The Filesystem

In Linux, everything is a file

• Processes
• Audio devices
• Kernel data structures and tuning parameters
• Interprocess communication channels

Main components

• A namespace
• An API
• Securiy models
• An implemenation

Path names

• Single unified hierarchy start at root: /
• Absolute path: path name starts from root
• Relative path: path name starts from current directory: . or subdirectory name

Mouning and unmounting

• The root filesystem is composed of smaller trunks (smaller filesystems)
• Smaller filesystems are attached to the tree with the mount command, which …
  • Maps a directory within the existing filesystem tree, called the mount point, to the root of the newly attached filesystem.

Who is doing what on which file system?

• fuser

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man fuser
sudo fuser -cv /users
 

• Instead of rebooting, perhaps unmounting/remounting of offending device drivers.

Organizaion of the file system tree

Filetype encoding

• Character/block device file: standard communication interface provided by device drivers.
• Local domain sockets: connections between processes that allow them to communicate hygienically.
• Named pipes allow communication between two processes running on the same host.
• Symbolic links: point to a file by name
• Hard links: create an illusion that a file exists in more than one place at the same time.

File attributes

• Traditionally 12 bits for each file: the file’s mode (plus 4 more bits : file’s type)
• 9 permission bits - read, write, execute for owner, group, others
• setuid & setgid bits (4000 , 2000)
  • setgid on directory - newly created file has group ownership of the directory (not group ownership of a user creating it)
• sticky bit (1000)
  • on regular files ignored (original meaning: keep program text on swap device)
  • on directories - only the owner of the file and the owner of that directory may remove the file from that directory

Access control lists

• supported for ext2, ext3, ext4, reiserfs, XFS, JFS: mount -o [no]acl
• allows rwx to be set independently for any user.group combination: getfacl, setfacl ( plus man acl)
• NFSv4 - superset of POSIX ACLs plus all permission bits and most semantics from Windows