how to create shell command 'd' as equivalent to 'drush'?
If you are using Drush-5 (recommended), then see examples/example.bashrc for a bunch of very useful ways to enhance your experience in your shell when using Drush.  Installation instructions are in the file.  One of the things it does is create an alias 'dr' for Drush; if you wanted this to be 'd' on your system, it would be easy enough to make that change.