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How many bits on a linux file system is taken up for the permissions of a file?

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Do you literally mean the traditional permissions bits, or do you care about other permission-like metadata? – mattdm Apr 1 '11 at 1:36
up vote 7 down vote accepted

To add to the other answers:

Traditional Unix permissions are broken down into:

  • read (r)
  • write (w)
  • execute file/access directory (x)

Each of those is stored as a bit, where 1 means permitted and 0 means not permitted.

For example, read only access, typically written r--, is stored as binary 100, or octal 4.

There are 3 sets of those permissions, which determines the allowed access for:

  • the owner of the file
  • the group of the file
  • all other users

They are all stored together in the same variable, e.g. rw-r-----, meaning read-write for the owner, read-only for the group, and no access for others, is stored as 110100000 binary, 640 octal.

So that makes 9 bits.

Then, there are 3 other special bits:

  • setuid
  • setgid
  • sticky

See man 1 chmod for details of those.

And finally, the file's type is stored using 4 bits, e.g. whether it is a regular file, or a directory, or a pipe, or a device, or whatever.

These are all stored together in the inode, and together it makes 16 bits.

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Thank you for a great visual explanation to the answer! – KerxPhilo Apr 1 '11 at 3:21
After I was confused by this blog post about linux file permissions I almost wanted to ask about it in UL-SE but lucky I found your explanation. – humanityANDpeace Feb 23 '14 at 7:55

Which permissions? Basic permissions fit in 16 bits; ext2 uses 32 bits, plus another 32 bits for file flags (chattr(1)); then POSIX ACLs use variable space in addition. See /usr/include/linux/ext2_fs.h for details. (ext3 and ext4 build on ext2 and mostly use the same structure.)

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Information about files are stored in a data structure called an inode. There is a field in this structure for the mode, which contains the permissions. This field on my system is an unsigned short which is 2 bytes and 16 bits.

Take a look at fs.h in the Linux source to see for yourself.

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