Types of files

• There are 7 different types of files.
• ls -l

  -rw-rw-r--    1 bennett  bennett       244 Feb 26 09:41 foo.C
  drwxrwxr-x    2 bennett  bennett      4096 Nov 27 07:28 games
  brw-rw----    1 bennett  floppy     2,   0 Jan 30  2003 /dev/fd0
  crw-------    1 root     root      10,   1 Mar  5 11:06 /dev/psaux
  lrwxrwxrwx    1 bennett  bennett         4 Mar 14 11:27 blah -> /tmp
  srwxrwxrwx    1 root     root            0 Mar  5 11:06 X0
  prw-rw-r--    1 bennett  bennett         0 Mar 14 11:29 apip
     

  • The first character tells us what it is .
    • - regular file
    • d directory
    • b block special
    • c character special
    • l symbolic link
    • s unix domain socket
    • p pipe/fifo
  • The other letters tell us permissions
    • Groups of three
    • read, write, and execute permissions for user, group, other
    • The most applicable permission set applies,
      • If you own the file, user permissions apply
      • If you are in the group, but don't own the file, group permissions apply
      • Otherwise other permissions apply.
    • Basic permission meanings
      • r means that you can read the file
      • w means that you can write/modify the file
      • x means that you can try to run (execute) the file.
        • If it is a binary, it is just run
        • If it is not a binary, it looks at the first line of the file. If it is a structured comment, it tries to run it with the command interpreter supplied.
        • look at any of the files in /etc/rc.d

          #!/bin/sh
          		     

    • These are different for directories (but not really)
      • read means you can do an ls on the directory
      • write means that you can create/delete files inside of the directory.
      • execute means that you can make this your current working directory. (or cd to it, or under it)
    • Permissions are changed with chmod
      • chmod permissions path
      • Permissions are made up of three pieces.
        • a combination of ugo
        • a + or -
        • a combination of rwx
      • An older form is
        • chmod ddd path
        • where ddd are digits for u g and o
        • the digits are
          • 100 - read
          • 010 - write
          • 001 - execute
        • so chmod 761 foo
        • is u = rwx, g=rw, o=x
        • These are actually hex digits.
        • 1--- is used for suid, sgid and stickey bit.
      • suid, sgid, stickey bit
        • chmod u+s, g+s, o+s
        • For u+s this is suid or set userid.
          • chmod u+s filename
          • chmod 900 filename (s+x)
          • When this is set, and the file is executable
          • When someone other than the owner executes the file, it is run as the owner.
          • This is the mechanism used to let normal users change things they normally don't have access to.
          • passwd for example.

            -r-s--x--x    1 root     root        16336 Feb 13  2003 /usr/bin/passwd
            		    

          • In this case, when password is run, it is run as root, no matter who runs it.
          • If the user does not have permissions to execute then the s is a S
        • sgid
          • chmod g+s, chmod 080
          • normal files, not much
          • Directories, all files created in that directory are created with the directories group ownership.
        • Stickey bit
          • not much any longer
          • On directories, (like /tmp) you can't delete or move files you don't own, even if the permissions are open.
• File Types
  • regular files
    • These are the files normal files
    • text, executables, and everything else.
    • You know about these.
  • directories
    • You should be much better with these now.
  • character and block
    • these are the files in /dev
    • they are where unix interacts with devices
    • character are for keyboard like things
    • block are for disk drive like things
    • Each has a major and minor number

      [bennett@mirkwood bennett]$ ls -l /dev/hda?
      brw-rw----    1 root     disk       3,   1 Jan 30  2003 /dev/hda1
      brw-rw----    1 root     disk       3,   2 Jan 30  2003 /dev/hda2
      brw-rw----    1 root     disk       3,   3 Jan 30  2003 /dev/hda3
      brw-rw----    1 root     disk       3,   4 Jan 30  2003 /dev/hda4
      brw-rw----    1 root     disk       3,   5 Jan 30  2003 /dev/hda5
      brw-rw----    1 root     disk       3,   6 Jan 30  2003 /dev/hda6
      brw-rw----    1 root     disk       3,   7 Jan 30  2003 /dev/hda7
      brw-rw----    1 root     disk       3,   8 Jan 30  2003 /dev/hda8
      brw-rw----    1 root     disk       3,   9 Jan 30  2003 /dev/hda9
      	   

    • The major number tells the kernel what routine to call to access the device. (notice it is the same for all /dev/hda?)
    • The minor number is a parameter
    • Created with mknod
  • unix domain sockets
    • Provide a mechinism for interprocess communications.
    • socket() system call makes them
    • rm delets them.
  • named pipes
    • Another process communications mechinism.
    • mknod pipename p
  • symbolic links
    • This is a way to have one file point to another.
    • ln -s oldfile newfile
    • It is not a copy of a file, it is just a pointer to it.
• Hard Links
  • ln oldfile newfile
  • Only within a partition.
  • They are now the same file.
  • Link count in ls -l