Why do directories need the executable (X) permission to be opened?

Question

In my CMS, I noticed that directories need the executable bit (+x) set for the user to open them. Why is the execute permission required to read a directory?

Answer 1

When applying permissions to directories on Linux, the permission bits have different meanings than on regular files.

• The write bit allows the affected user to create, rename, or delete files within the directory, and modify the directory's attributes
• The read bit allows the affected user to list the files within the directory
• The execute bit allows the affected user to enter the directory, and access files and directories inside
• The sticky bit states that files and directories within that directory may only be deleted or renamed by their owner (or root)

Answer 2

First, think: What is a directory? It's just a list of items (files and other directories) that live within. So: directory = list of names.

Read bit = If set, you can read this list. So, for example, if you have a directory named poems:

• You can ls poems and you'll get a list of items living within (-l won't reveal any details!).
• You can use command-line completion i.e. touch poems/so <TAB> poems/somefile.
• You cannot make poems your working directory (i.e. cd into it).

Write bit = If set, you can modify this list i.e. you can {add,rename,delete} names on it. But! You can actually do it only if the execute bit is set too.

Execute bit = Make this directory your working directory i.e. cd into it. You need this permission if you want to:

• access (read, write, execute) items living within.
• modify the list itself i.e. add, rename, delete names on it (of course the write bit must be set on the directory).

Interesting case 1: If you have write + execute permissions on a directory, you can {delete,rename} items living within even if you don't have write perimission on those items. (use sticky bit to prevent this)

Interesting case 2: If you have execute (but not write) permission on a directory AND you have write permission on a file living within, you cannot delete the file (because it involves removing it from the list). However, you can erase its contents e.g. if it's a text file you can use vi to open it and delete everything. The file will still be there, but it will be empty.

Summary:

Read bit = You can read the names on the list.
Write bit = You can {add,rename,delete} names on the list IF the execute bit is set too.
Execute bit = You can make this directory your working directory.

PS: The article mentioned by KAK is a good read.

Answer 3

Good article on this. Summary: A directory with its x bit set allows the user to cd into this directory, and access the files in it.