1 / 216

UNIX System Programming with Perl: A Practical Overview

Perl, designed by Larry Wall in 1986, is a powerful tool for reports and web interactions. Learn Perl's syntax, structure, data types, and how to use it effectively for scripts, data file parsing, and more.

lkondo
Télécharger la présentation

UNIX System Programming with Perl: A Practical Overview

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Perl • Practical Extraction and Report Language • Not designed for the Web • Designed by Larry Wall in 1986 to create reports • Drew upon useful features in other programming languages • (Pathologically Eclectic Rubbish Lister) UNIX System Programming

  2. Perl overview • Originally called PEARL, but a conflict with an existing graphics language resulted in the shortening of the name • Language includes pattern matching, file handling, and scalar data • It has syntax patterned after C and was originally designed to operate as a shell script • Its power is that it views programs and files as data • It has proven to be an effective language for interacting with Web pages UNIX System Programming

  3. Perl structure ► Variables in Perl are either integers or strings and begin with the symbol $. ► A simple Perl program consists of a series of print statements. ► It has the usual sequence of control structures such as for, while, and until loops and if conditional. UNIX System Programming

  4. Perl is good for … • Quick scripts, complex scripts • Parsing & restructuring data files • CGI-BIN scripts • High-level programming • Networking libraries • Graphics libraries • Database interface libraries UNIX System Programming

  5. Bad for ….. • Compute-intensive applications (use C, C++, Java) • Hardware interfacing (device drivers…) UNIX System Programming

  6. Perl structure • Comment lines begin with: # • First line should look like: #!/bin/perl • File Naming Scheme • filename.pl (programs) • filename.pm (modules) • filename.ph (old Perl 4 header file) • Example prog: print “Hello, World!\n”; UNIX System Programming

  7. Perl structure • Statements must end with semicolon. • $a = 0; • Should call exit() function when finished. • Exit value of zero means success • exit (0); # successful • Exit value non-zero means failure • exit (2); # failure UNIX System Programming

  8. Perl structure ► Perl shares an ability to process regular expressions like several other process languages Example use: $ENV{'USER'} = ‘mvz’ will be true if login name mvz is the string $ENV{'USER'} (i.e., is the user running this program). This shows how Perl can interact with the system environment UNIX System Programming

  9. Perl: Example script if ($food eq "spinach") { print "You ate spinach, so you get dessert!"; } elsif ($food eq "broccoli") { print "Broccoli's OK. Maybe you'll get dessert."; } else { print "No spinach, no dessert!"; } UNIX System Programming

  10. Perl: The learning process • Access to Perl Interpreter on a server • Can download and install for free • Perl on UNIX server (needed to learn basic UNIX commands). • Basic Text Editor (e.g., Vi, Vim, Emacs, etc.) UNIX System Programming

  11. Perl UNIX System Programming

  12. Perl UNIX System Programming

  13. Perl UNIX System Programming

  14. Data Types • Integer • 25 750000 1000000000 • Nondecimal Integer • 0567 (Octal 567) • 0xff (Hex FF) • 0b10111011 (Binary 10111011) UNIX System Programming

  15. Data Types • Perl allows underscores for readability: • 0b1011_1011 • 0x0000_FFFF UNIX System Programming

  16. Data Types • Floating Point • 1.25 50.0 6.02e23 -1.6E-8 • String • ‘hi there’ “hi there, $name” qq(tin can) • print “Text Utility, version $ver\n”; UNIX System Programming

  17. Perl UNIX System Programming

  18. Perl strict - Perl pragma to restrict unsafe constructs use ‘refs’; // generate run-time errors if symbolic refs used use ‘vars’; // generate compile-time errors if var is not declared or out of scope use ‘subs’ // if bareword is used to call subs e.g., $SIG{PIPE} = Plumber; instead of $SIG{PIPE} = "Plumber” use strict; no strict ‘vars’ ; UNIX System Programming

  19. Data Types • Boolean • 0 0.0 “” represents False • all other values represents True UNIX System Programming

  20. Data Types • Scalar • $num = 14; • $fullname = “Cass A. Nova”; • Variable Names are case-sensitive • Underlines Allowed: $Program_Version = 1.0; UNIX System Programming

  21. Perl UNIX System Programming

  22. Perl UNIX System Programming

  23. Perl UNIX System Programming

  24. Perl UNIX System Programming

  25. Basic I/O – Input from STDIN Evaluating this operator in a scalar context gives you the next line of input, or undef, if there are no more lines. $a = <STDIN>;# read the next line UNIX System Programming

  26. Basic I/O – Input from STDIN ► Evaluating this operator in an array context gives you all of the remaining lines as a list. ► One line per element. @a = <STDIN>;# read the next line UNIX System Programming

  27. Basic I/O – Input from STDIN ► To read a scalar value for <STDIN> and use value as the controlling expression of a loop: while (<STDIN>) {# like while ($_ = <STDIN>) chomp;# like chomp ($_ ) # process $_ here ; } UNIX System Programming

  28. Input from Diamond Operator <> Unlike <STDIN>, < > gets data from file(s) specified on the command line that invoked the Perl program. E.g., if Perl file is called foo, consisting of #!/usr/bin/perl -w while (<>) { print $_ ; } UNIX System Programming

  29. Input from Diamond Operator <> And you invoke foo with: foo file1 file2 file3 < > will read each line of file1 followed by each line of file2 followed by each line of file3. Acts like UNIX’s cat(enate) command UNIX System Programming

  30. Input from Diamond Operator <> ►< > does not look at command line arguments. ► Uses the @ARGV array ►$#ARGV returns the number of arguments ► Set by Perl to contain the list of command-line arguments Example: set @ARGV in-program. @ARGV = (“passwd1”,”passwd2”,”passwd3”); while (<>) { #process files passwd? print “This line is $_ “; } UNIX System Programming

  31. <> example: ►print lines that are not _________ ? while (<>) { chop; if ($_ ne “zatoichi”) { print “$_ \n“; } UNIX System Programming

  32. Output to STDOUT ► print or printf: ► Parenthes is optional, needed for clarification & readability. E.g., print (2+3),”hello”; # wrong! Prints 5, ignores “hello” print ((2+3),”hello”); # right, prints 5hello print 2+3,”hello”; # also right, prints 5hello UNIX System Programming

  33. Output to STDOUT ► printf for formatted output (from C) E.g., format width printf “%15s %5d %10.2f\n”, $s, $n, $r; string integer floating point UNIX System Programming

  34. Built-in Perl Operators: chop • Scalar Context • Chop off and return the last character of a string $s = “string”; $c = chop($s); $s  “strin” $c  “g” • Array Context • Chop off the last character of every string in the array, but return the last character of the last element of the array. @s = (“s1”, “s2”, “s3”); chop(@s)  @s = (“s”, “s”, “s”) UNIX System Programming

  35. chomp • Scalar Context • Remove and return the last character of a string, if and only if it equals the current record separator value (\n). $s = “string\n”; $c = chomp($s); $s  “string” $c  “\n” • Array Context • Remove the last character of every string in an array only if it is the current record separator value. @s = (“hello\n”, “world”); chomp(@s)  @s = (“hello”, “world”) UNIX System Programming

  36. Perl UNIX System Programming

  37. Perl UNIX System Programming

  38. Perl UNIX System Programming

  39. Examples with strings….. $fred = ‘hi’ ; $barney = “a test of ”.’$fred’; # yields ‘a test of $fred’ $barney2 = “a test of \$fred”; # same result as above UNIX System Programming

  40. Examples with strings….. $fred = “pay”; $fredday = “wrong!”; $barney = “It’s $fredday” # not It’s payday, but It’s wrong! $barney = “It’s ${fred}day” # now, $barney = “It’s payday” $barney2 = “It’s $fred” . “day” # another way to do it, and another … $barney3 = “It’s”.$fred.”day”; UNIX System Programming

  41. Arrays • List (one-dimensional array) @memory = (16, 32, 48, 64); @people = (“Alice”, “Alex”, “Albert”); • First element numbered 0 (can be changed) • Single elements are scalar: $names[0] = “Fred”; • Slices are ranges of elements @guys = @people[1..3]; • How big is my list? print “Number of people: $#people\n”; UNIX System Programming

  42. Examples of Arrays (1, 2, 3) # array of three values (“fred”,4.5) # two values Elements can be expressions: ($a, 17) # two values, current value # of $a, and 17 ($b+$c, $a+$d) # two values Empty array: ( ) UNIX System Programming

  43. Examples of Arrays Ranges allowed: (1..3) # same as (1, 2, 3) (2..4,10,12) # (2,3,4,10,12) ($a..$b) # depends on those values Float ranges: (1.2..5.2) # (1.2,2.2,3.2,4.2,5.2) (1.2..6.1) # (1.2,2.2,3.2,4.2,5.2) UNIX System Programming

  44. Examples of Array Variables @fred = (1,2,3) # fred array gets 3 elements @barney = @fred # that is copied to $barney Array variable name may appear in array literal list: @fred = (“one”,”two”); @barney = (4,5,@fred,6,7); # = (4,5,”one”,”two”,6,7) @barney = (8, @barney); # puts 8 in front # of @barney UNIX System Programming

  45. Examples of Array Variables When array variable is assigned a scalar variable, number assigned is length of array: @fred = (4,5,6); #initialize @fred $a = @fred; # $a gets 3, current length # of @fred UNIX System Programming

  46. Examples – Element Access @fred = (7,8,9); $b = $fred[0]; # $b = 7 $fred[0] = 5 # @fred = (5,8,9) $fred[2]++ ; # increment 3rd element of # @fred ($fred[0],$fred[1]) = ($fred[1],$fred[0]) # swap first two UNIX System Programming

  47. Examples – Element Access Slice: access list of elements @fred[0,1] ; # same as ($fred[0],$fred[1]) @fred[0,1] = @fred[1,0] # swap @fred[0,1,2] = @fred[1,1,1] # make all 3 elements same as # 2nd element @fred[1,2] =(9,10) # change last 2 values # 9 and 10 Note: @ instead of $ UNIX System Programming

  48. Examples: Index Expressions Index can be any expression that returns a number, e.g., @fred = (7,8,9); $a = 2; $b = $fred[$a]; # $fred[2] = 9 $c = $fred[$a-2] # c = $fred[0] = 7 UNIX System Programming

  49. Examples: Out-of-bounds access undef: value that represents something that is not defined. @fred = (1,2,3) $fred[i] for 0 > i > 2 yields undef value e.g., $barney = $fred[7] # $barney is now undef UNIX System Programming

  50. Examples: Out-of-bounds assignment Assigning a value beyond the end of the current array automatically extends the array, giving undef to all intermediate values, if any. @fred = (1,2,3); $fred[3] = “hi”; # @fred is now (1,2,3,”hi”) $fred[6] = “ho”; # @fred = (1,2,3,”hi”,undef,undef,”ho”)) UNIX System Programming

More Related