Question 1

What does chmod 755 mean?

Accepted Answer

Octal chmod uses three digits for owner, group, and others. Each digit is the sum of read (4), write (2), and execute (1). 755 means the owner has read, write, and execute (4+2+1=7), while group and others have read and execute only (4+1=5). It is typical for directories and executable files on Unix servers.

Question 2

What is the difference between 644 and 755?

Accepted Answer

644 is rw-r--r--: the owner can read and write; everyone else can only read. It is standard for non-executable files. 755 is rwxr-xr-x: the owner can read, write, and execute; others can read and execute — common for scripts and directories that must be entered.

Question 3

Can I paste ls -l permission characters?

Accepted Answer

Yes. You can paste the nine permission characters (rwxr-xr-x), optionally with the leading file-type letter from ls -l (for example -rwxr-xr-x). The tool converts them to the numeric octal mode for the basic permission bits.

Question 4

What are the fourth octal digit and special bits?

Accepted Answer

A leading 4, 2, or 1 in a four-digit octal mode sets setuid, setgid, or the sticky bit on top of the usual three digits for rwx triplets. For example 4755 includes setuid. The calculator calls out those bits when you enter a four-digit octal value.

Question 5

Does this change files on my machine?

Accepted Answer

No. It only converts between representations for planning Dockerfiles, Ansible, Terraform, or shell scripts. Run chmod on your actual system or container when you are ready to apply permissions.

chmod Calculator

Unix file permissions