Chmod Calculator

What does it convert between?

Octal notation, symbolic notation and individual permission checkboxes — change any one and the others follow. It covers the three special bits as well: setuid, setgid and sticky.

How do the octal digits work?

Each digit is a sum of three bits: read is 4, write is 2, execute is 1. So 7 is all three, 6 is read plus write, 5 is read plus execute. The three digits are owner, group and others in that order — which makes 755 "owner does everything, everyone else may read and execute".

Why does the octal sometimes have four digits?

Because a leading digit carries the special bits: setuid is 4, setgid is 2, sticky is 1, summed the same way. The tool follows the usual convention of omitting it when it is zero, so 0755 and 755 mean the same thing while 4755 means setuid is set.

What do the special bits actually do?

setuid on an executable makes it run as the file's owner rather than the caller. setgid does the same for the group, and on a directory it makes new files inherit that directory's group. sticky on a directory means only a file's owner can delete it — the reason anyone can write to /tmp without deleting other people's files.

Why is setuid dangerous?

Because it hands the file owner's privileges to whoever runs it. A setuid-root binary with a bug is a privilege-escalation vulnerability, which is why it is worth being deliberate before setting that bit and why many systems ignore it on mounted filesystems.

What does the execute bit mean on a directory?

Permission to traverse it — to enter and reach what is inside. Read on a directory lets you list the names; execute lets you use them. A directory with read but not execute gives you a list of files you cannot open, which is a common source of confusion.

Is 777 ever the right answer?

Almost never. It grants every user on the system permission to modify and run the file. It is a common "fix" for a permissions error, and it usually replaces a small problem with a larger one — the actual fix is normally correcting ownership.

Does this apply to Windows?

No. Windows uses ACLs, a different and more granular model. These numbers are Unix, Linux and macOS.

Is anything sent to a server?

No. The arithmetic runs in your browser.