File Permissions and Access Control Lists in Linux.

Introduction:

Linux is a multi-user operating system, so it has security to prevent people from accessing each other’s confidential files. When you execute a “ls” command, you are not given any information about the security of the files, because by default “ls” only lists the names of files. You can get more information by using an “option” with the “ls” command.

Access control list (ACL) provides an additional, more flexible permission mechanism for file systems. It is designed to assist with UNIX file permissions. ACL allows you to give permissions for any user or group to any disc resource.

File System Hierarchy:

• In the Filesystem Hierarchy Standard (FHS), all files and directories appear under the root directory /, even if they are stored on different physical or virtual devices.

• Some of these directories only exist on a particular system if certain subsystems, such as the X Window System, are installed.

• Most of these directories exist in all UNIX operating systems and are generally used in much the same way; however, the descriptions here are those used specifically for the FHS and are not considered authoritative for platforms other than Linux.

• The root directory is "/" (forward slash) and it is the base directory.

Understanding the Directories:

Types of files in Linux Systems:

1. General Files – It is also called ordinary files. It may be an image, video, program, or simple text file. These types of files can be in ASCII or Binary format. It is the most commonly used file in the Linux system.

2. Directory Files – These types of files are a warehouse for other file types. It may be a directory file within a directory (subdirectory).

3. Device Files – In a Windows-like operating system, devices like CD-ROM, and hard drives are represented as drive letters like F: G: H whereas in the Linux system devices are represented as files. As for example, /dev/sda1, /dev/sda2, and so on.

These are the common top-level directories associated with the root directory:

Some other directories in the Linux system:

Some more basic symbols

File Permissions:

#ls -l

There’s a lot of information in those lines.

1. The first character = ‘-‘, which means it’s a file ‘d’, which means it’s a directory.

2. The next nine characters = (rwxrwxr) show the security

3. The next column shows the owner of the file. (Here it is ubuntu)

4. The next column shows the group owner of the file. (Here it is ubuntu which has special access to these files)

5. The next column shows the size of the file in bytes.

6. The next column shows the date and time the file was last modified.

7. Last Column = File_name or Directory_name. (For example, here are: createdirectory.sh , merikahani.sh )

File type:

The file type is determined by the first character of the permission sequence. These are the symbols and their meaning

File Permission:

The Read, Write and execute have been assigned some certain values. These values can be used to to set permissions for a file.

Read (r) - 4

Write (w)- 2

Execute (x) - 1

Above values are used to give permissions to files. we have one file createdirectory.sh and need to give all read , write & execute to users, groups and others. we will sum up the values i.e. 4+2+1 = 7. So the Value 7 resembles all the permissions are granted.

• We will use chmod (changemode) command here

  chmod 777 filename

  #chmod 777 createdirectory.sh

We can add/remove permissions anytime. Lets say here i'll remove write and execute permission permission for groups and other.

user - rwx , group- r , others- r i.e- user= 4+2+1 , group - 4 , others - 4

• Same way, we can modify permissions

  

  basic commands:

• chown - change ownership - #chown file.txt

• chgrp - change group. #chgrp file.txt

• chmod - change mod. #chmod 777 file.txt

ACL (Access Control List):

• Access control list (ACL) provides an additional, more flexible permission mechanism for file systems. It is designed to assist with UNIX file permissions. ACL allows you to give permissions for any user or group to any disc resource.

• setfacl and getfacl are used for setting up ACL and showing ACL respectively.

setfacl: #setfacl -m u:username:rw file.txt

-m option in setfacl stands for "modify," and it is used to modify the ACL of a file or directory by adding or changing ACL entries.

rw: Read and Write Permission

u: To specify the username

1. "setfacl" : This is used to set ACLs. For e.g. to grant read and write access to a file for a specific user:

#setfacl -m u:username:rw test.sh

-m option in setfacl stands for "modify," and it is used to modify the ACL of a file or directory by adding or changing ACL entries.

rw: Read and Write Permission

u: To specify the username.

1. "gefacl" : This command is used to view access control lists of a file or directory.

   # getfacl test.sh

   

   Removing ACLs:

   To remove ACLs from a file or directory, you can use the setfacl command with the -b option:

   #setfacl -b test.sh

   Task1:

   1. Create a simple file and do ls -ltr to see the details of the files refer to Notes

Each of the three permissions are assigned to three defined categories of users. The categories are:

• owner — The owner of the file or application.

• "chown" is used to change the ownership permission of a file or directory.

• group — The group that owns the file or application.

• "chgrp" is used to change the group permission of a file or directory.

• others — All users with access to the system. (outised the users are in a group)

• "chmod" is used to change the other users permissions of a file or directory.

  As a task, change the user permissions of the file and note the changes after ls -ltr

Task done!!

Task2: Write an article about File Permissions based on your understanding from the notes.

Done in above paragraph.

Task3:Read about ACL and try out the commands getfacl and setfacl.

Done in above paragraph.

Thank you for reading my blog:)