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Introduction to Scripting Languages

COMP284 Lectures

Scripting Languages

Scripts are user-readable and user-modifiable programs that perform simpler operations and control the operation of other programs.

They have the following properties:

  • Program code is present at run time and the starting point of execution:
    • Compilation is not needed.
    • Compilation to intermediary languages may be performed behind the scenes as an optimisation.
  • Presences of a suitable runtime environment is required for thee execution so scripts:
    • Includes one of:
      • Interpreter
      • Just-In-Time Compiler
      • Byte-code Compiler

    and a virtual machine.

  • Execution of scripts is typically slower than the execution of code that has been fully pre-compiled.
  • Rich and easy to use interface to the underlying operating system in order to run other programs and communicate with them.
    • Rich I/O including:
      • Pipes
      • Network Sockets
      • File I/O
  • Easy integration with larger systems:
    • Often used to glue other systems together.
    • Can be embedded into other applications.

Language Constructs

  • Variables, functions, and methods typically do not require type declarations. Automatic conversion between types:
    • Strings and numbers.
  • Lots of pre-defined functions and libraries.
    • Lots of pre-defined functions for the specific purpose the language was designed for.
  • Ability to generate, load, and interpret source code at run time through an eval function.

    • This can lead to self modifying and running code like the following:

        var x = 2;
  var y = 6;
  var str = " if ( x > 0) { z = y / x } else { z = -1 }";
  console . log ( ' z is ', eval ( str )); // Output : z is 3
  x = 0;
  console . log ( ' z is ', eval ( str )); // Output : z is -1

Language Evolution

The evolution of a scripting language typically starts with a limited set of languages constructs for a specific purpose:

  • PHP started as a set of functions for tracking web visits.

The language then accumulates more and more language constructs as it is used for a wider range of purposes:

  • These additional language constructs may or may not fit well together with the original core and may duplicate existing language constructs.
  • During this evolution of the language, backward compatibility may or may not be preserved.

Language design of scripting languages is often sub-optimal.