Scripting Languages that Begin with

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Scripting Languages that Begin with P Perl,
PHP and PythonLes LaCroix, 79Associate
Director of Network ServicesCarleton College
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This program is brought to you by the letter
Pand the numbers
5, 4 and 2
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Who is this guy?

• lt15 years as Network/Systems Administrator small
  programs (lt10,000 lines of code)
• lt10 years Software Engineer moderate
  (100,000-500,000 lines of code) to large systems
  (gt1,000,000 lines)
• Programmed in C, C, Fortran, Basic, Pascal,
  assembly languages, Java, Perl, PHP, Python

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Who is this guy?

• Worked in an AI group
• Won a technology innovation award while employed
  for a major computer manufacturer for designing a
  hardware configurator calculus

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What are scripting languages?

• Early scripting languages ran commands in
  batches (e.g. DOS .BAT files, Unix shells sh,
  csh)
• More recent languages are just high-level,
  practical programming languages
• Scripting languages are the bread and butter
  tools of network, system and web administrators

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Where did these particular languages come from?

• Larry needed to create reports from a bug
  tracking system
• Rasmus wanted to track users visiting is online
  resume
• Guido wanted hobby project to keep him busy
  during the week around Christmas

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Common features

• Typeless
• Similar available basic data types
• numbers
• strings
• arrays
• associative arrays (a.k.a. hashes or dictionaries)

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Common features

• Dynamically sized structures (strings, arrays,
  hashes)
• C-like structs emulated with hashes
• Classes, objects, methods
• Can be thought of as interpreted languages (as
  opposed to compiled)
• Portability

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What are they good for?

• Prototyping/Rapid development
• Small programs (lt100,000 lines)
• Web development

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What are they not so good for?

• Writing efficient code
• Software engineering moderate (100,000-500,000
  lines) to large programs
• no argument checking or type checking
• encapsulation is a convention, not enforced
• Distributing applications where you want to
  protect the source code

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Common features

• Extensibility in C
• Common extensions
• file-level input/output
• math functions
• command call-out (system, pipes)
• network service connectivity (e.g. databases,
  LDAP, IMAP)
• CGI parsing, web server integration

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Perl

• Larry Wall created Perl because awk didnt cut it
• Perl is to awk as Picard is to Kirk
• Perl is great with strings
• Strings are good a very common administration
  practice is to take the human-readable output of
  one program and munge it into a script

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Perl

• Perl 5.4 has been around for years relatively
  few innovations in 5.5 and 5.6
• Took off with Web/CGI
• OOP, extensibility as an afterthought.
  (Implementation is reasonable anyway.)

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Perl

• There is more than one way to do it.
• accommodates multiple programming paradigms
• can lead to obfuscated coding (see later
  examples)
• Perl Poetry poems have been written in Perl and
  have appeared in non-computer publications (e.g.,
  The Economist)

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Why is Perl good with strings?

• Simple, built-in operations for string
  transliteration, replication, and
  regular-expression pattern matching (with or
  without substitution)
• transliteration variable tr/A-Z/a-z/
• replication variable x 5
• r.e. matching variable /pattern/
• with subs. var s/pattern/replacement/

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Examples

• reverse the letters of each word in a string,
  leaving everything else intact
• v s/\w/reverse()/eg
• exchange first two words in the string, leaving
  spacing and punctuation intact
• v s/(\w)(\W)(\w)/321/

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So what?

• Simple operations are trivial
• Complex patterns can be expressed as terms with
  spacing and coding to make things legible.
  (Other languages insist that patterns are
  fundamentally strings.)

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So what?

• Matching is always a multi-valued operation (did
  anything match, what matched, what came before
  and after, and were there submatches)
• Results imported transparently into local name
  space or expression stack. Other languages have
  baggage to get to any of it just in case you
  wanted to get to all of it.

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PHP

• Started as a macro language for HTML
• Feels like a toy language
• Clumsy string manipulation, OOP, extensibility
• Wonderfully integrated with HTML seamless
  intermixing of PHP and HTML

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Example PHP program

• ltHTMLgtltHEADgtltTITLEgtSample PHP Programlt/TITLEgtlt/HE
  ADgtltBODYgtltPgtThe day of the week islt?php
  current_date getdate(time()) print
  current_dateweekday?gtlt/Pgtlt/BODYgtlt/HTMLgt

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Python

• Designed by a credible programming language
  designer
• Objects, classes, exception handling and
  extensibility are core features, not add-ons
• All things are First Class Objects None,
  numbers, strings, compound data types, functions,
  classes, bound methods

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Python

• Used for RAD (Rapid Application Development), GUI
  programming, Web programming, system
  administration
• Not great at string handling
• Very good with class inheritance, object
  persistence

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Python

• There are fewer ways to do it
• language specifically reduces the number of
  different, natural ways to code an expression
• Program block structuring enforced by indentation
  rather than punctuation or keywords
• Supposedly leads to more readable programs

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Sample Python program

• Mail all Python source files (.py) in the
  current directory to me!/usr/local/bin/python
  import glob, osfor filename in
  glob.glob(.py) print filename
  os.system(mail me_at_carleton.edu -s s lts
  (filename,filename))

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Conclusion

• Use the best tool dont drive a screw with a
  hammer
• Avoid religious wars no one is ever completely
  wrong or right
• Have fun
• Write home often