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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?

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...uh, because that's what the "+x" flag is for :) –  badp Sep 22 '11 at 19:44
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Think like this: the directory entry contains file names, so "reading" a directory is listing the files, "using" the directory is accessing the files. –  tylerl Sep 23 '11 at 9:12

3 Answers 3

up vote 94 down vote accepted

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)
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6  
Great answer, but I think the last sentence is misleading. None of these permissions can be overridden per-file actually. Here “access” is a bit ambiguous: +x on the directory grants access to files inodes through this specific directory (nothing less, nothing more, well… maybe chdir needs +x too). To read or write the contents of one file, the user also needs +r/+w on this file, but those are distinct permissions (they do not override anything). –  Stéphane Gimenez Sep 22 '11 at 13:06
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The immutable flag is filesystem specific and it won't precisely override those permissions, that's why I think the last sentence is misleading :-) –  Stéphane Gimenez Sep 22 '11 at 13:18

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.

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Thinking about directory as a list makes things more clear and logical. –  Trismegistos Jan 9 at 8:51
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+1 for the file listing image. Love it! –  Euloiix Aug 15 at 19:33
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Great answer, but too focused on the term "working directory". I need the x bit for any access to this file: for cat a/b/c/d, I need the x bit on all a, b and c, even if I don't use them as cwd. –  glglgl Nov 3 at 9:24
    
This is where I found out you can't write unless it's executable too! Case 2 is also interesting, great answer! –  Mirko Nov 30 at 1:56

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.

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