Master Linux File Permissions: chown, chmod, and Permission Bits Explained

Linux is a multi‑user operating system where users have different privileges.

To protect security, Linux defines separate permissions for different users on the same file or directory.

The two main commands to modify file ownership and permissions are chown (change owner) and chmod (change mode).

Use ll or ls -l to view file attributes, owner, and group.

# ls -l
total 64
drwxr-xr-x 2 root root 4096 Dec 14 2012 bin
drwxr-xr-x 4 root root 4096 Apr 19 2012 boot
...

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

Positions 0‑9 encode the file type and permissions; positions 1‑3 are owner permissions, 4‑6 group permissions, 7‑9 others permissions. The numeric mode is the sum of r=4, w=2, x=1 for each group (e.g., 770 = rwx for owner and group, none for others).

Changing file ownership and group

chgrp – change group

Syntax: chgrp [-R] group file. The -R option applies the change recursively.

chown – change owner (and optionally group)

Syntax: chown [-R] owner file or chown [-R] owner:group file.

# cd ~
# chown bin install.log
# ls -l install.log
-rw-r--r-- 1 bin users 68495 Jun 25 08:53 install.log
# chown root:root install.log
# ls -l install.log
-rw-r--r-- 1 root root 68495 Jun 25 08:53 install.log

chmod – change mode

Permissions can be set numerically or symbolically. Numeric mode is the sum of r=4, w=2, x=1 for each class.

# chmod 777 .bashrc
# ls -l .bashrc
-rwxrwxrwx 1 root root 395 Jul 4 11:45 .bashrc

To set permissions to -rwxr-xr-- (754): chmod 754 file or symbolically chmod u=rwx,g=rx,o=r file.

# touch test1
# ls -l test1
-rw-r--r-- 1 root root 0 Nov 15 10:32 test1
# chmod u=rwx,g=rx,o=r test1
# ls -l test1
-rwxr-xr-- 1 root root 0 Nov 15 10:32 test1

To remove execute permission from all users: chmod a-x file.

# chmod a-x test1
# ls -l test1
-rw-r--r-- 1 root root 0 Nov 15 10:32 test1