System Administration Introduction to Scripting, Perl Session 5 – Fri 23 Nov 2007

1. System AdministrationIntroduction to Scripting, PerlSession 5 – Fri 23 Nov 2007 • References: • Perl man pages Albert Lingelbach, Jr. alingelb@yahoo.com

2. Review • Intro to shell • Intro to perl • Questions from exercises ?

3. Arrays: introduction • Arrays store a series of values that can be accessed by "index" • Index is an integer • starting at 0 or 1; Perl array indexing starts at 0 • the upper limit depends on the language or computer • In Perl, array variables begin with @ • Example: my @namearray; • To access an element: $namearray[i] • Example: $namearray [3] = "testing";

4. Arrays: initialization • The contents of an array can be initialized at declaration: my @namearray = ("geoff", "foibe", "oscar"); • This is equivalent to: my @namearray; $namearray[0] = "geoff"; $namearray[1] = "foibe"; $namearray[2] = "oscar";

5. Arrays: size • The index of the last element can be found using the expression $#array • Notice that this is a scalar expression • What is an expression that will return how many elements are in an array ? • What statements will print out every element of an array ?

6. Arrays: practicum • Number of elements in an array: $#array + 1 @arrayin scalar context • Print every element of an array: for (my $idx = 0; $idx < @array; $idx++) { print "array[$idx] = $array[$idx]\n"; }

7. Arrays: multiple dimensions • Arrays can have more than one dimension: my @gameboard = ((" ", " ", " "), (" ", " ", " "), (" ", " ", " ") ); $gameboard [1, 1] = "X"; $gameboard [0, 0] = "O"; $gameboard [1, 2] = "X"; $gameboard [1, 0] = "O";

8. Arrays: miscellaneous • An array can be sorted my @sorted = sort @myarray; • An array can be reversed my @reversed = reverse @myarray; • Seeman -M /usr/perl5/man perldatafor more information

9. Hashes: introduction • Hashes store a series of values that can be accessed by "key" • Key is a scalar (string, number) which is unique among the set of keys in the hash • Value is another scalar, associated with the key • Hashes begin with % • Example: my %phonebook; • To access an element: $hash{key} • Example: $phonebook {"mangula"}

10. Hashes: initialization • The contents of a hash can be initialized at declaration: my %phonebook = ( "albert" => "0787487449", "mangula" => "0756181255" ); • This is equivalent to: my %phonebook; $phonebook {"albert"} = "0787487449"; $phonebook {"mangula" = "0756181255";

11. Hashes: useful functions • The set of keys in a hash: my @keys = keys %myhash; • The set of values in a hash: my @values = values %myhash; • Expression to return the number of elements in a hash ? • Code to display the contents of a hash ?

12. Hashes: practicum • Number of elements in a hash: my @arr = keys %hash; my $length = @arr; my $length = $#arr + 1; • Display contents of a hash: my @keys = keys %hash; for (my $idx = 0; $idx < @arr; $idx++) { print "hash {$keys [$idx]} = " ."$hash {$keys [$idx]}\n"; }

13. Hashes: miscellaneous • Seeman -M /usr/perl5/man perldatafor more information

14. Regular Expressions: introduction • Regular expressions (sometimes shortened to "regexp"s) are used for matching text to patterns • Recall shell "wildcards": * ? • Excellent for searching, data validation

15. Regular Expressions: statement syntax • "Matching" operator: =~ • Pattern markers: / / • Example: if ($s =~ /$pattern/) { print "$s matches $pattern\n"; } else { print "$s does not match $pattern\n"; }

16. Regular Expressions: pattern syntax • any letter or number character represents itself • "abc" matches /abc/ • | or: one of many sub-patterns • () group sub-patterns • "one" or "two" or "three" match /(one|two|three)/ • "one" matches /(one)/, /(on)(e)/ • [] group single character options • "a" or "b" or "c" match /[abc]/ • "af" or "ag" match /[abc][fgh]/ • "ab" does not match /[abc][fgh]/

17. Regular Expression pattern syntax continued • [^ ] not in set • "a" or "b" matches /[^fgh]/ • "f" does not match /[^fgh]/ • Placement: • ^ beginning of string • $ end of string • "abc" matches /abc/, /^abc/, /abc$/, /^abc$/ • "xabc" matches /abc/, /abc$/ • "xabc" does not match /^abc/ or /^abc$/ • "abcx" matches /abc/, /^abc/ • "abcx" does not match /abc$/, /^abc$/

18. Regular Expression pattern syntax continued • Quantifiers • ( )? zero or one • "caa", "abad" match /ab?a/ • "abba" does not match /ab?a/ • "acb", "yafoob3" match /a(foo)?c/ • "afoofooc" does not match /a(foo)?c/ • ( )* zero or more • "xaa", "yabbbbbbba" match /ab*a/ • ( )+ one or more • "aa", "abbbb" do not match /ab+a/

19. Regular Expression pattern syntax continued • More quantifiers • ( ){n} exactly n occurrences • "afoofoofoob" matches /a(foo){3}b/ • "afoofoob" does not match /a(foo){3}b/ • ( ){i, j} between i and j occurrences • "afoobarbarb" matches /a(foo){1,3}(bar){2,4}b/ • "afoobarbarb" does not match /a(foo){2,3}(bar){2,4}b/ • ( ){i, } i or more occurrences • "afoob" does not match /a(foo){2,}b/ • "afoofoofoofoofoofoob" does match /a(foo){2,}b/

20. Regular Expression pattern syntax continued • \ is "escape" character • use in front of special character to match it literally • "a(b" matches /a\(b/ • /a]b/ is not a correct regular expression • use as marker for pre-defined sets/patterns (next slide)‏ • . (dot, period) matches any one character • "abb", "a(b" match /a.b/ • "abx" does not match /a.b/

21. Regular Expression pattern syntax continued • Pre-defined patterns • \s whitespace (space, tab, newline)‏ • \S non-whitespace character (same as [^\s])‏ • \d digit • \w word character (a-z, A-Z, 0-9, _)‏

22. Regular Expressions: miscellaneous • There are more pre-defined characters • There are more features • For more information:man -M /usr/perl5/man perlrequickman -M /usr/perl5/man perlretutman -M /usr/perl5/man perlre

23. Review • Arrays: @array = ("zero", "one", "two"); $array[idx]; $#array; sort @array; reverse @array; • Hashes: %hash = ("a" => 1, "b" => 2);$hash {$key};keys %hash; values %hash; • Regular expressions: =~ // . ? + * () [] [^] ^ $ {,}

24. Next up: • Process management • No class until January • See e-mail for exercises • e-mail any questions