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Scripting Languages

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					Scripting Languages

  CS351 – Programming Paradigms
Common Characteristics of Scripting
languages.
 1.   Both Batch and Interactive use.
 2.   Economy of Expression.
 3.   Lack of declarations; simple scoping
      rules
 4.   Flexible dynamic typing.
 5.   Easy access to other programs.
 6.   Sophisticated Pattern matching.
 7.   High-level data types.
1 – Batch and Interactive Use

  Perl has a JIT compiler that reads the
   entire program before execution.
  Other languages are quite happy to only
   execute a line as soon as it has been
   read.
  Python, Tcl and Ruby will also accept
   commands from the keyboard.
2 – Economy of Expression
    To support rapid development and interactive use,
     scripting languages require very little boilerplate code.
    This can be illustrated very clearly by the following
     example:
 public static void main(String args[])
 {
     System.out.println(“Hello!”);
 }

 print “Hello!\n”


    Another example is reading a file word by word.
3 – Lack of Declarations and simple
scoping rules
  Most scripting languages dispense with declarations.
  They also use simple rules to govern the scopes of
   names ( identifiers ).
  In Perl every name is global by default.
  In Tcl and PHP, everything is local by default.
  Python has a unique implementation in that any
   variable that is assigned a value is local to the block in
   which the assignment appears.
  Special syntax is required to access the variable from
   a surrounding scope.
4 – Flexible Dynamic Typing
    Most scripting languages are dynamically typed.
    In PHP, Ruby and Python, the type of a variable is
     checked immediately prior to use.
    In others such as Tcl and Perl, a variable can be
     interpreted differently in different contexts.
    E.g.
     $a = “4”;
     print $a . 3 . “\n”;
     print $a + 4 . “\n”;
5 – Easy access to other programs

    Most programming languages provide a way to
     interact with the underlying commands of the OS.
    However this can be quite convoluted, as anyone who
     has had to do work with the Runtime.exec command
     knows.
    Scripting languages support these OS commands and
     their outputs much more cleanly and directly.
    Perl provides 100 built-in commands for the OS while
     the os module in Python is an excellent library for
     executing commands.
6 – Pattern Matching
    As scripting language ancestors were used for text
     processing, the support for pattern matching in text
     files is impressive.
    Scripting languages have very good facilities for
     pattern matching, searching and string manipulation.
    This is all based upon the notion of extended regular
     expressions.
    sed is a very powerful scripting language for text
     processing.
    What does sed '1!G;h;$!d' do?
7 – High level data types
    High level data types such as sets, bags, dictionaries,
     lists and tuples are some of the very convenient
     features provided by scripting languages.
    In C++, it is possible to use ``operator overloading’’ to
     create advanced user-defined types but scripting
     languages go one step further by building high-level
     types into the syntax and semantics.
    E.g. in Python a dictionary can map a key to a value:

     dict([(x, x**2) for x in (2, 4, 6)])
     {2: 4, 4: 16, 6: 36}
Scripting Languages Evolution
    Due to the dynamism of the open source community
     and the freedom from working within an ISO published
     standard, scripting languages have been at the recent
     forefront of programming language development.
    Most scripting languages have had only a single
     person driving the initial development.
    All of the most interesting features and powerful
     features appear in Python:
    E.g. true iterators, array slices, multiway assignment,
     anonymous first-class functions and functional type
     lists.
Problem Domains for Scripting
Languages
         Some languages such as Perl, Python and Ruby are
          intended for general purpose use as they support
          features such as modules, separate compilation,
          reflection etc.
         However a lot of scripting languages are intended
          for use within a well defined domain.
         We will look in detail at the following:
     1.     Shell Languages
     2.     Text Processing
     3.     Glue Languages
Shell Languages
    In the early 1970’s a command language for
     automating the control of Unix programs was written.
    This was known as ``shell’’ or sh.
    This was extended to allow for variables and control
     flow and was known as bash. This is the default
     Unix command shell.
    Shell languages allow for many operations including
     manipulating file names, arguments and commands
     and for tying other programs together.
Shell Languages cont…
    Some features provided by a shell languages.
            List all file ending in .txt -> ls *.txt
    This feature is known as filename expansion or
     globbing.
    There are other alternatives to this command:
            ls fig?.eps
            ls fig[0-9].eps
            ls fig3.{eps,pdf}


    What do each of these commands do?
Shell Languages cont…
    Such simple commands do not even hint at the
     flexibility provided by shell languages.
    It is possible to provide loops within shell languages.
    These can be very useful.
    E.g.

     for file in *.eps
     do
        ps2pdf $file
     done
    What is going on here?
Shell Languages cont…
    We can also use conditionals within our bash scripts:

     for file in *.eps
      do
        if [ ! test –e ${file%.eps}.pdf ]
        then
           ps2pdf $file
        fi
      done
    The test command has many features, type man test
     to get a list of them all.
Shell Languages cont…
    It is possible to pass the results of one program as the input to
     another very easily through the use of pipes.

     find . –name *.txt | ls | wc –l

    What does this do?
    It is is also possible to send output to and from files using re-
     directs.

     ls *.txt > files
     for file in *.txt; do echo $file >> files; done

    What do these commands do?
Shell Languages cont…
    There are some simple rules to bear in mind when
     using shell scripts.

        foo=bar
        single=‘$foo’
        double=“$foo”
        echo $single $double
    What is printed?
    When using other programs in a shell script, they must be
     placed within backticks.
        bname=`basename $file .txt`
Text Processing
    Shell languages are heavily string oriented.
    Commands are strings parsed into single words.
    All variables are string-valued and there are elaborate
     quoting conventions.
    However shell languages cannot be used like a text
     editor.
    How can we account for interactive features such as
     insertion, deletion, replacement, bracket-matching etc.
    Through the use of special ``online’’ editors such as
     sed and awk.
sed
    Stands for stream editor.
    Very powerful and can be used to write complex
     scripts but is more commonly used for one-line
     programs.
    E.g.

      sed –e ‘/^[[:space:]]*$/d’


    awk is an attempt to modify sed to look more like a
     programming language.
Glue Languages
    These general purpose scripting languages inherit a
     rich set of features form both the shell languages and
     the text processing languages.
    Imagine writing a script to kill a named process in *nix.
    We could do it ( hack it ) via a bash script or we could
     use a general purpose language to do it.
    We will discuss Python in detail on Thursday.

				
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