Master Linux File Permissions: Users, Groups, and chmod/chown Explained

1 Linux File Permission Management Overview

1.1 Identity

In Linux, three identities can access a file or directory: owner (user), group (users belonging to the same group), and others (all remaining users).

1.2 Permission Types

Three permission categories exist: read (r) – commands like ls, cat; write (w) – commands like cp, mv, rm; execute (x) – commands like cd.

1.3 Root User

The root (superuser) has unrestricted privileges and can manage all other users.

2 Viewing Permissions

2.1 Using ls -l

ls -l
# Example output showing permission bits, owner, group, size, date, name

The first character indicates the file type (d for directory, - for regular file, l for symlink, etc.). Characters 2‑4 represent owner permissions, 5‑7 group permissions, and 8‑10 others permissions. rwx denote read, write, execute; a dash (-) means the permission is absent.

3 Modifying Permissions

3.1 chmod Command

3.1.1 Syntax

chmod [option] filename/dirname
# The executor must be the owner or root

3.1.2 Who to Set

u – owner (user)
g – group
o – others
a – all (u,g,o)
# If omitted, defaults to all

3.1.3 Symbolic Mode

r – read
w – write
x – execute
- – no permission
+ – add permission
- – remove permission
= – set exact permission

3.1.4 Numeric Mode

r = 4
w = 2
x = 1
0 = no permission
# Example: 754 means owner 7 (rwx), group 5 (r-x), others 4 (r--)

3.1.5 Recursive Option -R

chmod -R 777 dir1   # Apply permissions recursively to directory and its contents

4 Changing Owner and Group

4.1 chown Command

Changes the owner and/or group of a file or directory.

4.1.1 Syntax

chown [-R] newuser dirname
# For files, -R is not needed; for directories, use -R for recursion
# Executor must be root

chown user:group filename   # Set both owner and group
chown user filename         # Set only owner
chown :group filename       # Set only group