OS: 14.0-RELEASE-p6
Shell: tcsh
$ mkdir ~/beginner_s_introduction_to_perl
$ cd ~/beginner_s_introduction_to_perl
$ command -v perl; type perl; which perl; whereis perl
/usr/local/bin/perl
perl is /usr/local/bin/perl
/usr/local/bin/perl
perl: /usr/local/bin/perl /usr/local/lib/perl5/5.30/perl/man/man1/perl.1.gz /usr/src/contrib/file/magic/Magdir/perl
$ command -v env; type env; which env; whereis env
/usr/bin/env
env is /usr/bin/env
/usr/bin/env
env: /usr/bin/env /usr/share/man/man1/env.1.gz /usr/src/usr.bin/env
$ vi first.pl
$ cat first.pl
#!/usr/bin/env perl
print "Hello, World!\n";
$ ls -lh first.pl
-rw-r--r-- 1 dusko dusko 45B Jul 24 11:02 first.pl
$ chmod 0744 first.pl
$ ls -lh first.pl
-rwxr--r-- 1 dusko dusko 45B Jul 24 11:02 first.pl
$ ./first.pl
Hello, World!
Variables
Perl has three types of variables: scalars, arrays and hashes. Think of them as “things”, “lists” and “dictionaries”.
In Perl, all variable names are a punctuation character, a letter or underscore, and one or more alphanumeric characters or underscores.
Scalars are single things.
The name of a scalar begins with a dollar sign ($), such as $i or $abacus.
About the only basic operator that you can use on strings is concatenation. The concatenation operator is the period (.). Concatenation and addition are two different things:
$ cat concat.pl
#!/usr/bin/env perl
$a = "8"; # Note the quotes. $a is a string.
$b = $a + "1"; # "1" is a string too.
$c = $a . "1"; # But $b and $c have different values!
print "$b\n";
print "$c\n";
$ ./concat.pl
9
81
Just remember, the plus sign adds numbers and the period puts strings together.
Arrays are lists of scalars.
Array names begin with @. You define arrays by listing their contents in parentheses, separated by commas:
@lotto_numbers = (1, 2, 3, 4, 5, 6);
@months = ("July", "August", "September");
The contents of an array are indexed beginning with 0. To retrieve the elements of an array, you replace the @ sign with a $ sign, and follow that with the index position of the element you want. (It begins with a dollar sign because you’re getting a scalar value.) You can also modify it in place, just like any other scalar.
If you want to find the length of an array, use the value $#array_name. This is one less than the number of elements in the array. If the array just doesn’t exist or is empty, $#array_name is -1. If you want to resize an array, just change the value of $#array_name.
$ cat months.pl
#!/usr/bin/env perl
@months = ("Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec");
print $months[0]; # This prints "Jan"
print "\n"; # Newline must go within quotes
print "$months[9]\n"; # Oct, not Sep because indexing starts with 0
$winter_months[0] = "Dec"; # This implicitly creates @winter_months
print "$winter_months[0]\n";
print "\n";
print "$#months\n";
$ ./months.pl
Jan
Oct
Dec
11
Hashes are called “dictionaries” in some programming languages, and that’s what they are: a term and a definition, or in more correct language a key and a value. Each key in a hash has one and only one corresponding value.
The name of a hash begins with a percentage sign (%), like %parents. You define hashes by comma-separated pairs of key and value:
%days_in_summer = ( "Jul" => 31, "Aug" => 31, "Sep" => 30 );
You can fetch any value from a hash by referring to $hashname{key}, or modify it in place just like any other scalar.
print $days_in_summer{"Sep"}; # 30
$days_in_summer{"Feb"} = 29; # Let's assume it's a leap year
If you want to see what keys are in a hash, you can use the keys function with the name of the hash. This returns a list containing all of the keys in the hash. The list isn’t always in the same order, though; while we could count on @months to always return Jul, Aug, Sep in that order, keys %days_in_summer might return them in any order whatsoever.
@month_list = keys %days_in_summer;
$ cat hashes.pl
#!/usr/bin/env perl
%days_in_summer = ( "Jul" => 31, "Aug" => 31, "Sep" => 30 );
print $days_in_summer{"Sep"}; # 30
print "\n";
$days_in_summer{"Mar"} = 31;
print $days_in_summer{"Mar"};
print "\n";
@month_list = keys %days_in_summer;
# @month_list is now ('Jul', 'Sep', 'Aug') !
print "\n";
print @month_list[0,1,2];
print "\n";
$ ./hashes.pl
30
31
AugJulMar
$ ./hashes.pl
30
31
MarJulAug
$ ./hashes.pl
30
31
JulSepAug
The three types of variables have three separate namespaces. That means that $abacus and @abacus are two different variables, and $abacus[0] (the first element of @abacus) is not the same as $abacus{0} (the value in abacus that has the key 0).