chmod-laskuri

Unix file permissions control who can read, write and execute a file or directory, split across three groups: the owner, the group, and everyone else. This calculator converts between interactive checkboxes, the numeric octal form like 755, and the symbolic form like rwxr-xr-x, keeping all three in sync.

Each group gets three bits, read (4), write (2) and execute (1), and you add them up to get one digit. So 7 is read plus write plus execute (rwx), 5 is read plus execute (r-x), and 6 is read plus write (rw-). Three digits, one per group, give you the familiar permission number.

The common values are worth memorizing. 755 suits directories and executables: the owner has full access while group and others can read and execute but not write. 644 suits normal web files: the owner can read and write, everyone else can only read.

For sensitive files, 600 keeps things tight: only the owner can read or write, and nobody else has any access, which is what SSH private keys and similar secrets need.

There is also an optional fourth leading digit for special bits, setuid, setgid and the sticky bit, which change how programs run or how a shared directory behaves. When one is set, the execute position shows as s or t instead of x.

The calculator gives you the ready-to-run chmod command to copy, and runs entirely in your browser. It is a reference and converter, so it does not change any files itself.