Understand Linux File and Directory Permissions

A chmod mode describes permission bits for the owner, group and others. Use the visual calculator to read the bits before constructing an absolute mode.

Work through the task

1. For each class, add read 4, write 2 and execute/search 1. Read plus write is 6; all three is 7.
2. Read the digits in owner/group/others order. 644 is owner read/write and read-only for group and others.
3. Choose Directory to understand search/traversal rather than program execution. Write on a directory concerns directory entries.
4. Inspect special bits separately. Lowercase s/t combines a special bit with execute; uppercase S/T means execute is absent.

Checked examples

Calculation method

The three basic octal digits encode three independent read/write/execute groups. An optional preceding digit encodes setuid 4, setgid 2 and sticky 1. These are absolute mode bits, not a measurement of effective access.

Assumptions and limits

Ownership, ACLs, parent directories, mounts and system policy also matter. Presets are examples, not universal security advice. File commands use numeric modes; directory commands use GNU chmod's equals extension to request clearing unselected special bits. Both use a fixed quoted placeholder and -- before the path, without a recursive option. This site never executes commands or changes permissions.