Chmod Calculator

How it works

Each octal digit = read(4) + write(2) + execute(1) for owner, group, others · optional leading digit = setuid(4) + setgid(2) + sticky(1)

A Unix file mode packs nine permission bits — read, write and execute for each of owner, group and others — into three octal digits, because each group of three bits is exactly one octal digit (0–7). Read is worth 4, write 2 and execute 1, so you add them: 7 is rwx, 6 is rw-, 5 is r-x, 4 is r--. A fourth, leading digit sets the special bits: setuid (4) makes an executable run as its owner, setgid (2) as its group, and the sticky bit (1) on a directory stops users deleting each other’s files. Those special bits reuse the execute slot in the symbolic string — shown as s/t when execute is also set, or capital S/T when it is not.

Worked example

755 breaks into 7, 5, 5. Owner = 7 = 4+2+1 = rwx (read, write, execute); group = 5 = 4+1 = r-x (read, execute); others = 5 = r-x — giving the symbolic string rwxr-xr-x, the classic mode for an executable or a public directory. 644 gives rw-r--r-- (owner reads and writes, everyone else reads); 777 gives rwxrwxrwx (everyone can do everything).

Sources: chmod — POSIX / Wikipedia

Frequently asked questions