chmod 500: Owner Only Read and Execute, Nothing Else

500 is 700 with the write bit taken away. On a file that means the owner can run it and read it but an editor will refuse to save, which is exactly what you want on a personal script that has bitten you once by being edited by accident, or on a program you keep in ~/bin and would rather not modify without thinking about it.

On a directory it produces something more interesting: the owner can list the names and traverse in, and cannot create, rename or delete anything. The set of entries is frozen while the files themselves remain editable according to their own modes. That is a neat way to protect the shape of a directory without protecting every file in it.

As with 555, this is a guard rail and not a control. The owner can run chmod u+w at any moment, because the ability to change a mode comes from owning the file rather than from having write permission on it. Against yourself it works well; against anything adversarial it works not at all.

The private half is what distinguishes it from 555. Nothing outside the owning account can read, run or traverse, so a 500 script keeps its contents private in a way a 555 one does not. For a personal automation script that embeds a path or a hostname you would rather not publish to every account on a shared machine, this is the right shape.

Watch the interaction with backups and syncing. Tools that recreate a directory tree will happily fail partway through when they cannot write into a 500 directory, and some report the failure only in a summary line. If you freeze a directory, make sure whatever writes into it is expected to fail, not merely likely to.