chmod Calculator
Unix file permissions in every spelling — octal 755, ls column -rwxr-xr-x, symbolic u=rwx,g=rx — per-class table, setuid/setgid/sticky, chmod commands, umask.
Octal (with the special-bit digit), symbolic, the ls column, binary and decimal, read/write/execute per class, the chmod commands that set the mode, and the umask that yields it by default.
Example: 755 = rwxr-xr-x = u=rwx,g=rx,o=rx, umask 022; -rwxr-sr-x is 2755 with setgid; 1666 shows rw-rw-rwT (sticky without execute) and is world-writable; a=rw,o-w gives 664.
Three classes,
three bits each.
How the spellings are read and written, what the special bits do, and where umask fits.
Reading a mode
Octal: three digits for owner, group and others (4 read, 2 write, 1 execute), with an optional leading digit for the special bits (4 setuid, 2 setgid, 1 sticky). The ls column: nine characters rwxrwxrwx, with a leading file-type character allowed and s/S, t/T standing in for the execute position when a special bit is set. Symbolic: clauses like u=rwx, g+w, o-x, a=r applied in order to an empty mode, so the result is what chmod would set from scratch.
Special bits
setuid on an executable runs it as the file's owner; setgid runs it as the group, and on a directory makes new files inherit the directory's group; sticky on a directory lets only a file's owner delete it (/tmp). In the ls column a lower-case s or t means the execute bit is also set; a capital S or T means it is not — usually a mistake.
umask and limits
The umask shown is the mask that would produce this mode for a new directory (777 minus the mode) and the file mode it would give (666 minus the mask). Access-control lists, immutable flags and platform-specific bits are outside the classic nine-bit model. Nothing here reads or changes any file. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- POSIX chmod: read 4, write 2, execute 1 per class; setuid 4000, setgid 2000, sticky 1000; s/S and t/T display per ls(1); umask = 777 minus the directory mode
Last reviewed 22 September 2026. How results are checked: How we verify.