File Permissions

File permissions in Linux determine who can read, write, or execute a file or directory. Understanding these permissions is essential for managing system security and functionality.

By executing the ls -l command, a list of details from a file or directory will be displayed. The first part represents the permissions associated with the file. It looks like a combination of symbols - and letters d r c w s. These can be understood as follows:

• First indicates the type of file: Not special (-), directory (d)

• Then, we find three sections of three letters, each one indicating: owner, group, and other

• Each letter indicates the permissions: read (r), write (w), or execute (x) for the respective section

drw-r--r-x     #Is a Directory, the Owner can read and write, the Group can read, and Other can read and execute
-rwsr--r-x     #This has SUID permissions because it has s on the Owner section
-rw-r-sr-x     #This has SGID permissions because it has s on the Group section

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• The permissions can also be represented by adding the numbers in each section, where r=4, w=2 and x=1

-rw-r--r-x      #Can be represented as 645
-rwxrwxr--      #Can be represented as 774

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Special permissions

Two classes of special permission, known as SUID (Set User ID) and SGID (Set Group ID), allow running specific applications as the root user for certain users or groups without being the owners.

• The letter s indicates SIUD or SGID permissions

//-rw-r--r-s      #The letter s represents the special permission

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Also, we can find an extra permission for directories known as Sticky bit, which ensures that only the owner of the file/directory, or the root user, can delete or rename files within the directory.

• The letters t represents the Sticky bit, but users still have execution permissions

• If shown capitalized (T) all other users do not have access permissions to the folder contents

drw-rw-r-t    #Sticky with execute permissions
drw-rw-r-T    #Sticky with no access permissions