Perl 学习手札之二： Guide to experienced programmers

open a file and return its number of lines.

Version 1st:

#!/usr/bin/perl
use strict;
use warnings;

my $filename = "linesfile.txt"; # the name of the file

open(FH, $filename);    # open the file
my @lines = <FH>;       # read the file
close(FH);              # close the file

my $count = scalar @lines;  # the number of lines in the file
print("There are $count lines in $filename\n");

Version 2nd

#!/usr/bin/perl
use strict;
use warnings;
use IO::File;

my $filename = "linesfile.txt"; # the name of the file

# open the file - with simple error reporting
my $fh = IO::File->new( $filename, "r" );
if(! $fh) {
    print("Cannot open $filename ($!)\n");
    exit;
}

# count the lines
my $count = 0;
while( $fh->getline ) {
    $count++;
}

# close and print
$fh->close;
print("There are $count lines in $filename\n");

Version 3rd:

#!/usr/bin/perl
use strict;
use warnings;
use IO::File;

main(@ARGV);

# entry point
sub main
{
    my $filename = shift || "linesfile.txt";
    my $count = countlines( $filename );
    message("There are $count lines in $filename");
}

# countlines ( filename ) - count the lines in a file
# returns the number of lines
sub countlines
{
    my $filename = shift;
    error("countlines: missing filename") unless $filename;

    # open the file
    my $fh = IO::File->new( $filename, "r" ) or
        error("Cannot open $filename ($!)\n");
    
    # count the lines
    my $count = 0;
    $count++ while( $fh->getline );

    # return the result
    return $count;    
}

# message ( string ) - display a message terminated with a newline
sub message
{
    my $m = shift or return;
    print("$m\n");
}

# error ( string ) - display an error message and exit
sub error
{
    my $e = shift || 'unkown error';
    print("$0: $e\n");
    exit 0;
}

note： 以上的三个script都是在做一个事情，统计一个文件的行数， 相比下第三个更具备扩展性，更易于维护。

　注意此时shift的用法；

type "perldoc IO:FIle" in terminal

type "perldoc -f" in terminal

if using eclipse, right click and choose Perldoc, and will get the same as in terminal