Chmod Calculator

The owner may read, write and execute; everyone else may read and execute. Each digit is a sum: 4 for read, 2 for write, 1 for execute, so 7 is all three and 5 is read plus execute. It is the usual mode for a program or a directory, where execute on a directory means the right to enter it rather than to run it.

The execute bit. 644 is the default for ordinary files — the owner can edit, everyone can read — and 755 adds execute for everyone, which is what a program or a directory needs. Applying 644 to a directory is a common mistake: you can list the names inside it and nothing more, since without execute you cannot enter it or read any file within.

It makes a program run as its owner rather than as whoever launched it, which is how passwd can update a file only root may write. It is correct for a handful of system binaries and a liability anywhere else, and on a world-writable file it is a direct privilege escalation: anyone can replace the contents of something that then runs as root. Every modern kernel ignores it on shell scripts precisely because it was too dangerous.

On a directory, it means only a file's owner may delete or rename it, even though the directory itself is writable by everyone. That is exactly what makes /tmp safe to share, and it is the right companion to any world-writable directory. On a file it means nothing on any current system; it once asked the kernel to keep the program in swap.

Because the special bit is set on something that cannot execute. A lower-case s means both setuid and execute are on; a capital S means setuid is on and execute is not, which does nothing at all and is nearly always a mistake. The same applies to T for the sticky bit.

Almost never. It is what people reach for when a permission problem is not understood, and it replaces one problem with a different and worse one: anyone on the system can change the file, and anything that runs or trusts it becomes a way in. The right fix is usually ownership or group membership rather than opening the file to everybody.