Story for lecture 3: Directories, Users, File attributes
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1. Intro: using ls to learn about directories and file attributes

   Last time we looked at the who command and learned about
   how to read and write files.  We studied the system calls
   open, read, write, creat, close, lseek.  These allow a
   programmer to operate on the CONTENTS of a file.

   Here's a picture.

   BUT, files are not islands.  They are organized on the disk
   into directories, and they have attributes.  Today we shall
   study directories and file attributes.  To do so, we shall
   write a version of the unix utility called ls.

2. Using ls

   If we are going to write a version of ls, we should become
   familiar with what it does.

      $ ls
      $ ls /tmp 
      $ ls / ls1.c samples.03
   
      $ ls -l
      $ ls -l /tmp
      $ ls -l ls1.c

      $ ls -a
      $ ls -a /
      $ ls -lu

   Note: args can be nothing: defaults to current directory
                       files: shows info about files
                 directories: shows contents of directory

3. A brief review of the Unix directory tree

   Any hard disk is organized into a tree of directories, each of
   which contains files and/or directories. Such a structure is
   called a file system

   cd, pwd, ls allow you to explore the file system

4. Ok, so to write ls, we just need to display the files contained
   in a directory... Maybe it will be like who: 

         open, read_struct, display_info, close

5. What is a Directory?

   Fuzzy Ans: a collection of files and/or dirs
    Real Ans: a special kind of file that contains a list of f a/o d
        Note: it also contains two special items, . and ..
       Again: it is just a file that contains a list of filenames

6. Can we use open, read, close to get the contents of a directory?

   Ans1: on old versions of Unix, that was the only way
   Ans2: on some versions of Unix, you still can, but not for all dirs
   demo: cat / ; cat /tmp ; cat /home
         od -c / ; od -c .
   Ans3: Some versions of Unix (e.g. Linux, never)
   Why?: Unix allows various disk formats to appear as part of a
         single tree.  It supports Mac HFS, FAT, FAT32, lots of Unix
         flavors.  So there IS NO SINGLE FORMAT, no utmp struct to
         read.

7. OK, OK, so how DO I read the contents of a directory?

   $ man -k direct
   $ man -k direct | grep read

   Answer: opendir, readdir, closedir

   Application: ls1, just like who, sort of

8. How close are we to a complete version of ls?

   Problems:   a) no columns
               b) no sorting
               c) no -a
               d) no -l

   Solutions:  a) boring, but doable
               b) use qsort, build a table
               c) not too hard
               d) hmmm

9. How do we add the -l option?

   Real Question: what does -l display?
          Answer: size, times, owner, group type, permissions

        Question: Where do we get file info?
          Search: $ man -k file 
                  $ man -k file | grep information | more

          Answer: stat()

10. How does stat() work?

     Diagram: the disk stores the contents of the file
              AND the info about a file.
              read() gets the contents
              stat() gets the info
              It copies the info from the disk 
                  into a buffer in user space
     example: statdemo.c

11. What info do we need?

       type, access: mode
              owner: owner
              group: group
              mtime: mtime
               size: size
              links: links
     Soln: we write stat1.c 

12. How close are we?

          We have the info, but it is still in code.
          Our first version of who was like this.
          All we needed to do was to figure out how to
          decode the fields:

              size: ok
             links: ok
             mtime: ok (use ctime)
       owner/group: need to translate number to name
              type: ?
              perm: ?

13. Where is type and permissions encoded?

    Ans: in the mode value
    How: the 16-bit number stores type, rwxrwxrwx and three extras
    How does that work?
    Answer: A brief into to binary math
            The bitwise AND operator
            A brief intro to octal math
            The idea of a mask: grading multiple-choice tests
    Result: decoding the permission bits
            decoding the type - 

14. How do we convert uid to name?

    Intro to /etc/passwd 

	    $ man -k uid
	    $ man getpwuid
            
    The library function getpwuid() converts a uid to a struct
    with a name.

15. How do we convert gid to group name?

    Same idea.  Meet /etc/group

16. stat2.c - the detail line from ls -l

17. How can we combine stat2.c with ls1.c ?
    soln: stat2 `ls1 | sort`
    But what about ls2 -l filename ?