Printing Environment Variables Using Perl One Liner

Solution 1

C:\Documents and Settings\sivkumar>perl

use Data::Dumper;
print Dumper \%ENV;


Solution 2 - Perl One Liner

perl -MData::Dumper -e "print Dumper \%ENV"

Tilde-Tilde Operator

A bit obscure is the tilde-tilde "operator" which forces scalar context.

print ~~ localtime;

is the same as

print scalar localtime;

and different from

print localtime;

Spaceship Operator.

Easy with the Spaceship Operator.

$a = 2 <=> 5;  # $a is set to -1
$a = 5 <=> 2;  # $a is set to 1
$a = 2 <=> 2;  # $a is set to 0

Non-Obvious features in perl.

For example, did you know that there can be a space after a sigil?

 $ perl -wle 'my $x = 3; print $ x'
3

Or that there you can give subs numeric names if you use symbolic references?

$ perl -lwe '*4 = sub { print "yes" }; 4->()' 
yes

There's also the "bool" quasi operator, that return 1 for true expressions and the empty string for false:

$ perl -wle 'print !!4'
1
$ perl -wle 'print !!"0 but true"'
1
$ perl -wle 'print !!0'
(empty line)

Other interesting stuff: with use overload you can overload string literals and numbers (and for example make them BigInts or whatever).

Many of these thins are actually documented somewhere, or follow logically from the documented features, but nonetheless some are not very well known.

Update: Another nice one. Below the q{...} quoting constructs were mentioned, but did you know that you can use letters as delimiters?

$ perl -Mstrict  -wle 'print q bJet another perl hacker.b'
Jet another perl hacker.

Likewise you can write regexes

m xabcx
# same as m/abc/

Error : Can't do inplace edit without backup

I am writing a command line perl program to replace text content in a file .I know that the following instruction executes successfully on one of the unix machine. I am trying to execute it through cygwin.

perl -pi -e 's/siva/prabu/g;' TestScript.xml

But I get following error.
Can't do inplace edit without backup.

I tried to run:
perl -pi 'Temp.bak' -e 's/siva/prabu/g;' TestScript.xml

It gives me
Can't open -e: No such file or directory.
Can't open s/siva/prabu/g;: No such file or directory.
Can't do inplace edit without backup.

Solution :

perl -pi.bak -e 's/siva/prabu/g;' TestScript.xml

or

perl -pi.bak -e "/siva/prabu/g;" TestScript.xml

Special Variables - @ARGV

@ARGV

Short Name   :          @ARGV

Scope        :          always global

This variable is an array of the arguments passed to the script. he first element of this array is the first argument (not the program name). As the arguments are processed, the value of this variable can alter.

Example:

$TestString = "There were $#ARGV arguments first arguments @ARGV[0]\n";

print $TestString;

Perl Handles Numbers

Perl can handle both whole numbers (integers, like 37) and floating-point numbers (real numbers with decimal points, like 17.5 or -235.2). Internally, Perl handles both as 'double precision floating-point values', but in Perl code they are treated the same way and can be used interchangeably. Here are some examples of number literals :

128 (positive integer)
-127 (negative integer)
0
17.5 (positive floating number)
-4.6E13 (negative 4.6 times 10 to the 13th power. E denotes exponential notation)

The last numeric literal above is an example of exponential or scientific notation, used when you need to work with very large or very small numbers.

In addition to decimal literals, Perl supports octal (base 8) and hexadecimal (base 16) literals.

Octal literals are denoted by a leading 0, and hex literals by a leading 0x or 0X.

For example:
0177 # 177 octal, same as 127 decimal
0xf0 # f0 hexadecimal, same as 240 decimal
- 0Xff # negative ff hexadecimal, same as -255 in decimal

Many people writing Perl programs will never need to work with octal or hex numbers - but be careful not to specify decimal numbers with a leading zero, because Perl will interpret them as octal.