Running Linux, 5th Edition - Matthias Kalle Dalheimer [416]
19 print "User $user, total login time ";
20 # Perl has printf, too
21 printf "%02d:%02d, ", $hours{$user}, $minutes{$user};
22 print "total logins $logins{$user}.\n";
23 }
Line 1 tells the loader that this script should be executed through Perl, not as a shell script. Line 3 is the beginning of the program. It is the head of a simple while loop, which C and shell programmers will be familiar with: the code within the braces from lines 4 to 10 should be executed while a certain expression is true. However, the conditional expression Perl reads input one line at a time (unless you tell it to do otherwise). It also reads by default from standard input unless you tell it to do otherwise. Therefore, this while loop will continuously read lines from standard input until there are no lines left to be read. The evil-looking mess on line 5 is just an if statement. As with most programming languages, the code within the braces (on lines 7-9) will be executed if the expression that follows the if is true. But what is the expression between the parentheses? Those readers familiar with Unix tools such as grep and sed will peg this immediately as a regular expression: a cryptic but useful way to represent a pattern to be matched in the input text. Regular expressions are usually found between delimiting slashes (/.../). This particular regular expression matches lines of the form: mdw ttypf loomer.vpizza.co Sun Jan 16 15:30 - 15:54 (00:23) This expression also "remembers" the username (mdw) and the total login time for this entry (00:23). You needn't worry about the expression itself; building regular expressions is a complex subject. For now, all you need to know is that this if statement finds lines of the form given in the example, and splices out the username and login time for processing. The username is assigned to the variable $1, the hours to the variable $2, and the minutes to $3. (Variables in Perl begin with the $ character, but unlike the shell, the $ must be used when assigning to the variable as well.) This assignment is done by the regular expression match itself (anything enclosed in parentheses in a regular expression is saved for later use to one of the variables $1 through $9). Lines 6 through 9 actually process these three pieces of information. And they do it in an interesting way: through the use of an associative array. Whereas a normal array is indexed with a number as a subscript, an associative array is indexed by an arbitrary string. This lends itself to many powerful applications; it allows you to associate one set of data with another set of data gathered on the fly. In our short program, the keys are the usernames, gathered from the output of last. We maintain three associative arrays , all indexed by username: hours, which records the total number of hours the user logged in; minutes, which records the number of minutes; and logins, which records the total number of logins. As an example, referencing the variable $hours{'mdw'} returns the total number of hours that the user mdw was logged in. Similarly, if the username mdw is stored in the variable $1, referencing $hours{$1} produces the same effect. In lines 6 to 9, we increment the values of these arrays according to the data on the present line of input. For example, given the input line: jem ttyq2 mallard.vpizza.c Sun Jan 16 13:55 - 13:59 (00:03) line 7 increments the value of the hours array, indexed with $1 (the username, jem), by the number of hours that jem was logged in (stored in the variable $2). The Perl increment operator += is equivalent to the corresponding C operator. Line 8 increments the value of minutes for the appropriate user similarly. Line 9 increments the value of the logins array by one, using the ++ operator. Associative arrays are one of the most useful features of