on directory permissions

from https://askubuntu.com/questions/862289/difference-between-executable-directory-vs-executable-files

It’s useful to think of a directory just as a file which is a list of file names.

Read bit – If set, you can read this list. If you have a directory named books:

• You can ls books and you’ll get a list of the files it contains (-l won’t work however).
• You can use command-line completion i.e. touch books/bo+Tab to get books/bookfile.
• You cannot make books your working directory, cd won’t work.

Write bit – You can modify this list names on it. You can only do this if the execute bit is also set.

Execute bit – You need this permission if you want to:

• Have any access to files within the directory.
• Modify details of the list itself. You can add, rename or delete names on the list, but this also requires write permission on the directory.

Setting the execute bit on a directory does not in any way affect the files themselves, but it does affect your access to them. For instance if you have write and execute access for a directory, you can move, rename and delete files, even if you can’t write into the files themselves.