Master Linux File Permissions: chown, chmod, and Numeric Modes Explained

Linux File Ownership and Permissions

Linux is a multi‑user operating system; each file has an owner, a group, and permission bits that control read, write, and execute access for the owner, the group, and other users.

Viewing file attributes

# ls -l
total 64
drwxr-xr-x 2 root root 4096 Dec 14 2012 bin
...

Permission string format

First character indicates the file type: d directory, - regular file, l symbolic link, b block device, c character device.

The next nine characters are three groups of rwx representing read, write, and execute permissions for owner, group, and others. A dash ( -) means the permission is absent.

Numeric (octal) representation

Permission values: r=4, w=2, x=1. Add the values for each class to obtain a three‑digit octal code.

Owner rwx → 7

Group rwx → 7

Others --- → 0

Resulting mode: 770. The chmod syntax is:

chmod [-R] xyz file_or_directory

Changing ownership

chgrp

changes the group; chown changes the owner and optionally the group.

# Change group
chgrp [-R] group_name file_name

# Change owner only
chown user file_name

# Change owner and group
chown user:group file_name

Modifying permissions with chmod

Numeric example – give full access to a file:

# Show current mode
ls -al .bashrc
# Set mode to 777 (rwx for all)
chmod 777 .bashrc
# Verify change
ls -al .bashrc

Symbolic example – set mode to -rwxr-xr--: chmod u=rwx,g=rx,o=r file_name Remove a permission without affecting others (e.g., strip execute bits from everyone):

chmod a-x file_name

Symbolic mode components

Classes: u (owner), g (group), o (others), a (all).

Operators: + (add), - (remove), = (set).

Permissions: r, w, x.

Recursive changes

Both chgrp and chown accept the -R flag to apply the change recursively to all files and sub‑directories.

Practical examples

# Change owner of install.log to user "bin"
chown bin install.log

# Change back to root:root
chown root:root install.log

# Change group recursively
chgrp -R developers /opt/project

# Set permissions recursively (755 = rwxr-xr-x)
chmod -R 755 /var/www/html