welcome to the video tutorial on setting
file permissions on Linux security is a
big concern for Linux which is a clone
of Unix
the multi-user operating system as Linux
is used in mainframes and servers it is
vital to keep it safe from a maligned
user who can corrupt change or remove
crucial data this is why for effective
security Linux divides authorization
into two levels ownership and permission
the concept of permissions and ownership
is crucial and Linux here we will
discuss both of them let's start with
ownership and the next files every file
in directory in your UNIX or Linux
system is assigned three types of owner
the first one among them is user a user
is the owner of the file by default the
person who created a file becomes its
owner hence a user is also sometimes
called an owner the next one is a group
a user group can contain multiple users
all users belonging to a group will have
the same access permissions to the file
so you can add many users to a group and
assign group permission to a file so
that only the group members can read or
modify them
furthers other who is any other user who
has access to a file
this person has neither created the file
nor does he belong to a user group which
owns the file practically it means
everybody else hence when you set the
permission for others it is also
referred as set permissions for world
now the big question arises how does
Linux distinguishes between these three
user types so that a user a cannot
affect a file which contains some other
user B's vital information or data it is
like you do not want your colleague who
works on your Linux computer to view
your personal images this is where
permissions set in and they define user
behavior let us understand the
permission system on Linux every file in
directory and your UNIX or Linux system
has three permissions read write and
execute define for all the three users
that we discussed earlier
this permission gives you the authority
to open and read a file read permission
on a directory gives you the ability to
list its content the write permission
gives you the authority to modify the
contents of the file the write
permission on a directory gives you the
authority to add remove and rename files
stored the directory in windows an
executable program usually has an
extension exe which you can easily run
in UNIX or Linux you cannot run a
program unless the execute permission is
set for example in the sample file here
the user does not have the permission to
execute it hence we would get an error
message if we try to open it therefore
if the execute permission is not set you
will not be able to run the file let us
learn more about Linux security by
examples running the LS L command on
terminal gives us the following result
by picking the details for the first
file we get to know that the file types
and access permissions are displayed by
this weird-lookin code let us learn what
it is about let's first learn about the
characters R stands for read permission
W is for write permission X is execute
permission and the - stage no permission
here the first - implies that we have
selected a file else if it were a
directory D would have been shown the
next part of the code is our W - and it
tells us the permissions for the Amna
this suggests that the owner can read
the file write or edit the file but he
cannot execute the file since the
execute bit is set to - which is no
permission the next part is our w - it
is for the user group which is going 99
and the group members can read the file
and write or edit the file by design
many Linux distributions like fedora
sent us
Ubuntu etc we'll add users to a group of
the same group name as the username thus
a user Tom is added to a group named Tom
the third part is for the world which
means any user it says are - - this
means that the user can only read the
file
moving on let's say you do not want your
colleague to see your personal images
you can easily do so by changing file
permissions we can use the chmod command
which stands for change mode using the
command we can set permissions is that
read write and execute on a file or
directory for the owner group and the
world there are two ways to use the
command the first one is the absolute
mode and the second one is the symbolic
mode starting with the first one
in this mode file permissions are not
represented as characters but a
three-digit octal number the table shows
numbers for all the permission types for
example the number 0 states no
permission and the symbol for it is
three hyphens successively the number 1
would be 4 execute and the symbol for
the same is - - X let us understand this
by an example
here we have a file with the main test
we will change the permissions for this
file using the absolute mode of chmod
command here we use the seven six for
absolute code and now the permissions
for test file would change let's examine
the changes to the permissions seven six
for absolute code says the following
read write and execute for the user or
the owner read and write for the user
group and read only for other users so
this is how we can change the
permissions using the absolute mode of
the chmod command moving on to the next
way in the absolute mode we change
permissions for all three owners but on
the symbolic mode you can modify
permissions of a specific owner as well
it makes use of mathematical symbols to
modify the file permissions so the plus
sign would mean adding a permission to a
file or directory - would suggest
removing the permission and equal to
we'll set the permission and override
the permissions set earlier the various
owners are represented as u G o and a
let us learn it with some examples again
we would use the same test file and
change its permissions using the
symbolic mode by using oh is equal to r
WX we have changed permissions for the
other users
let us check what the new permissions
are as you can see here the other users
have the read write and execute
permission for this test file with G + X
we have added execute permission to the
group now if you will check the test
file it will show the read write and
execute permission for the crew by
running u- r we will remove the read
permission for the user or the owner of
the file let us see how it looks now as
we can see the permissions for test file
do not show any read permission for the
owner this is how we can use the two
methods for chmod command and change
permissions for changing the ownership
of a file or directory you can use this
command in case you want to change the
user as well as group for a file or
directory
you need to use this command that is try
them on terminal let us change the file
ownership for the file command right now
it shows that the owner of the file is
guru 99 by running the town command with
sudo we would be able to change the
owner of this file to root
as you can see the new owner for the
file commands is root now let's try
changing the user group and group
ownership of the same file right now the
owner of the file is root and the user
group of the file is also root we will
change it to guru 99 for this we would
need to run the command sudo shower on
the owner name then the group name and
then the name of the file let us see the
changes in the user and group ownership
as you can see here the commands file is
now owned by guru 99 and it also belongs
to the same user group in case you want
to change only the group owner of a file
you will need to run the following
command here we will again change the
group owner of the file commands to root
the chgrp command stands for change
group now let us go through some
important tips on user groups the file
etc' group contains all the groups
defined in the system in order to reach
this file you need to go to the file
system click on EDC directory and then
search for the group file upon opening
this file you would be able to see all
the groups present on your Linux system
you can use the command groups to find
all the groups you are a member of
so it shows here that the guru 99 user
is a member of the following groups you
can use the command new GRP to work as a
member of a new group other than your
default group let's see how it works let
us now change the user group for the
user guru 99 to cd-rom let's create a
new file as test
upon checking the detailed information
for this file you will find that they
use a group for the test file is cd-rom
instead of the default guru 99 the next
step says that you cannot have two
groups owning the same file the one
group can be a subgroup of another which
can own a file let's now quickly
summarize what we have learnt the next
being a multi-user system uses
permissions and ownerships
for security they are free user types on
a Linux system user group and other
Linux divides the file permissions in to
read write and execute denoted by are W
and X the permissions on a file can be
changed by ch mod command which can be
further divided into absolute and
symbolic mode the CH o W and command can
change the ownership of a file or a
directory use the following command for
changing the user or the user and the
user group at the same time the chgrp
command can change the group ownership
the command is chgrp group and the file
name thank you for watching I will see
you in the next tutorial