Master Linux File Permissions: Owner, Group, and Mode Management Explained

Introduction

Linux supports multiple users and multitasking, so it never forces a shutdown and protects each user's privacy. Every file or directory has three identities—owner, group, others—each with three permissions: readable, writable, executable.

Document Attributes

Use ls -al --full-time or its alias ll to view all file attributes. The output has seven columns:

File type and permission bits (first character indicates type, next nine represent rwx for owner, group, others).

Link count.

Owner name.

Group name.

Size in bytes.

Last modification timestamp.

File name (names starting with a dot are hidden).

Changing Owner (chown)

Location

etc/passwd

Only accounts listed in /etc/passwd can be set as owners.

Syntax

chown [-R] [user] [file|dir]

chown [-R] [user]:[group] [file|dir]

Use -R for recursive changes. Although chown can also change the group, chgrp is recommended for that purpose.

Examples

chown daemon test          # change owner of "test" to daemon

chown daemon:root test     # change group of "test" to root

chown root.users test      # set owner to root and group to users

chown .root test           # change only the group to root

Changing Group (chgrp)

Location

etc/group

This file lists all groups.

Syntax

chgrp [-options] [group] [path]

Examples

chgrp -R users test       # recursively set group of "test" and its contents to users

If the group does not exist, an "invalid group" error is returned.

Changing Permissions (chmod)

Linux permissions consist of read, write, and execute for each of owner, group, and others, yielding nine possible bits. Permissions can be modified using symbolic or numeric methods.

Symbolic Method

Use u, g, o (or a for all) for identities, r, w, x for permissions, and +, -, = for adding, removing, or setting.

Set (=)

chmod u=rwx,g=rwx,o=rwx test

chmod ugo=rwx test

chmod a=rwx test

Remove (-)

chmod u-x,g-x,o-x test

chmod ugo-x test

chmod a-x test

Add (+)

chmod u+x,g+x,o+x test

chmod ugo+x test

chmod a+x test

Numeric Method

Permissions are represented by numbers: read=4, write=2, execute=1. Sum them for each identity.

chmod 777 test   # rwx for owner, group, others

chmod 666 test   # rw- for all

chmod 755 test   # rwx for owner, r-x for group and others

File vs Directory Permission Differences

Files

readable : can read file contents.

writable : can modify file contents.

executable : can be executed as a program.

Note: Write permission on a file does not allow deletion; deletion is controlled by directory permissions.

Directories

readable : can list directory contents (e.g., ls).

writable : can create, move, delete, or rename entries inside the directory.

executable : can enter the directory (e.g., cd).

To allow anyone to browse a directory, at least r or x must be set; reading file contents requires directory x plus file r .

Summary

Each Linux file can assign rwx permissions to owner, group, and others. Use chgrp to change groups, chmod to modify permissions, and chown to change owners, thereby protecting data security through proper permission management.