Users and Files

• Users
  • Are uniquely identified by a userid
  • Userid 0 is special, it is root.
  • Root can
    • Access any file on the system.
    • Send a signal to any process on the system
    • Generally you should only use the superuser account to do system admin type stuff.
  • The username is associated with this in /etc/passwd (possibly)
  • Your home directory and login shell are listed here as well.
  • There are groups as well.
• File System
  • As pictures on page 27, the file system is (sort of) a tree.
  • It is definitely built as a single hierarchical structure.
  • But we can form loops and point back up the tree (but this is not the default behavior)
  • There are many different types of files.
    • Normal text files.
    • Directories or folders
    • Symbolic links
    • Hard links
    • Pipes
    • Character special devices
    • Block special devices
      • File names
        • can contain anything but a / and the null character
        • But you would be silly to use much other than digits, letters and .
        • - for example becomes somewhat problematic as most shells think it is something else.
        • . and .. are used by the os so they are out.
      • Path names
        • There are two (and a half) ways to specify a path.
        • Absolute comes from root down (pwd)
        • relative involves .. to go up
        • ~ is a shell thing
          • ~/ means the current user's home directory
          • ~user means from another user's home directory
      • Each process has an associated current working directory.
      • Each file has an owner and permissions.
    • File I/O model
      • They claim unix/linux has a universal I/O model
      • Everything is a file
      • All files are manipulated in the same way
        • open, read, write, close
      • Files in programs are represented by file descriptors
        • A small positive integer (or 0)
        • Which is really just a pointer into a kernel table.
        • Three default stdin, stdout, stderr
        • These map to cin, cout and cerr in C++