drush bootstrap error on ubuntu

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Drush was installed using PEAR. Drush by default was installed in
/usr/share/php/drush folder. I am trying to run drush from my site root.
Could someone please help to resolve this error?

root@devdrupal:/var/www/drupal# drush pml
Command pm-list needs a higher bootstrap level to run - you will need to
[error]
invoke drush from a more functional Drupal environment to run this
command.
The drush command 'pml' could not be executed.
[error]

                                                                     [error]

Drush was attempting to connect to:
 Drupal version         :  7.23
 Site URI               :  http://default
 Database driver        :  mysql
 Database username      :  drupaluser
 Database name          :  acquia_drupal
 Default theme          :  garland
 Administration theme   :  garland
 PHP executable         :  /usr/bin/php
 PHP configuration      :  /etc/php5/cli/php.ini
 PHP OS                 :  Linux
 Drush version          :  6.2.0
 Drush configuration    :
 Drush alias files      :
 Drupal root            :  /var/www/drupal
 Site path              :  sites/default
 File directory path    :  sites/default/files

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