How to …

How to flush the DNS cache

Mac OS X:

  dscacheutil -flushcache

Windows:

  ipconfig /flushdns

How to list the installed packages on Debian or Ubuntu

Get the list of installed packages:
```
  dpkg --get-selections
  dpkg --get-selections | grep php
```
Get a list of files installed with a package:
```
  dpkg -L php5-gd
```