Master Linux File Permissions: A Hands‑On Guide to Using chmod

Linux’s chmod command changes or assigns permissions for files and directories. Permissions determine who can read, write, or execute a resource and are based on three classes—owner (u), group (g), and others (o)—each with read (r), write (w), and execute (x) bits.

Viewing Permissions

Use ls -l to list files in long format; the leftmost column shows the file type and nine permission bits, grouped in three triples for user, group, and others. A leading ‘-’ indicates a regular file, ‘d’ a directory, and ‘l’ a symbolic link. linuxmi@linuxmi:~/www.linuxmi.com$ ls -l Example output:

-rw-rw-r-- 1 linuxmi linuxmi 1087 Oct 3 20:23 linuxmi.com.cpp

The first three characters ( rw-) are the owner’s permissions, the next three ( rw-) the group’s, and the final three ( r--) others’ permissions.

chmod Syntax

$ chmod [options] mode file

Only the root user or a user with sudo privileges can modify permissions.

Example 1 – Numeric (Octal) Mode

Permissions can be set with a three‑digit octal number where read=4, write=2, execute=1. The sum of the desired bits for each class forms the mode.

Read = 4

Write = 2

Execute = 1

For owner=rwx (7), group=rx (5), others=r (4), the mode is 754: chmod 754 filename Common usages:

chmod 744 www.linuxmi.com.txt   # owner rwx, group & others r

chmod 750 www.linuxmi.com.txt   # owner rwx, group rx, others ---

chmod 755 linuxmi.txt           # owner & group rwx, others r-x

Example 2 – Recursive Directory Changes

Apply permissions to a directory and all its contents with -R:

chmod -R 755 /home/linuxmi/linux

Example 3 – Symbolic Mode

Use symbolic notation to modify specific classes: chmod [ugoa][+-=][rwx] file Classes:

u – user (owner)

g – group

o – others

a – all (u,g,o)

Operators:

- removes the specified permissions

+ adds the specified permissions

= sets exactly the specified permissions, clearing others

Examples:

chmod o=r file.txt               # give others read only

chmod ug+x linuxmi.txt           # add execute to owner and group

chmod u=rwx,g=rw,o=r file       # owner rwx, group rw, others r

chmod o= file.txt                # remove all permissions from others

Example 4 – Reference File

Copy permissions from a reference file:

chmod --reference=ref_file target_file

Example 5 – Apply Execute Only to Directories

Use capital X to set execute bits only on directories:

chmod a+X *

Summary

The article covered the purpose and syntax of chmod, demonstrated how to view permissions, explained numeric and symbolic modes, showed recursive changes, reference‑file copying, and the special a+X usage for directories, providing a comprehensive guide for managing Linux file permissions.