Chmod Calculator

Type a mode. Get what it actually does — for the owner, the group and everyone else — plus the chmod commands that produce it and a warning for every way it does not mean what it looks like it means.

Everything is computed on your device. No account, no upload, works offline.

Read any mode
755, rwxr-xr-x, or a whole ls -l line — the app takes any of them and shows
the octal, the letters and a plain-English sentence. A capital S or T is
called out for what it is: a special bit with no execute behind it, doing
nothing at all.

Build one by tapping
A three-by-three grid: owner, group and others against read, write and
execute. Each row shows its digit and the sum it comes from, so the octal
arithmetic stops being something to memorise. setuid, setgid and the sticky
bit get their own switches — with a note wherever a bit does nothing on your
kind of target.

Evaluate any chmod argument
u+x, go-w, a=r, g=u, +X, 2755 — type what you would type at the shell and see
the result before you run it, clause by clause, with the before and after of
every step. The umask is applied exactly where chmod applies it: only when you leave the who part out.

Linux and macOS do not run the same chmod
Several ordinary commands do different things under GNU and BSD, silently:
• chmod 755 on a setgid directory keeps the setgid bit under GNU — even
0755 does. It takes 00755, or an explicit g-s.
• chmod o+t sets the sticky bit on Linux and does nothing on a Mac.
• chmod a-X removes execute under GNU and can grant it under BSD.
• chmod a=u clears setuid under GNU and keeps it under BSD.

Switch implementations with one tap; whenever the two disagree, the app shows both answers side by side.

Every behaviour this app describes was measured by running tens of thousands of cases against both real chmod binaries — not read off a manual page.

Findings, before they bite
World-writable files. A setgid program its own group can rewrite. A directory
its owner cannot enter. A mode that gives everyone else more access than the
owner. Execute without read, which works for a binary and fails for a script.
Each finding says what actually happens, and one tap applies the fix.

umask, both directions
What 022 gives new files and directories, why the symbolic form reads as the
opposite of the octal one, why no umask can ever create an executable file —
and the reverse question: which mask produces the files you want.

Decode ls -l
Paste a whole listing. Every row becomes an octal mode and a sentence, ACL and extended-attribute markers are explained, and a symlink's meaningless mode is called out. The owner and group columns are never even read, and nothing you
paste leaves your device.

A guide worth reading
Not a manual-page mirror: the sticky bit and /tmp, why write on a directory
lets someone delete files they cannot even read, the SSH permissions sshd
actually enforces, and the chmod -R mistake that marks every file in a tree
executable — with the capital-X fix.