Minimal Perl For UNIX and Linux People 9 potx

356 CHAPTER 10 LOOPING FACILITIES

From a Perlish perspective, you can think of select as a special kind of interactive

variation on a foreach loop. But rather than having each list-value assigned auto￾matically to the loop variable for one iteration, select only assigns values as they are

selected by the user.

Next, you’ll see how you can avoid “re-inventing wheels” by using this loop.

10.7.1 Avoiding the re-invention of the

“choose-from-a-menu” wheel

Although Perl has no counterpart to the Shell’s handy select loop, its functionality

is provided by a CPAN module called Shell::POSIX::Select.

21 It provides its

services through source-code filtering, which means it extracts the select loops from

your program and rewrites them using native Perl features. As a result, you can use a

feature that’s missing from Perl as if it were there!

The benefit of bringing the select loop to Perl is that it obviates the need for ter￾minal applications to provide their own implementations of the choose-from-a-menu

code, which indulges the programmer’s noble craving for Laziness—and thereby

increases productivity.

Table 10.9 shows the syntax variations for the Shell’s version of the select loop.

If in LIST is omitted (as in Form 0), in "$@" is used by default to provide automatic

processing of the script’s (or function’s) argument list.

Some of the major forms of Perl’s select loop are shown in table 10.10. These

take their inspiration from the Shell and then add enhancements for greater friendli￾ness and, well, Perlishness.

As you can see, Perl’s select lets you omit any or even all of its components (apart

from the punctuation symbols). For example, if the loop variable is omitted, as in

Forms 0, 1, and 2, $_ is used by default. If the LIST is omitted, as in Forms 0 and 1,

the appropriate arguments are used by default (i.e., those provided to the script or the

21 Written by yours truly, a long-time Shell programmer turned Perl proponent, while writing this chap￾ter—so I wouldn't have to say “the best Shell loop is missing from Perl”.

Table 10.9 The Shell’s select loop

select var ; do commands; done # Form 0

select var in LIST; do commands; done # Form 1

Table 10.10 The select loop for Perl

use Shell::POSIX::Select;

select () { } # Form 0

select () { CODE; } # Form 1

select (LIST) { CODE; } # Form 2

select $var (LIST) { CODE; } # Form 3

THE CPAN’S select LOOP FOR PERL 357

enclosing subroutine), as with its Shell counterpart. And if CODE is omitted (as in

Form 0), a statement that prints the loop variable is used as the default code block.

Because system administrators have the responsibility for monitoring user activity

on their systems, they might find the following application of select to be of par￾ticular interest.

10.7.2 Monitoring user activity: the show_user script

This program allows the user to obtain a system-activity report for users who are cur￾rently logged in:

$ cat show_user

#! /usr/bin/perl –wl

use Shell::POSIX::Select;

# Get list of who's logged in

@users=`who | perl -wnla -e ' print \$F[0]; ' | sort -u`;

chomp @users; # remove newlines

# Let program's user select Unix user to monitor

select ( @users ) { system "w $_"; }

This script uses the who command to get the list of current users, and then a separate

Perl command to isolate their names from the first column of that report. Note the

need to backslash the $ to prevent the Perl script from providing its own (null) value

for $F[0] before the who | perl | sort pipeline is launched. sort is used with the

“unique lines” option to remove duplicate user names for those logged in more than

once. The w command, which reports the selected user’s activity, won’t appreciate

finding newlines attached to the ends of those names, so the @users array is chomp’d

to remove them.

Here’s a sample run of the script:

$ show_user

1) phroot 2) tim

Enter number of choice: 2

3:51pm up 4 days, 17:57, 7 users, load average: 0.00, 0.00, 0.00

USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT

tim pts/1 lumpy Mon10am 3days 18.91s 1.19s -bash

tim pts/3 stumpy Mon10am 28:16m 0.48s 0.48s bash -login

tim tty5 grumpy Sun 3pm 28:16m 1.71s 1.04s slogin lumpy

tim pts/0 bumpy Sun 4pm 1.00s 4.03s 0.14s w tim

<ENTER>

1) phroot 2) tim

Enter number of choice: <^D>

358 CHAPTER 10 LOOPING FACILITIES

Note that the user pressed <ENTER> to redisplay the menu and <^D> to exit the loop,

just as she’d do with the Shell’s select.

22

Next, you’ll see how select can facilitate access to Perl’s huge collection of

online man pages.

10.7.3 Browsing man pages: the perlman script

One of the obstacles faced by all Perl programmers is determining which one of Perl’s

more than 130 cryptically named man pages covers a particular subject. To make this

task easier, I wrote a script that provides a menu interface to Perl’s online documentation.

Figure 10.1 shows the use of perlman, which lets the user choose a man page

from its description. For simplicity’s sake, only a few of Perl’s man pages are listed in

the figure, and only the initial lines of the selected page are displayed.23

22 For those who don’t like that behavior (including me), there’s an option that causes the menu to be

automatically redisplayed before each prompt. I wish the Shell’s select also had that feature!

23 The select loop is a good example of the benefits of Perl’s source-code filtering facility, which is de￾scribed in the selected man page, perlfilter.

Figure 10.1 Demonstration of the perlman script

THE CPAN’S select LOOP FOR PERL 359

Before we delve into the script’s coding, let’s discuss what it does on a conceptual level.

The first thing to understand is that man perl doesn’t produce “the” definitive

man page on all things Perlish. On the contrary, its main purpose is to act as a table

of contents for Perl’s other man pages, which deal with specific topics.

Toward this end, man perl provides a listing in which each man page’s name is

paired with a short description of its subject, in this format:

perlsyn Perl syntax

As illustrated in the figure, the role of perlman is to let the user select a man-page

name for viewing from its short description.

Listing 10.7 shows the script. Because it’s important to understand which of its

elements refer to the man-page names versus their corresponding descriptions, dis￾tinctive highlighting with bold type (for man-page names) and underlined type (for

descriptions) is used.

1 #! /usr/bin/perl -w

2

3 use Shell::POSIX::Select;

4

5 $perlpage=`man perl`; # put name/description records into var

6

7 # Man-page name & description have this format in $perlpage:

8 # perlsyn Perl syntax

9

10 # Loop creates hash that maps man-page descriptions to names

11 while ( $perlpage =~ /^\s+(perl\w+)\s+(.+)$/mg ) { # get match

12

13 # Load ()-parts of regex, from $1 and $2, into hash

14 $desc2page{$2}=$1; # e.g., $hash{'Perl syntax'}='perlsyn'

15 }

16

17 select $page ( sort keys %desc2page ) { # display descriptions

18 system "man $desc2page{$page}"; # display requested page

19 }

The script begins by storing the output of man perl in $perlpage on Line 5. Then

a matching operator, as the controlling condition of a while loop (Line 11), is used

to find the first man-page name (using “perl\w+”) and its associated description

(using “.+”) in $perlpage. The m modifier on the matching operator allows the

pattern’s leading ^ to match the beginning, and its $ the end, of any of the lines

within the variable (see table 3.6 on multi-line mode).

Capturing parentheses (see table 3.8) are used in the regex (Line 11) to store

what the patterns matched in the special variables $1 and $2 (referring to the first

Listing 10.7 The perlman script

360 CHAPTER 10 LOOPING FACILITIES

and second set of parentheses, respectively), so that in Line 14 the man-page name

can be stored in the %desc2page hash, using its associated description as the key.

The next iteration of the loop will look for another match after the end of the pre￾vious one, due to the use of the matching operator’s g modifier in the scalar context

of while’s condition.24

Finally, in Lines 17–19, select displays the numbered list of sorted man-page

descriptions in the form “7) Perl source filters”. Then it obtains the user’s selection,

retrieves its corresponding page name from the hash, and invokes man to display the

requested page (in the case of figure 10.1, “perlfilter”).

As you might imagine, this script is very popular with the students in our classes,

because it lets them find the documentation they need without first memorizing lots

of inscrutable man-page names (such as “perlcheat”, “perltoot”, and “perlguts”).

TIP You can use the only Shell loop that Larry left out of Perl by getting the

Shell::POSIX::Select module from the CPAN.

10.8 SUMMARY

Perl provides a rich collection of looping facilities, adapted from the Bourne shell, the

C shell, and the C language.

The closely-related while and until loops continue iterating until the control￾ling condition becomes False or True, respectively. You saw while used to incremen￾tally compress images until a target size was reached (in compress_image, section

10.2.2) and to extract and print key/value pairs from a hash with the assistance of the

each function (in show_pvars, section 10.2.3).

Perl also provides bottom-tested loops called do while and do until, which

perform one iteration before first testing the condition. Although these aren’t “real”

loops, the savvy programmer can construct functional replacements using while and

until with continue blocks to allow loop-control directives to function properly

(as shown in confirmation, section 10.6.4).

The foreach loop provides the easiest method for processing a list of values,

because it frees you from the burden of managing indices. You saw it used to remove

files (rm_files, section 10.4.1) and to perform text substitutions for deciphering

acronyms in email messages (expand_acronyms, section 10.4.4).

The relatively complex for loop should be used in cases where iteration can be con￾trolled by a condition, and which benefit from its index-management services. An exam￾ple is the raffle script (section 10.5.1), which needs to process its arguments in pairs.

24 The meaning of the matching operator’s g modifier is context dependent—in list context, it causes all

the matches (or else the captured sub-matches, if any) to be returned at once. But in scalar context, the

matches are returned one at a time.