The Ability to Both Read and Modify a File.

Linux is a clone of UNIX, the multi-user operating organisation which tin be accessed past many users simultaneously. Linux tin can likewise be used in mainframes and servers without whatsoever modifications. But this raises security concerns every bit an unsolicited or malign user can decadent, modify or remove crucial data. For constructive security, Linux divides say-so into 2 levels.

1. Ownership
2. Permission

In this Linux file commands tutorial, you volition larn-

• Linux File Ownership
• Linux File Permissions
• Irresolute file/directory permissions in Linux Using 'chmod' command
• Absolute(Numeric) Mode in Linux
• Symbolic Way in Linux
• Changing Ownership and Group in Linux

The concept of Linux File permission and buying is crucial in Linux. Hither, nosotros will explicate Linux permissions and ownership and volition discuss both of them. Allow us first with the Buying.

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Linux File Buying

Every file and directory on your Unix/Linux system is assigned 3 types of owner, given below.

User

A user is the owner of the file. By default, the person who created a file becomes its possessor. Hence, a user is also sometimes called an owner.

Group

A user- group can contain multiple users. All users belonging to a grouping volition have the same Linux group permissions access to the file. Suppose you have a projection where a number of people crave access to a file. Instead of manually assigning permissions to each user, you could add all users to a group, and assign group permission to file such that only this group members and no ane else can read or modify the files.

Other

Any other user who has admission to a file. This person has neither created the file, nor he belongs to a usergroup who could own the file. Practically, it means everybody else. Hence, when you prepare the permission for others, it is likewise referred as prepare permissions for the world.

Now, the big question arises how does Linux distinguish betwixt these three user types so that a user 'A' cannot affect a file which contains another user 'B's' vital information/data. It is like y'all do not want your colleague, who works on your Linux estimator, to view your images. This is where Permissions prepare in, and they define user behavior.

Allow us understand the Permission system on Linux.

Linux File Permissions

Every file and directory in your UNIX/Linux organization has following iii permissions defined for all the 3 owners discussed above.

• Read: This permission give you the authority to open up and read a file. Read permission on a directory gives you the power to lists its content.
• Write: The write permission gives you the authority to modify the contents of a file. The write permission on a directory gives you the authorization to add, remove and rename files stored in the directory. Consider a scenario where yous take to write permission on file but do not have write permission on the directory where the file is stored. You volition exist able to change the file contents. But y'all will not be able to rename, move or remove the file from the directory.
• Execute: In Windows, an executable program usually has an extension ".exe" and which you can easily run. In Unix/Linux, you cannot run a program unless the execute permission is prepare. If the execute permission is not set, you might still be able to encounter/modify the plan code(provided read & write permissions are set), simply not run information technology.

File Permissions in Linux/Unix

Let's run across file permissions in Linux with examples:

ls – fifty on concluding gives

ls - l

Here, we have highlighted '-rw-rw-r–'and this weird looking code is the one that tells us nigh the Unix permissions given to the owner, user group and the world.

Here, the beginning '–' implies that we have selected a file.p>

Else, if it were a directory, d would take been shown.

The characters are pretty easy to recollect.

r = read permission
w = write permission
ten = execute permission
– = no permission

Let u.s.a. wait at it this mode.

The first part of the code is 'rw-'. This suggests that the owner 'Home' can:

• Read the file
• Write or edit the file
• He cannot execute the file since the execute chip is set to '-'.

By design, many Linux distributions like Fedora, CentOS, Ubuntu, etc. volition add users to a group of the same group name equally the user name. Thus, a user 'tom' is added to a group named 'tom'.

The 2d role is 'rw-'. Information technology for the user group 'Home' and grouping-members tin:

• Read the file
• Write or edit the file

The 3rd part is for the globe which means any user. It says 'r–'. This means the user can only:

• Read the file

Irresolute file/directory permissions in Linux Using 'chmod' command

Say you lot practice not want your colleague to see your personal images. This tin be achieved by changing file permissions.

We can employ the 'chmod' command which stands for 'change mode'. Using the control, we can gear up permissions (read, write, execute) on a file/directory for the owner, group and the earth.

Syntax:

chmod permissions filename

There are ii ways to use the control –

1. Accented mode
2. Symbolic way

Accented(Numeric) Fashion in Linux

In this mode, file permissions are non represented as characters merely a 3-digit octal number.

The table below gives numbers for all for permissions types.

Number	Permission Type	Symbol
0	No Permission	—
1	Execute	–ten
two	Write	-w-
iii	Execute + Write	-wx
four	Read	r–
five	Read + Execute	r-10
half-dozen	Read +Write	rw-
7	Read + Write +Execute	rwx

Let's see the chmod permissions control in activeness.

In the above-given concluding window, we have inverse the permissions of the file 'sample to '764'.

'764' absolute code says the following:

• Possessor can read, write and execute
• Usergroup can read and write
• World can simply read

This is shown as '-rwxrw-r–

This is how y'all tin can change user permissions in Linux on file by assigning an absolute number.

Symbolic Way in Linux

In the Accented mode, you change permissions for all iii owners. In the symbolic mode, you lot can modify permissions of a specific owner. It makes employ of mathematical symbols to modify the Unix file permissions.

Operator	Clarification
+	Adds a permission to a file or directory
–	Removes the permission
=	Sets the permission and overrides the permissions set earlier.

The diverse owners are represented every bit –

User Denotations
u	user/possessor
g	group
o	other
a	all

Nosotros volition not be using permissions in numbers like 755 but characters like rwx. Let's expect into an example

Changing Ownership and Group in Linux

For changing the buying of a file/directory, you tin utilise the post-obit command:

chown user filename

In case yous want to change the user as well as group for a file or directory apply the command

chown user:grouping filename

Let's run into this in action

In case you want to change group-owner only, use the command

chgrp group_name filename

'chgrp' stands for alter group.

Tip

• The file /etc/group contains all the groups defined in the system
• You can employ the command "groups" to find all the groups you lot are a member of



• You lot can use the command newgrp to work as a fellow member a group other than your default group



• You lot cannot have two groups owning the same file.
• You do not accept nested groups in Linux. I grouping cannot be sub-group of other
• ten- eXecuting a directory ways Being allowed to "enter" a dir and gain possible access to sub-dirs
• There are other permissions that you lot can set up on Files and Directories which volition be covered in a afterwards advanced tutorial

Summary:

• Linux beingness a multi-user organization uses permissions and buying for security.
• There are three user types on a Linux system viz. User, Group and Other
• Linux divides the file permissions into read, write and execute denoted by r,w, and ten
• The permissions on a file can exist changed by 'chmod' control which can be further divided into Absolute and Symbolic mode
• The 'chown' command can change the ownership of a file/directory. Use the following commands: chown user file or chown user:group file
• The 'chgrp' command tin can change the group buying chrgrp group filename
• What does x – eXecuting a directory mean? A: Being immune to "enter" a dir and gain possible admission to sub-dirs.

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Source: https://www.guru99.com/file-permissions.html

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