Master Linux File Permissions: A Complete Guide to Using chmod

What is chmod?

The chmod (change mode) command modifies file and directory permissions on Linux/Unix systems, controlling which users can read, write, or execute a given object.

Linux permission model

Each file or directory belongs to an owner (user), a group, and others. Permissions are expressed as read ( r), write ( w), and execute ( x) for each of these three categories. u – owner permissions g – group permissions o – others permissions

Viewing permissions

Use ls -l to list files with their permission strings. The first character indicates the type ( - regular file, d directory, l symlink). The next nine characters are three groups of rwx triples.

linuxmi@linuxmi:~/www.linuxmi.com$ ls -l

Numeric (octal) notation

Permissions can be set with a three‑digit (or four‑digit) octal number where each digit is the sum of read = 4, write = 2, execute = 1.

Read → 4

Write → 2

Execute → 1

Example: owner rwx = 7, group rx = 5, others r‑ = 4 → 754. $ chmod 754 filename Common shortcuts:

$ chmod 744 file.txt   # rwx for owner, r-- for group & others

$ chmod 750 file.txt   # rwx for owner, r-x for group, --- for others

$ chmod 755 file.txt   # rwx for owner & group, r-x for others

Symbolic notation

Symbolic mode uses [ugoa][+-=][rwx] to modify specific classes. $ chmod u+rwx,g=rw,o=r filename Operators: + – add permission - – remove permission = – set exact permission (clears previous)

Common examples

Assign read permission only to others:

$ chmod o=r file.txt

Add execute permission for owner and group:

$ sudo chmod ug+x file.txt

Set different permissions for each class:

$ sudo chmod u=rwx,g=rw,o=r filename

Remove all permissions from others (equivalent forms): $ sudo chmod o= filename or

$ sudo chmod o-rwx filename

Recursive changes

Use -R to apply permissions to a directory and all its contents.

$ chmod -R 755 /path/to/dir

Copy permissions from a reference file

The --reference option copies the mode of a template file to the target.

$ sudo chmod --reference=ref_file target_file

Apply execute permission only to directories

Uppercase X adds execute permission only to directories.

$ chmod a+X *

Summary

The chmod command provides flexible ways to view and modify file and directory permissions using numeric or symbolic notation, recursive options, and reference files, enabling precise access control in Linux environments.