On CentOS 6 with PHP 5.3.3 and Drupal 7:
Drush 6.2 installed using Pear works fine.
Then I needed Drush 7 so:
Drush 7 installed using Composer doesn't work.
but
Drush 6.6 installed using Composer works fine.

Drush 7 can't find the site, that's the error I get just trying to clear cache. Doesn't matter if I use an alias, or if I'm in httpdocs or sites/default.

I changed localhost to 127.0.0.1 in settings.php but no change.

I've also noticed that 7 doesn't show any database info. Probably just another symptom.

I'll try to illustrate all three versions of Drush using image-flush, sorry for the long post:
(Note that drush cc all reports No Drupal Site Found using Drush 7, the third command down. Another symptom I'm sure...)

root@venus > drush --version
 Drush Version   :  7.0.0

/var/www/drupal7/EXAMPLE.com/httpdocs
root@venus > drush status
 Drupal version         :  7.38                                            
 Site URI               :  http://default                                  
 PHP executable         :  /usr/bin/php                                    
 PHP configuration      :  /etc/php.ini                                    
 PHP OS                 :  Linux                                           
 Drush script           :  /root/.composer/vendor/drush/drush/drush.php    
 Drush version          :  7.0.0                                           
 Drush temp directory   :  /tmp                                            
 Drush configuration    :                                                  
 Drush alias files      :  /root/.drush/aliases.drushrc.php                
 Drupal root            :  /var/www/drupal7/EXAMPLE.com/httpdocs 
 Site path              :  sites/default 

/var/www/drupal7/EXAMPLE.com/httpdocs/sites/default
root@venus > drush cc all
No Drupal site found, only 'drush' cache was cleared.                                       [warning]
'all' cache was cleared.                                                                    [success]


/var/www/drupal7/EXAMPLE.com/httpdocs
root@venus > drush image-flush
Command image-flush needs a higher bootstrap level to run - you will need to invoke drush from a more functional Drupal environment to run this command.            [error]
The drush command 'image-flush' could not be executed.                                                                                                              [error]
Drush was not able to start (bootstrap) the Drupal database.                                                                                                        [error]
Hint: This may occur when Drush is trying to:
 * bootstrap a site that has not been installed or does not have a configured database. In this case you can select another site with a working database setup by
specifying the URI to use with the --uri parameter on the command line. See `drush topic docs-aliases` for details.
 * connect the database through a socket. The socket file may be wrong or the php-cli may have no access to it in a jailed shell. See http://drupal.org/node/1428638
for details.

Drush was attempting to connect to: 
 Drupal version         :  7.38                                            
 Site URI               :  http://default                                  
 PHP executable         :  /usr/bin/php                                    
 PHP configuration      :  /etc/php.ini                                    
 PHP OS                 :  Linux                                           
 Drush script           :  /root/.composer/vendor/drush/drush/drush.php    
 Drush version          :  7.0.0                                           
 Drush temp directory   :  /tmp                                            
 Drush configuration    :                                                  
 Drush alias files      :  /root/.drush/aliases.drushrc.php                
 Drupal root            :  /var/www/drupal7/EXAMPLE.com/httpdocs 
 Site path              :  sites/default   

drush cache-clear drush
composer global require drush/drush:6.*
composer global update
drush cache-clear drush

root@venus > drush --version
 Drush Version   :  6.6.0 

root@venus > drush status
 Drupal version                  :  7.38                                            
 Site URI                        :  http://default                                  
 Database driver                 :  mysql                                           
 Database username               :  xxx                                          
 Database name                   :  xxx                                          
 Database                        :  Connected                                       
 Drupal bootstrap                :  Successful                                      
 Drupal user                     :  Anonymous                                       
 Default theme                   :  at_one                                          
 Administration theme            :  seven                                           
 PHP executable                  :  /usr/bin/php                                    
 PHP configuration               :  /etc/php.ini                                    
 PHP OS                          :  Linux                                           
 Drush version                   :  6.6.0                                           
 Drush configuration             :                                                  
 Drush alias files               :  /root/.drush/aliases.drushrc.php                
 Drupal root                     :  /var/www/drupal7/EXAMPLE.com/httpdocs 
 Site path                       :  sites/default                                   
 File directory path             :  sites/default/files                             
 Temporary file directory path   :  /tmp  

/var/www/drupal7/EXAMPLE.com/httpdocs
root@venus > drush image-flush
Choose a style to flush.
 [0]   :  Cancel                                    
 [1]   :  all                                       
 [2]   :  flexslider_full                           
 [3]   :  flexslider_thumbnail                      
 [4]   :  thumbnail                                 
 [5]   :  medium                                    
 [6]   :  large                                     
 [7]   :  3x1_breakpoints_theme_at_one_mobile_1x    
 [8]   :  3x1_breakpoints_theme_at_one_narrow_1x    
 [9]   :  3x1_breakpoints_theme_at_one_normal_1x    
 [10]  :  3x1_breakpoints_theme_at_one_wide_1x

/var/www/drupal7/EXAMPLE.com/httpdocs
root@venus > drush62 --version
 Drush Version   :  6.2.0 

/var/www/drupal7/EXAMPLE.com/httpdocs
root@venus > drush62 status
 Drupal version                  :  7.38                                            
 Site URI                        :  http://default                                  
 Database driver                 :  mysql                                           
 Database username               :  xxx                                          
 Database name                   :  xxx                                          
 Database                        :  Connected                                       
 Drupal bootstrap                :  Successful                                      
 Drupal user                     :  Anonymous                                       
 Default theme                   :  at_one                                          
 Administration theme            :  seven                                           
 PHP executable                  :  /usr/bin/php                                    
 PHP configuration               :  /etc/php.ini                                    
 PHP OS                          :  Linux                                           
 Drush version                   :  6.2.0                                           
 Drush configuration             :                                                  
 Drush alias files               :  /root/.drush/aliases.drushrc.php                
 Drupal root                     :  /var/www/drupal7/EXAMPLE.com/httpdocs 
 Site path                       :  sites/default                                   
 File directory path             :  sites/default/files                             
 Temporary file directory path   :  /tmp                                            


/var/www/drupal7/EXAMPLE.com/httpdocs
root@venus > drush62 image-flush
Choose a style to flush.
 [0]   :  Cancel                                    
 [1]   :  all                                       
 [2]   :  flexslider_full                           
 [3]   :  flexslider_thumbnail                      
 [4]   :  thumbnail                                 
 [5]   :  medium                                    
 [6]   :  large                                     
 [7]   :  3x1_breakpoints_theme_at_one_mobile_1x    
 [8]   :  3x1_breakpoints_theme_at_one_narrow_1x    
 [9]   :  3x1_breakpoints_theme_at_one_normal_1x    
 [10]  :  3x1_breakpoints_theme_at_one_wide_1x 

Comments

brayfe’s picture

Having this exact same issue on a RHEL server with php 5.3.3 using composer to install drush 7.1. Have no idea why it can't bootstrap the database.

mr. chips’s picture

I don't know why I can't find the requirements for Drush, but on my CentOS 6 server with PHP 5.3.3 Drush 6 works but Drush 7 doesn't.

Drush 7 works on CentOS 7 with PHP 5.4.16