Question 1

What is chmod?

Accepted Answer

chmod is a Linux/Unix command to change file and directory access permissions. It sets Read, Write, and Execute permissions for three groups: Owner, Group, and Others.

Question 2

What does chmod 755 mean?

Accepted Answer

chmod 755 grants the owner full permissions (read, write, execute), and group/others read and execute permissions. Commonly used for web server directories and scripts.

Question 3

How are permission numbers calculated?

Accepted Answer

Read=4, Write=2, Execute=1. Sum the values: read+execute = 4+1 = 5, all permissions = 4+2+1 = 7. The three digits represent Owner, Group, Others respectively.

Permission	Symbolic	Use Case
644	rw-r--r--	HTML, CSS, images
600	rw-------	Config files, passwords
400	r--------	SSH private key (id_rsa)
755	rwxr-xr-x	CGI, shell scripts

Permission	Symbolic	Use Case
755	rwxr-xr-x	Public directories (web root)
700	rwx------	Home directory, private folders
750	rwxr-x---	Group shared directories

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	Read (4)	Write (2)	Execute (1)
Owner
Group
Others