Chmod Calculator

Every file and directory on a Unix-like system has three permission sets — for the owner, the group, and others (everyone else). Each set contains three bits: read (r, worth 4), write (w, worth 2), and execute (x, worth 1). Adding the values of the enabled bits for each set produces one octal digit, so three sets become a three-digit number like 755.

Worked example — chmod 754 script.sh: the owner digit 7 = 4+2+1 (read, write, execute → rwx), the group digit 5 = 4+1 (read, execute → r-x), and the others digit 4 = read only (r--). The full symbolic form is rwxr-xr--. For directories, “execute” means permission to enter the directory and access files inside it.

A fourth, leading digit sets the special bits: setuid (4) makes an executable run with the file owner’s privileges, setgid (2) runs it with the group’s privileges (on directories, new files inherit the directory’s group), and the sticky bit (1) on a shared directory like /tmp means only a file’s owner can delete it. In symbolic notation these appear as s or t in the execute position — lowercase when execute is also set, uppercase (S/T) when it is not.