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linux_file_permissions.md

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File Permissions Documentation

Preface: Duely note that every file or directory in Linux has an owner and a group (one group).

  • Permissions are set for the owner.
  • Permissions are set uniformly for everyone in the group.
  • Permissions are set for everyone else.

This is three distinct permission sets being applied to any file or directory.

Permissions to be applied include read, write, and execute- shortened to "rwx" for easy reference. The administrators job is to take combinations of these three permissions (rwx) and apply a combination to each kind of user (owner, group, and everyone else).

This match-making process is enabled via the following implementation process:

Octal Value, File Permissions Set, Permissions Description

Octal Value File Permissions Set Permissions Description
0 --- No permissions
1 --x Execute permission only
2 -w- Write permission only
3 -wx Write and execute permissions
4 r-- Read permission only
5 r-x Read and execute permissions
6 rw- Read and write permissions
7 rwx Read, write, and execute permissions

File Permission Pattern (Framework for human-memory)

One notable pattern is observed during the incremention of the octal value, following the binary representation:

  • Imagine a set of three binary place values, filled from right to left.
    • When incrementing the set, you flip the right-most 0 into a 1.
    • When assigning a value to indices with existing values (1s) to the left, those values to the left are retained.
    • When assigning a value to indices with existing values (1s) to the right, those values to the right are dropped.

Example:

Octal Value Example Binary Representation Notes
0 --- 000 Initial state
1 --x 001 First increment
2 -w- 010 Middle index filled, values to the right dropped
3 -wx 011 Right-most index filled, values to the left retained
4 r-- 100 Left-most index filled, values to the right dropped
5 r-x 101 Right-most index filled, values to the left retained
6 rw- 110 Middle index filled, values to the right dropped
7 rwx 111 Right-most index filled, values to the left retained

This results in a total of 8 possible states, making octal (0-7) a suitable method for representing any state. When setting permissions for a file or directory in Linux, the octal format is used to express permissions for all three (read, write, and execute) via a single value.

Usage in Linux File System

Linux file system commands utilize a set of three octal values: 0-7 0-7 0-7. For instance, 644.
Each position out of the three characters represents a user category to apply the permissions to:

  • Position one (x--): File owner
  • Position two (-x-): Associated group
  • Position three (--x): Everyone else

Permissions Explanation

  • Permissions applied to the owner: Permissions that the file or directory owner is allowed.
  • Permissions applied to groups: Permissions for the group associated with the file or directory.
  • Permissions for everyone: Permissions for all other users who are not the owner or in the designated groups.

Access Control

Owner: The file or directory owner's permissions are determined by the first position (e.g., x--) in the permission set. The owner typically has the highest level of control over the file or directory, including the ability to read, write, and execute files.

Group permissions are represented by the second position (e.g., -x-) in the permission set. The file or directory has one associated group, and all members of this group share the same set of permissions. Administrators manage group permissions using commands like chgrp to change group ownership or chmod to adjust permissions to the named file or directory for all members of its assigned group.

Everyone: Permissions for all users not the owner or members of the designated group are governed by the third position (e.g., --x) in the permission set. This category encompasses any user not part of the owner or any associated groups.

Examples

  1. Setting Permissions for a Shared Directory: Suppose you have a directory named "shared" that needs to be accessed by all users in a group called "team." Here's how you can set the permissions and associate the "team" group with the "shared" directory:

    # Create the "shared" directory
$ mkdir shared

# Associate the "team" group with the "shared" directory
$ chgrp team shared

# Set the permissions to allow the owner full access, the "team" group full access, and deny all access to others
$ chmod 774 shared

    This series of commands first creates the "shared" directory and then associates the "team" group with it using the chgrp command. Finally, it sets the permissions using the chmod command. The owner is given full permissions (7, AKA rwx), and the "team" group is also given full permissions (7, AKA rwx). Everyone else has read-only access (4, AKA r--). Therefore, the permission set is 774, as illustrated by chmod 774 shared.

  2. Restricting Access to Sensitive Files: Let's say you have a file named "confidential.txt" containing sensitive information. You want to ensure that only the owner can read and modify the file. You can set the permissions as follows:

    $ chmod 600 confidential.txt

    This command gives the owner read and write permissions (rw-), while denying all permissions to groups and others.