowed permission at the time of directory creation (777)
Substract : Max allowed Permission (777) - umask value (022) : 755 (Default value)

Chaning the umask Command

Temporary umask change

umask <umask value>

Permanently umask change

CASE I: User-specific umask change
- Go to .bashrc and add the line umask <umask value>
CASE II: System-wide umask change
- Go to /etc/.basrc and the line umask <umask value>

How to calculate the required umask value for file ?

Suppose we want a file to have only read write permission for the owner. (-rw-------)
Using the Substraction method :
What is the substractor ( 666 - ??? = 600 )
( 666 - 066 = 600 ), 066 is the umask value.

Example : Temporarily changing Umask value for file

 
# Default umask value
[sanjeeb@client ~]$ mkdir class && cd class && touch file1 && ls -lh 
total 0
-rw-r--r--. 1 sanjeeb sanjeeb 0 Sep 21 09:51 file1
[sanjeeb@client class]$ umask 
0022
 
# Changing the umask value temporarily
[sanjeeb@client class]$ umask 066
[sanjeeb@client class]$ umask 
0066
 
# Creating a file and viewing permission
[sanjeeb@client class]$ touch file2 && ls -lh file2 
-rw-------. 1 sanjeeb sanjeeb 0 Sep 21 09:53 file2

How to calculate the required umask value for directory ?

Suppose we want a file to have only read write permission for the owner. (-rwx------)
Using the Substraction method :
What is the substractor ( 777 - ??? = 700 )
( 777 - 077 = 700 ), 077 is the umask value.

Example : Temporarily changing Umask value for directory

 
# Default permission of folder
[sanjeeb@client class]$ mkdir dir1 | ls -lh 
total 0
drwx--x--x. 2 sanjeeb sanjeeb 6 Sep 21 09:56 dir1
 
# Changing the umask value temporarily
[sanjeeb@client class]$ umask 077
 
# Viewing the permission after change
[sanjeeb@client class]$ mkdir dir2 && ls -lh dir2
total 0
drwx------. 2 sanjeeb sanjeeb 6 Sep 21 09:58 dir2

Manipulating Permissions

There are also operators to manipulate the permissions:

Task	Operator
Grant a level of access	+
Remove a level of access	-
Set a level of access	=

 
[sanjeeb@server Documents]$ ls -l 
total 0
-rw-r--r--. 1 sanjeeb sanjeeb 0 Apr  6 21:25 file1

 
[sanjeeb@server Documents]$ chmod 777 file1 
[sanjeeb@server Documents]$ ls -l 
total 0
-rwxrwxrwx. 1 sanjeeb sanjeeb 0 Apr  6 21:25 file1

 
[sanjeeb@server Documents]$ chmod u=rwx,g=rw,o=r file1 
[sanjeeb@server Documents]$ ls -l
total 0
-rwxrw-r--. 1 sanjeeb sanjeeb 0 Apr  6 21:25 file1

Example 1: Change the permission of the /home/sanjeeb/Documents/file1 such that only the owner gets full permission and groups members & others do not get any permission. Use `symbolic` method to change the permission.

 
[sanjeeb@server Documents]$ ls -lh
total 0
----------. 1 sanjeeb sanjeeb 0 Apr  6 21:25 file1
[sanjeeb@server Documents]$ chmod u=rwx,g=,o= file1 
[sanjeeb@server Documents]$ ls -lh
total 0
-rwx------. 1 sanjeeb sanjeeb 0 Apr  6 21:25 file1

Example 2: Change the permission of the /home/sanjeeb/Documents/file2 such that the owner and the group members get read and execute permission and others get read only permission. Use `numeric` method to change the permission.

 
[sanjeeb@server Documents]$ chmod 554 file2
[sanjeeb@server Documents]$ ls -lh
total 0
-r-xr-xr--. 1 sanjeeb sanjeeb 0 Apr  7 06:39 file2
[sanjeeb@server Documents]$

Example 3: Grant all privileges to all users to the directory /home/sanjeeb/Documents/folder1. Make sure that full privileges is granted to all users on that directory as well as to the contents and sub-contents of the directory at one shot. Use `numeric` method to change the permission.

 
[sanjeeb@server Documents]$ chmod -R 777 folder1/
 
[sanjeeb@server Documents]$ ls -lh
total 0
drwxrwxrwx. 2 sanjeeb sanjeeb 45 Apr  7 06:45 folder1
 
[sanjeeb@server Documents]$ cd folder1
[sanjeeb@server folder1]$ ls -lh
total 0
-rwxrwxrwx. 1 sanjeeb sanjeeb 0 Apr  7 06:45 file1
-rwxrwxrwx. 1 sanjeeb sanjeeb 0 Apr  7 06:45 file2
-rwxrwxrwx. 1 sanjeeb sanjeeb 0 Apr  7 06:45 file3

Example 4: Removing rwx permission from file1 for other users

 
[sanjeeb@server folder1]$ chmod o-rwx file1
[sanjeeb@server folder1]$ ls -lh
total 0
-rwxrwx---. 1 sanjeeb sanjeeb 0 Apr  7 06:45 file1

Example 5: Removing execute permission from owner, write-execute permission from group and everything from other in file2

 
[sanjeeb@server folder1]$ chmod u-x,g-wx,o= file2 
[sanjeeb@server folder1]$ ls -lh | grep file2
-rw-r-----. 1 sanjeeb sanjeeb 0 Apr  7 06:45 file2

Lesson 1.10: Basics of Users, Groups & Permissions

Users

Type of Users

Adding User

User's Database file : /etc/passwd

Groups

Type of Groups

Permissions

Conversions

Identity & Position

Viewing Permissions on a file/directory

Permissions Field

UMASK

Formula to Calculate default permission of file

Formula to Calculate default permission of directory

Chaning the umask Command

Temporary umask change

Permanently umask change

How to calculate the required umask value for file ?

Example : Temporarily changing Umask value for file

How to calculate the required umask value for directory ?

Example : Temporarily changing Umask value for directory

Manipulating Permissions

There are also operators to manipulate the permissions:

Example 1: Change the permission of the /home/sanjeeb/Documents/file1 such that only the owner gets full permission and groups members & others do not get any permission. Use symbolic method to change the permission.

Example 2: Change the permission of the /home/sanjeeb/Documents/file2 such that the owner and the group members get read and execute permission and others get read only permission. Use numeric method to change the permission.

Example 3: Grant all privileges to all users to the directory /home/sanjeeb/Documents/folder1. Make sure that full privileges is granted to all users on that directory as well as to the contents and sub-contents of the directory at one shot. Use numeric method to change the permission.

Example 4: Removing rwx permission from file1 for other users

Example 5: Removing execute permission from owner, write-execute permission from group and everything from other in file2

User's Database file : `/etc/passwd`

Example 1: Change the permission of the /home/sanjeeb/Documents/file1 such that only the owner gets full permission and groups members & others do not get any permission. Use `symbolic` method to change the permission.

Example 2: Change the permission of the /home/sanjeeb/Documents/file2 such that the owner and the group members get read and execute permission and others get read only permission. Use `numeric` method to change the permission.

Example 3: Grant all privileges to all users to the directory /home/sanjeeb/Documents/folder1. Make sure that full privileges is granted to all users on that directory as well as to the contents and sub-contents of the directory at one shot. Use `numeric` method to change the permission.