title: How can check if particular application/software is installed in Mac OS
body: <p>I want to check if particular application is installed in Mac OS using Perl/Shell scripts. 
  I am writing package using PackageMaker in which i need to check user machine for few applications before installing the application.
  So am planning to write a script that will check this for me. 
  Please advice if I can perform this in better way.</p>