So Linux operating system are not only multitasking but also multi-user(different users).
multi-user means that more than one person can be operating the computer at the same time.
Now let's see how to read and set file permissions in different format.
File Permissions
Linux system has permission for different files and directory's assigned access rights for the owner or user(u) of the file, the members of a group(g) of related users, and everybody(o) else.
Linux system divides the authorization in to two
Ownership
Permission
Linux File or Dir Ownership
There are 3 types of owner
User By default, the person who created a file becomes its owner
Group contains multiple user and all users in a group have the same permission.
Other everybody else, anybody has permission
Linux File or Dir Permissions
There are 3 types of permission
Read gives permission to open and read files and the ability to lists its content on directory.
Write gives permission to modify a file and on a directory to add, remove and rename files stored in the directory.
Execute gives permission to execute or run a file.
To view the permission of a file or Dir use the command:
ls -l
r = read permission
w = write permission
x = execute permission
– = no permission
The first ‘–‘ implies that we have selected a file.
We use the command chmod to set permission
chmod <permission> <file and dir>
There are two ways of setting a permission
Absolute mode
Symbolic mode
Absolute(Numeric) Mode in Linux
The absolute(numeric) mode uses numeric format to specify ownership and permission.
To really learn how find the number we need to learn how to turn binary's into numbers, don't worry if you don't get it you can just memorize the numbers but turning binary into numbers is a really easy way to do it.
Binary to numbers
It is really easy to turn binary to number just multiply each bit with 2^n then add all the number to get the decimal.
Example
110 = 2^2 * 1 + 2^1 * 1 + 2^0 *0 = 4*1+2*1+1*0 = 4+2+0 = 6
100 = 2^2 * 1 + 2^1 * 0 + 2^0 *0 = 4*1+2*0+1*0 = 4+0+0 = 4
Here is how absolute(numeric) mode works
rwx rwx rwx = 111 111 111 = 777
rw- rw- rw- = 110 110 110 = 666
rwx --- --- = 111 000 000 = 700
So basically we consider - as 0 bit and r,w or x as 1 bit
rwx = 111 = 2^2*1 + 2^1*1 + 2^0*1 = 7
rw- = 110 = 2^2*1 + 2^1*1 + 2^0*0 = 6
r-x = 101 = 2^2*1 + 2^1*0 + 2^0*1 = 5
r-- = 100 = 2^2*1 + 2^1*0 + 2^0*0 = 4
--- = 000 = 2^2*0 + 2^1*0 + 2^0*0 = 0
Now that we understand how the mode works we need to use it
To give permission of -rwx--xr-x = 715 use the command
chmod 715 <file_name or dir name>
Symbolic Mode in Linux
The Symbolic mode uses Symbols to modify permissions of a specific owner and use of mathematical symbols to modify the Unix file permissions.
+ Adds a permission to a file or directory
– Removes the permission
= Sets and overrides the permissions set earlier.
Owners are represented as
u user/owner
g group
o other
a all
How to use this
Adding permission
to add execute permission to the user use the + operator
chmod u+x <file or dir>
Remove permission
to remove read permission to the group use the - operator
chmod g-r <file or dir>
setting permission
to set a read and write permission to all use the operator =
chmod a=rw <file or dir>
Now that we seen how permission work, we will see some commands and how to use them.
chmod
The first command we will see is chmod which is used to change the permissions of a file or directory.
To use the command:
chmod <PERMISSION_MOD> <FILE_OR_DIR>
Usually implemented options include:
-R Recursive, i.e. include objects in subdirectories.
-v verbose, show objects changed (unchanged objects are not shown).
Use --reference=REF_FILE to set the permission of the new file relative to the ref_file.
chown --reference=REF_FILE FILE
Tip: for more info checkout chmod or use man chmod
su
su is a program that can give you temporary access to the superuser's privileges.
To exit the superuser session, type exit and we will return to your previous session.
su
su <USER_NAME>
To exit a shell use exit command
exit
sudo
sudo command is used to execute a command as the superuser, the desired command is simply preceded with the sudo command.
sudo <SOME_COMMAND>
chown
We use the chown command to change the ownership of a file, like changing the owner of file1 form me to you.
chown [OPTIONS] USER[:GROUP] FILE(s)
chown <CHANGED_OWNER> <FILE>
you can also change the owner and the group at the same time
chown USER:GROUP FILE
To recursively operate on all files and directories under the given directory, use the -R (--recursive) option.
chown -R USER:GROUP DIRECTORY
The --reference=ref_file option allows you to change the user and group ownership of given files to be same as those of the specified reference file (ref_file).
chown --reference=REF_FILE FILE
chgrp
We use the chgrp command to change the group ownership of a file or directory.
chgrp <NEW_GROUP> <FILE>
Tip: for more info use the man page
id
We use the id command to print the user and group name and ID of the current user or any other user in the server.
id [OPTION]… [USER]
use the man page for more info on options
groups
We use the groups command to prints the names of the primary and any supplementary groups for each given username and manage users with the same security and access privileges.
groups [username]...
use the man page for more info
whoami
we use the whoami to displays user, group and privileges information for the user who is currently logged on to the local system.
whoami
use the man page for more info
adduser
we use the adduser to add a new user to your current Linux machine.
But you need to install adduser using the command
sudo apt-get install adduser
adduser <username>
for more info read adduser
useradd
we use the useradd to add user accounts to your system.
useradd [options] name_of_the_user
for more info read useradd
addgroup
we use the addgroup to add a new group to your current Linux machine.
sudo addgroup <groupname>
for more info read addgroup
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