Unlock Linux File Permissions: A Complete Guide to Managing Access Rights

Linux Permission Management

Permissions define what actions a user can perform on a file; the root user has all permissions.

1. Permission Classification

Permissions are divided into three parts, each with a numeric code:
    Read (r)    : 4
    Write (w)   : 2
    Execute (x) : 1
    No permission (-) : 0

Executing a script == running a script

2. Detailed Permission Information

Permission bits are split into three sections: owner, group, others.
Example: drwxr-xr-x
    d   : file type
    rwx : owner
    r-x : group
    r-x : others

3. Adding Permissions by Summation

Using drwxr-xr-x as an example:
    Owner : read, write, execute
    Group : read, execute
    Others: read, execute

rwxr-xr-x is equivalent to 755

4. Modifying Permissions

Command: chmod
Format: chmod [options] permission_expression target

Options:
    -R : apply changes recursively to directories and their contents

Example: chmod -R 755 /root/text/   # sets all files to rwxr-xr-x

5. Ownership Categories

Owner : u

Group : g

Others: o

Purpose: change permissions for a specific class.
Example: chmod g-r,g+w 1.txt   # remove read from group, add write

6. Why Testing Permissions Matters for Users

Regular users must strictly follow permission rules.

Root user overrides permission restrictions.

Changes take effect after re‑login (su or su -).

7. Significance of Permissions on Directories

To read a file, the path must have execute (x) permission on each directory and read (r) on the file.

To write a file, the path needs execute on directories and write (w) on the file.

To execute a file, directories need read and execute, and the file itself needs execute.

8. Default Permissions for New Files

Check default file permissions with: /etc/profile

Common defaults:
    Files : 666 (modified by umask)
    Directories : 777 (modified by umask)

Default file permission = 644 (666 - 022)
Default directory permission = 755 (777 - 022)

Useful commands:
    /usr/bin/id -un   # username
    /usr/bin/id -gn   # group name