Perl 学习手札之七： special variables

perl uses special variable for a number of purpose.

default value for function and loops

error codes and messages

information about the system and its enviroment

over 70 special variables

　　full list in perl documentation page "perlvar"

$_ 　　　　　　　　　　Default input

$1, $2, $3, etc 　　　　　Pattern results

$!　　　　　　　　　　　system error number or string

$@ 　　　　　　　　　　eval() error

$$　　　　　　　　　　　process ID(PID)

$0　　　　　　　　　　　programe name

@_　　　　　　　　　　 list of arguments for subroutine

@ARGV　　　　　　　　list of command line arguments

@INC　　　　　　　　　List of path perl searches for libraries and modules

%ENV　　　　　　　　　Hash of environment variables

example.pl

#!/usr/bin/perl

use strict;
use warnings;

main(@ARGV);

sub main
{
#    message(join(":",@ARGV));
    message(join(":",@_));
}

sub message
{
    my $m = shift or return;
    print("$m\n");
}

sub error
{
    my $e = shift || 'unkown error';
    print("$0: $e\n");
    exit 0;
}

this code will demo @ARGV

run this code in commandline, perl example.pl one two three four five

and the result: one: two: three: four: five

in this code, substitude the  @ARGV with @_ , we will get the same result.

example1.pl

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

main("five","six","seven");

sub main
{
#    message(join(":",@ARGV));
    message(join(":",@_));
}

sub message
{
    my $m = shift or return;
    print("$m\n");
}

sub error
{
    my $e = shift || 'unkown error';
    print("$0: $e\n");
    exit 0;
}

in this code, the default array is main("five","six","seven");

example2.pl

#!/usr/bin/perl

use strict;
use warnings;

main("five","six","seven");
#main(@ARGV);

sub main
{
    foreach (@_){
    #foreach $_ (@_){
        print;
    }
}

sub message
{
    my $m = shift or return;
    print("$m\n");
}

sub error
{
    my $e = shift || 'unkown error';
    print("$0: $e\n");
    exit 0;
}

notice: the foreach loop use default $_, 不用这个$_结果一样。

filehandle.pl

#!/usr/bin/perl

use strict;
use warnings;

main(@ARGV);

sub main
{
    message("This is the template.pl exercise file from Perl 5 Essential Training.");
    error("This is the template.pl exercise file from Perl 5 Essential Training.");
}

sub message
{
    my $m = shift or return;
    #print(STDOUT "$m\n");
    print(STDOUT "$m\n");# if we want to redirect the output">", STDERR will not get it
}

sub error
{
    my $e = shift || 'unkown error';
    print(STDERR "$0: $e\n");
    exit 0;
}

notice: the default output is STDOUT.

if we replace the STDOUT with STDERR， 我们将得到一个报错的结果： 文件名+报错信息。

filehandle1.pl

#!/usr/bin/perl

use strict;
use warnings;

main(@ARGV);

sub main
{
    while(<>){
        print "$. $_ ";
    }
}

notice: $. represent line No

或者我们可以用直接一个print 语句，则默认输出$_。

constant.pl

#!/usr/bin/perl

use strict;
use warnings;

main(@ARGV);

sub main
{
    message("the filename is ". __FILE__);
    message("the line number is : ". __LINE__);
    message("the package is ". __PACKAGE__);
}

#__END__

sub message
{
    my $m = shift or return;
    print("$m\n");
}

sub error
{
    my $e = shift || 'unkown error';
    print("$0: $e\n");
    exit 0;
}
__END__

notice: __END__ is end of this script label.