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File Permissions in Linux / Unix: How to Read, Write & Change?

Linux is a clone of UNIX, the multi-user operating system which can be accessed by many users simultaneously. Linux can also be used in mainframes and servers without any modifications. But this raises security concerns as an unsolicited or malign user can corrupt, change or remove crucial data. For effective security, Linux divides authorization into 2 levels.

In this Linux file commands tutorial, you will learn-

The concept of Linux File permission and ownership is crucial in Linux. Here, we will explain Linux permissions and ownership and will discuss both of them. Let us start with the Ownership.

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Linux File Ownership

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 owner. Hence, a user is also sometimes called an owner.

Group

A user- group can contain multiple users. All users belonging to a group will have the same Linux group permissions access to the file. Suppose you have a project where a number of people require 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 one else can read or modify the files.

Other

Any other user who has access 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 set the permission for others, it is also referred as set permissions for the world.

Now, the big question arises how does Linux distinguish between these three user types so that a user ‘A’ cannot affect a file which contains some other user ‘B’s’ vital information/data. It is like you do not want your colleague, who works on your Linux computer, to view your images. This is where Permissions set in, and they define user behavior.

Let us understand the Permission system on Linux.

Linux File Permissions

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

• Read: This permission give you the authority to open and read a file. Read permission on a directory gives you the ability 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 authority to add, remove and rename files stored in the directory. Consider a scenario where you have to write permission on file but do not have write permission on the directory where the file is stored. You will be able to modify the file contents. But you 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 set. If the execute permission is not set, you might still be able to see/modify the program code(provided read & write permissions are set), but not run it.

Let’s see file permissions in Linux with examples:

ls – l on terminal gives

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

Here, the first ‘–‘ implies that we have selected a file.p>

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

The characters are pretty easy to remember.

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

Let us look at it this way.

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 bit is set to ‘-‘.

By design, many Linux distributions like Fedora, CentOS, Ubuntu, etc. will add users to a group of the same group name as the user name. Thus, a user ‘tom’ is added to a group named ‘tom’.

The second part is ‘rw-‘. It for the user group ‘Home’ and group-members can:

The third part is for the world which means any user. It says ‘r–‘. This means the user can only:

Changing file/directory permissions in Linux Using ‘chmod’ command

Say you do not want your colleague to see your personal images. This can be achieved by changing file permissions.

We can use the ‘chmod’ command which stands for ‘change mode’. Using the command, we can set permissions (read, write, execute) on a file/directory for the owner, group and the world.

chmod permissions filename

There are 2 ways to use the command –

Absolute(Numeric) Mode in Linux

In this mode, file permissions are not represented as characters but a three-digit octal number.

The table below gives numbers for all for permissions types.

Number	Permission Type	Symbol
0	No Permission	—
1	Execute	–x
2	Write	-w-
3	Execute + Write	-wx
4	Read	r–
5	Read + Execute	r-x
6	Read +Write	rw-
7	Read + Write +Execute	rwx

Let’s see the chmod permissions command in action.

In the above-given terminal window, we have changed the permissions of the file ‘sample to ‘764’.

‘764’ absolute code says the following:

• Owner can read, write and execute
• Usergroup can read and write
• World can only read

This is shown as ‘-rwxrw-r–

This is how you can change user permissions in Linux on file by assigning an absolute number.

Symbolic Mode in Linux

In the Absolute mode, you change permissions for all 3 owners. In the symbolic mode, you can modify permissions of a specific owner. It makes use of mathematical symbols to modify the Unix file permissions.

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

The various owners are represented as –

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

We will not be using permissions in numbers like 755 but characters like rwx. Let’s look into an example

Changing Ownership and Group in Linux

For changing the ownership of a file/directory, you can use the following command:

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

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

chgrp group_name filename

‘chgrp’ stands for change group.

Tip

• The file /etc/group contains all the groups defined in the system
• You can use the command “groups” to find all the groups you are a member of

Summary:

• Linux being a multi-user system uses permissions and ownership 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 x
• The permissions on a file can be changed by ‘chmod’ command 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 can change the group ownership chrgrp group filename
• What does x – eXecuting a directory mean? A: Being allowed to “enter” a dir and gain possible access to sub-dirs.

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Assign Read/Write Access to a User on Specific Directory in Linux

In a previous article, we showed you how to create a shared directory in Linux. Here, we will describe how to give read/write access to a user on a specific directory in Linux.

There are two possible methods of doing this: the first is using ACLs (Access Control Lists) and the second is creating user groups to manage file permissions, as explained below.

For the purpose of this tutorial, we will use following setup.

Operating system: CentOS 7 Test directory: /shares/project1/reports Test user: tecmint Filesystem type: Ext4 

Make sure all commands are executed as root user or use the the sudo command with equivalent privileges.

Let’s start by creating the directory called reports using the mkdir command:

# mkdir -p /shares/project1/reports

Using ACL to Give Read/Write Access to User on Directory

Important: To use this method, ensure that your Linux filesystem type (such as Ext3 and Ext4, NTFS, BTRFS) support ACLs.

1. First, check the current file system type on your system, and also whether the kernel supports ACL as follows:

# df -T | awk '' | grep "^/dev" # grep -i acl /boot/config*

From the screenshot below, the filesystem type is Ext4 and the kernel supports POSIX ACLs as indicated by the CONFIG_EXT4_FS_POSIX_ACL=y option.

2. Next, check if the file system (partition) is mounted with ACL option or not:

# tune2fs -l /dev/sda1 | grep acl

From the above output, we can see that default mount option already has support for ACL. If in case it’s not enabled, you can enable it for the particular partition (/dev/sda3 for this case):

# mount -o remount,acl / # tune2fs -o acl /dev/sda3

3. Now, its time to assign a read/write access to a user tecmint to a specific directory called reports by running the following commands.

# getfacl /shares/project1/reports # Check the default ACL settings for the directory # setfacl -m user:tecmint:rw /shares/project1/reports # Give rw access to user tecmint # getfacl /shares/project1/reports # Check new ACL settings for the directory

In the screenshot above, the user tecmint now has read/write (rw) permissions on directory /shares/project1/reports as seen from the output of the second getfacl command.

For more information about ACL lists, do check out our following guides.

Now let’s see the second method of assigning read/write access to a directory.

Using Groups to Give Read/Write Access to User on Directory

1. If the user already has a default user group (normally with same name as username), simply change the group owner of the directory.

# chgrp tecmint /shares/project1/reports

Alternatively, create a new group for multiple users (who will be given read/write permissions on a specific directory), as follows. However, this will create a shared directory:

2. Then add the user tecmint to the group projects as follows:

# usermod -aG projects tecmint # add user to projects # groups tecmint # check users groups

3. Change the group owner of the directory to projects:

# chgrp projects /shares/project1/reports

4. Now set read/write access for the group members:

# chmod -R 0760 /shares/projects/reports # ls -l /shares/projects/ #check new permissions

That’s it! In this tutorial, we showed you how to give read/write access to a user on a specific directory in Linux. If any issues, do ask via the comment section below.

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