The format of the report should exactly match the format of the sample
report. Since this script is to be standalone, it might be easier as a
shell script that can pipe the results of "last" into awk, rather than
a pure awk script.
It is important to note that the output of ttylast does not contain
multiple uses of a tty by a user -- only the most recent. This makes
your life much easier.
In order to keep the assignment fun, you are allowed to use only the
awk (awk, nawk or gawk) and last commands -- well, and, of course, a
shell.
First, begin by looking at the output from my program. Then, take a look
at the output of last. Make sure you understand the name of the game.
To help you out, I included a copy of last's output captured from
within my script, as well as my script's output.
There are many ways of solving this problem. As a hint, I'll tell you
how I did it. As I worked my way through last's output one line at a time,
I kept three arrays:
- one of users (by user)
- one of ttys (by tty)
- one of the "whole lines" indexed by their "userid,tty" pair
The first two arrays amount to nothing more than a list of users and a list
of ttys, respectively. In reality, I really only cared about the keys, not
the values.
The next array gave me the mapping. By building the "userid,tty" key", I
gave myself an easy way to solve the problem. Essentially, I could find
any pair. Invalid pairs would be the empty string "". So,
if I'd look up "gkesden, pts/8" and gkesden didn't use pts/8, the result
would be "".
So, to produce the output, I used two nested loops. The outer loop walked
through the list of ttys. The inner loop walked through the lists of users.
The code inside printed out the line of last mapped to "user,tty", well,
at least unless that lookup yielded the empty string ("").
The last detail was the annoying extraneous line in last's output:
wtmp begins Tue Sep 12 04:06:14 2006
To avoid this, I filtered by "action" so that it wouldn't happen
if the pattern contained a "wtmp" in the first field.
One detail that I haven't given you: How to keep the most recent,
rather than the least recent, use. Think about this one a bit. One
technqiue is to reverse the output of last. This can be done with
sed. Or, as an alternative, you can use nl to number
the lines and then sort in reverse, and then whack the line numbers
using, for example, awk or cut. Another technique is to
us an "if statement" within AWK to add the entry if, and only if, there
isn't already one. But, you surelly don't want to sort in AWK since,
so far as I know, there is no built-in sort function.
NAME
getimage - get images by keyword from www.flickr.com
SYNOPSIS
getimage [OPTION]... KEYWORD [KEYWORD ...]
DESCRIPTION
Query Flickr's Web site, by keyword, for images. Download images
matching all provided keywords, saving them into the current
working directory.
Saved files maintain their extension, but are named after the keywords,
and enumerated, beginning with 0, in the order downloaded. For example,
a query via the keywords "pony" and "hat" might result in the creation of
the files "pony_hat_0.jpg" and "pony_hat_1.jpg".
The use of multi-word keywords is not support. For example,
"George Washington" may only be expressed as two and'ed keywords, not an
ordered tuple.
-n n
Optional argument limiting the number of results to, at most,
n images. The default limit, absent this argument, is
10.
-t target_directory
Optional argument forcing files to be saved into the
target_directory instead of the current working directory.
RETURN VALUE
Returns 0 on success, non-zero on any other condition. For example, returns
1 if the target directory does not exist.
EXAMPLES
getimage pony hat
Download up to 10 images matching both of the keywords "pony" and
"hat", save them into the current working directory, and name them using the
convention as follows: pony_hat_0.jpg, pony_hat_1.jpg, &c. The limit of
at most 10 matching images is the default imposed in the absence
of the -n flag, which defines a user-specified limit.
getimage -n3 pony hat
As above, but save no more than three (3) matches.
getimage -n3 -timage_files pony hat
As above, but place results into the "images files" subdirectory.
We'll be using the get() and getstore() functions from the LWP library to
interact with the Web site. The get() function requests a page by URL. The
resulting object is returned as a string. The getstore() function has two
arguments, the second of which provides the name of the file to which the
object should be saved on disk.
The examples below illustrate these uses of get() and getstore(). In each of
the examples below, please notice the "use LWP::Simple;" line. It is an
tells PERL that we'll be using the Simple object from within
the LWP library. The same form is used to include other libraries in
PERL.
Example 1: Returning the page as a string
#!/usr/bin/perl
use LWP::Simple;
$url = "http://www.google.com";
print get($url);
Example 2: Storing the page into a file
#!/usr/bin/perl
use LWP::Simple;
getstore('http://www.cmu.edu', 'cmu.html');