CSCB09S Assignment 2

Due: Fri. Feb 16, 2001 (at noon)

Your employer just bought a broken Unix system off the street, and much to your dismay you discover that the ls command is missing. You decide to write a replacement called mydir which will do some of what ls does. You design the program as follows:

• The program reads a single directory, and prints information about all files in that directory.
• The program sorts its output alphabetically by file name, unless a switch (described below) is given.
• If no file argument is given, the program looks at the current working directory '.'. Otherwise it examines the directory named on the command line.
• The switch -t makes the program sort the files from most recently modified to least recently modified.
• The switch -r reverses the sense of the default sort, or the time sort indicated by the -t switch.
• The switch -a makes mydir print files whose names start with '.', which are ordinarily suppressed.
• The program prints out the following information in tabular form (i.e. in fixed-width fields): the ``mode'' of the file showing the permission bits and whether the file is actually a directory (exactly as ls would show it); the login-name and group of the file owner, the file size in bytes, the date of last modification, and the file name.

A partial example output is shown below:

drwx------     harper    users        512 Oct 27  2000 HOWTO
-rw-------     harper    users        4426 Sep 21  2000 HelloWorld.class
-rwx------     harper    other        1113 Sep 21  2000 HelloWorld.java
drwx------     harper    users       2048 Jan 31 11:15 Mail
drwxr-x---     harper    other        512 Jan  5 14:59 NEW
drwx------     harper    users        512 Nov 26  1999 News
-rw-------     harper    users       13354 Oct 23  2000 PW.C
drwx------     harper    users       1024 Dec 22 11:01 System

Observe the changing date format: for files more than about 90 days old, only the day and year are shown, for newer files, the year is suppressed and the time is shown. In either case the date occupies the same amount of space in the output. You will have to extract the required pieces from the ctime() output.

What to do

This is a program about command-line processing and system calls. You will have to read the man-pages on a number of library functions as noted below and determine how to use them. Pay particular attention to the include files listed, since these provide the function prototypes and structure definitions that you require.

Here are some of the library functions and man pages that will be of use in writing this program:

getopt(), chdir(), stat(), opendir(), readdir(), rewinddir(), qsort(), getpwuid(), getgrgid(). ctime(), strcmp(), time(), mknod.

What to hand in

Submit the code and the output of your program when run on the /etc directory on fissure using the -a, -t, and -r flags together.

Notes and Hints

As always, start NOW.

This program is easily built-up in stages, each of which is largely independent of the others. Thus, testing and development are straightforward. The final program, if well written, shouldn't be more than about 200 lines, in one file (not including comments).

• Since you must sort the data by various criteria, you need to have it all in hand before you sort or print. The easiest way to do this is to simply make two passes through the directory with readdir(), the first counts how many files there are, then you can allocate appropriate storage and load it with the second pass.
• The file name is obtained from the directory entry, but all other information comes from the inode as returned by the stat() call. Your storage needs to be able to store both sets of info (nested structures is an obvious way to do this) ¹.
• Once you've designed your storage, allocate an entire array of it, not just an array of pointers.
• qsort() takes the address of an array, the number of elements, and the size of each, and a function that provides the ordering information. This function gets two address (of two elements in the array), and can compare these elements (or parts thereof) any way it pleases. It returns 1 if the first element is considered ``greater'' than the second, -1 if less, and 0 if they are equal. You can have more than one sort function.
• Reversing the sort is trivial. If you seem to need lots of code to do it, rethink.
• You can deal with the permissions using bit-operations and the masks provided in the stat include file.
• Unix times are actually stored as a long int representing the number of seconds elapsed from Jan. 1 1970 GMT. The ordering of such time stamps is obvious, and the ctime() routine can be used to convert to a string representation of the time. Observe that each file has three times associated with it: the last access time, the last modify time, and the last change time. You are only interested in one of these.
• Watch this space on-line for any further notes or hints.

Footnotes

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Just because you're not using all the info now doesn't mean the program might not need it in the future. Plan ahead and keep most of the data.