The information that constitutes a file in the ext2/ext3 file system begins with the inode. The inode contains the following description of the file: the file type, its access rights, its owners, several different time stamps, the file size, and pointers to data blocks that hold the file information. When you want to access a file, the kernel uses those pointers to calculate where the data resides physically on the disk.

File attributes are also stored in the inode. The chattr command enables the root operator to manipulate some special attributes of files on an ext2/3 file system. One of the most interesting uses of the chattr command is to make a file immutable, meaning that it cannot be deleted, renamed, written to, or modified by anyone, even root (at least until the immutable attribute is unset). To make a file immutable (the word means unalterable; not capable of change):

# chattr +i filename

and to change it back:

# chattr -i filename

Another interesting flag is the s flag, which tells the system to zero out all the blocks used for that file when the file is deleted. To observe the existing attributes of a file, use:

# lsattr filename

The chattr utility is part of the e2fsprogs package.

A file's attributes include its access permissions. The traditional Unix system of permissions is controlled with chmod. Posix Access Control Lists extend this functionality.

ACL supports permissions for users and groups beyond the traditional owner/group/world scheme. They enable finer-grained control of permissions for files.

To learn more about Access Control Lists, visit the Posix ACLs for Linux website: http://acl.bestbits.at/.