Transcript Scripting Languages - Maynooth University Department of

Scripting Languages
CS351 – Programming Paradigms
Common Characteristics of Scripting
languages.
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6.
7.
Both Batch and Interactive use.
Economy of Expression.
Lack of declarations; simple scoping
rules
Flexible dynamic typing.
Easy access to other programs.
Sophisticated Pattern matching.
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.
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2 – Economy of Expression
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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”
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Another example is reading a file word by word.
3 – Lack of Declarations and simple
scoping rules
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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
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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 $a + 4 . “\n”;
5 – Easy access to other programs
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Most programming languages provide a way to
interact with the underlying commands of the OS.
However this can be quite convoluted, as anyone who
has had to do work with the Runtime.exec command
knows.
Scripting languages support these OS commands and
their outputs much more cleanly and directly.
Perl provides 100 built-in commands for the OS while
the os module in Python is an excellent library for
executing commands.
6 – Pattern Matching
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As scripting language ancestors were used for text
processing, the support for pattern matching in text
files is impressive.
Scripting languages have very good facilities for
pattern matching, searching and string manipulation.
This is all based upon the notion of extended regular
expressions.
sed is a very powerful scripting language for text
processing.
What does sed '1!G;h;$!d' do?
7 – High level data types
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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
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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
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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:
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Shell Languages
Text Processing
Glue Languages
Shell Languages
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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…
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Some features provided by a shell languages.
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This feature is known as filename expansion or
globbing.
There are other alternatives to this command:
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List all file ending in .txt -> ls *.txt
ls fig?.eps
ls fig[0-9].eps
ls fig3.{eps,pdf}
What do each of these commands do?
Shell Languages cont…
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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
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What is going on here?
Shell Languages cont…
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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
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The test command has many features, type man test
to get a list of them all.
Shell Languages cont…
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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
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What does this do?
It is is also possible to send output to and from files using redirects.
ls *.txt > files
for file in *.txt; do echo $file >> files; done
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What do these commands do?
Shell Languages cont…
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There are some simple rules to bear in mind when
using shell scripts.
foo=bar
single=‘$foo’
double=“$foo”
echo $single $double
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What is printed?
When using other programs in a shell script, they must be
placed within backticks.
bname=`basename $file .txt`
Text Processing
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Shell languages are heavily string oriented.
 Commands are strings parsed into single words.
 All variables are string-valued and there are elaborate
quoting conventions.
 However shell languages cannot be used like a text
editor.
 How can we account for interactive features such as
insertion, deletion, replacement, bracket-matching etc.
 Through the use of special ``online’’ editors such as
sed and awk.
sed
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Stands for stream editor.
Very powerful and can be used to write complex
scripts but is more commonly used for one-line
programs.
E.g.
sed –e ‘/^[[:space:]]*$/d’
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awk is an attempt to modify sed to look more like a
programming language.
Glue Languages
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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.