Linux is a multi-user operating system, which means that multiple users can use the system simultaneously and have their own files and directories. To ensure the security and integrity of the system and the files and directories on it, Linux uses a system of permissions to control access to resources.

• Understanding User and Group Ownership
• Types of Permissions: Read, Write, and Execute
• Setting Permissions using chmod
• Advanced Permissions: Setuid, Setgid, and Sticky Bit
• Tips for Managing Permissions in Linux
• Wrapping Up

In Linux, every file and directory has an owner, a group owner, and a set of permissions that control access to the resource. The owner of a resource is the user who created it, and the group owner is a group of users that the owner can assign to the resource. The permissions for a resource determine which users and groups have access to it and what they can do with it.

In this tutorial, we will explore how Linux permissions work and how you can use them to control access to resources on your system. We will also cover some advanced permissions and provide tips for managing permissions effectively. By the end of this tutorial, you should have a good understanding of how to use permissions in Linux to secure and manage your resources.

Understanding User and Group Ownership

In Linux, every file and directory has an owner and a group owner. The owner is the user who created the resource, and the group owner is a group of users that the owner can assign to the resource.

You can view the owner and group owner of a resource by using the ls -l command. This command will display a list of the files and directories in the current directory, along with their permissions, owner, and group owner. For example:

$ ls -l
total 8
-rw-r--r-- 1 user1 users  1234 Jan 1 12:34 file1.txt
-rw-r--r-- 1 user1 users  1234 Jan 1 12:34 file2.txt
drwxr-xr-x 2 user1 users  4096 Jan 1 12:34 dir1

In this example, the ls -l command displays a list of three files and directories in the current directory. The first column shows the permissions for each resource, the second column shows the owner, and the third column shows the group owner.

In this case, file1.txt and file2.txt are owned by user1 and are in the users group. dir1 is also owned by user1 and is in the users group.

By understanding user and group ownership, you can better control access to resources on your system. For example, you can use permissions to allow members of a group to access a shared resource, or you can use the chown command to change the owner of a resource.

Types of Permissions: Read, Write, and Execute

In Linux, there are three types of permissions that can be set for a resource: read, write, and execute. These permissions control which users and groups can access the resource and what they can do with it.

• Read permission: This permission allows a user or group to view the contents of a file or directory.
• Write permission: This permission allows a user or group to modify the contents of a file or directory.
• Execute permission: This permission allows a user or group to execute a file or access the contents of a directory.

The permissions for a resource are shown in the first column of the ls -l command output. Each resource is represented by a series of 10 characters, which are divided into three groups of 3 characters each. The first group represents the owner’s permissions, the second group represents the group owner’s permissions, and the third group represents the permissions for other users.

For example, the permissions for file1.txt in the ls -l command output above are -rw-r--r--. This means that the owner has read and write permissions (rw-), the group owner has read permissions (r--), and other users have read permissions (r--).

Setting Permissions using chmod

To set permissions for a resource in Linux, you can use the chmod command. The chmod command allows you to specify the permissions for a resource and the users and groups that the permissions apply to.

The chmod command uses a symbolic notation to specify permissions. In this notation, you specify the users or groups that the permissions apply to, followed by a + or - symbol to add or remove permissions, followed by the permission itself.

For example, to give the owner of a file read and write permissions, you can use the following chmod command:

$ chmod u+rw file1.txt

This command will add read (r) and write (w) permissions to the owner of file1.txt.

To remove write permission for the group owner of a file, you can use the following chmod command:

$ chmod g-w file1.txt

This command will remove write (w) permission for the group owner of file1.txt.

You can also use the chmod command to set permissions using an octal notation. In this notation, you specify the permissions using a three-digit octal number, with each digit representing the permissions for the owner, group owner, and other users, respectively.

For example, to give the owner of a file read and write permissions, and read permission to the group owner and other users, you can use the following chmod command:

$ chmod 644 file1.txt

This sets the file to rw-r--r--, which means that the owner has read and write permissions (rw-), the group owner has read permissions (r--), and other users have read permissions (r--).

By using the chmod command and the symbolic or octal notation, you can set the permissions for resources on your Linux system and control access to them.

It’s important to note that you must have the appropriate permissions to set the permissions of a resource. For example, if you are not the owner of a file or directory, you will need to have the write permission for the parent directory to set the permissions of the resource.

Octal	Symbolic	Permissions
0	– – –	– – –
1	– -x	Execute
2	-w-	Write
3	-wx	Write, Execute
4	r- –	Read
5	r-x	Read, Execute
6	rw-	Read, Write
7	rwx	Read, Write, Execute

In the table above, the octal notation is shown in the left column, and the corresponding symbolic notation is shown in the middle column. The permissions granted by each notation are shown in the right column.

For example, the octal notation 7 corresponds to the symbolic notation rwx, and grants read, write, and execute permissions. The octal notation 4 corresponds to the symbolic notation r--, and grants read permissions.

Advanced Permissions: Setuid, Setgid, and Sticky Bit

In addition to the standard read, write, and execute permissions, Linux also has advanced permissions known as the setuid, setgid, and sticky bit. These permissions can be used to further control access to resources on the system.

• Setuid (set user ID): This permission allows a file to be executed with the permissions of the owner of the file, rather than the permissions of the user executing the file. This can be useful for allowing users to execute a file with privileges that they do not normally have.
• Setgid (set group ID): This permission allows a file to be executed with the permissions of the group owner of the file, rather than the permissions of the user executing the file. This can be useful for allowing users to execute a file with privileges that they do not normally have as part of a group.
• Sticky bit: This permission is used on directories to prevent users from deleting or renaming files that they do not own. When the sticky bit is set on a directory, only the owner of the file or the owner of the directory can delete or rename the file.

To set the setuid, setgid, or sticky bit, you can use the chmod command and the symbolic notation. To set the setuid or setgid bit, you can use the u or g notation, followed by s. To set the sticky bit, you can use the o notation, followed by t.

For example, to set the setuid bit on a file, you can use the following chmod command:

$ chmod u+s file1.txt

This command will set the setuid bit on file1.txt, allowing it to be executed with the permissions of the owner.

To set the sticky bit on a directory, you can use the following chmod command:

$ chmod o+t dir1

This command will set the sticky bit on dir1, preventing users from deleting or renaming files in the directory that they do not own.

By using the setuid, setgid, and sticky bit, you can further control access to resources on your Linux system and enhance the security of your system. It’s important to use these advanced permissions with caution, as they can potentially compromise the security of your system if used improperly.

In the ls -l command output, the setuid, setgid, and sticky bit are represented by the letters s, S, and t, respectively. If the setuid or setgid bit is set, it will be displayed in the owner’s or group owner’s permissions, respectively. If the sticky bit is set, it will be displayed in the permissions for other users.

For example, the permissions -rwsr-sr-x indicate that the setuid bit is set for the owner, the setgid bit is set for the group owner, and the execute permission is set for all users. The permissions drwxrwxrwt indicate that the sticky bit is set on a directory, and the read, write, and execute permissions are set for all users.

Tips for Managing Permissions in Linux

Understand the types of permissions and how they are used to control access to resources. By understanding the different types of permissions and how they work, you can set the appropriate permissions for your resources and ensure that they are secure.

Use the chmod command to set permissions for resources. The chmod command allows you to set permissions using either symbolic or octal notation, making it easy to specify the permissions that you want.

Use the setuid, setgid, and sticky bit sparingly and with caution. These advanced permissions can be useful in certain situations, but they can also compromise the security of your system if used improperly.

Use group ownership to share resources with multiple users. By assigning resources to a group and setting the appropriate permissions, you can allow multiple users to access the resource without giving them full access to each other’s files.

Use the umask command to set default permissions for new resources. The umask command allows you to specify the default permissions for new files and directories, which can help to ensure that your resources are secure by default.

Wrapping Up

In Linux, permissions are used to control access to resources, such as files and directories. Every resource has an owner, a group owner, and a set of permissions that determine which users and groups have access to it and what they can do with it. There are three types of permissions: read, write, and execute. The chmod command is used to set permissions, and it can be specified using either symbolic or octal notation. In addition to the standard permissions, Linux also has advanced permissions known as the setuid, setgid, and sticky bit, which can be used to further control access to resources. By understanding and properly managing permissions, you can ensure the security and integrity of your Linux system.