Master Linux Permissions: From Basics to Advanced chmod Tricks

Linux Permission Management

Permission refers to the rights a user has on a specific file; the root (super‑user) has all permissions.

1. Permission categories

Permissions are divided into three parts, each represented by a numeric value:
    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, and others.
Example: drwxr-xr-x
    d   : file type
    rwx : owner permissions
    r-x : group permissions
    r-x : others permissions

3. Adding permissions by summing

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. Adding permissions (chmod command)

Command: chmod
Format: chmod [options] permission_expression target

Options:
    -R : apply recursively (e.g., chmod -R 755 /path/to/dir)

5. Ownership

Owner : u

Group : g

Others : o

Use cases: change only the group permission, e.g.,
chmod g‑r,g+w 1.txt

6. Testing permissions for users

Regular users strictly follow the set permissions.

Root user overrides permission restrictions.

Changes take effect after re‑login (e.g., using su or su ‑).

7. Testing permissions for directories

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

To write a file, the directory needs execute (x) permission and the file needs write (w) permission.

To execute a file, the directory needs read (r) and execute (x) permissions, and the file itself must have execute (x) permission.

8. Default file permissions

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

File permission = 666 – umask (default 022) => 644
Directory permission = 777 – umask (default 022) => 755

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