<Tr> <Td> r </Td> <Td> read </Td> <Td> read a file or list a directory's contents </Td> </Tr> <Tr> <Td> w </Td> <Td> write </Td> <Td> write to a file or directory </Td> </Tr> <Tr> <Td> x </Td> <Td> execute </Td> <Td> execute a file or recurse a directory tree </Td> </Tr> <Tr> <Td> X </Td> <Td> special execute </Td> <Td> which is not a permission in itself but rather can be used instead of x . It applies execute permissions to directories regardless of their current permissions and applies execute permissions to a file which already has at least one execute permission bit already set (either owner, group or other). It is only really useful when used with' +' and usually in combination with the - R option for giving group or other access to a big directory tree without setting execute permission on normal files (such as text files), which would normally happen if you just used "chmod - R a + rx .", whereas with' X' you can do "chmod - R a + rX ." instead </Td> </Tr>

Chmod a+x vs u+x