Follow the below steps to install Drush via Composer.
1. Install Composer globally.
2. Now add composer’s bin directory to the system path by placing ” export PATH=”$HOME/.composer/vendor/bin:$PATH” ” into your ~/.bashrc ( For Linux Users ) or into your ~/.bash_profile ( For Mac OS Users ).
3. Then install latest stable Drush using below command:
# composer global require drush/drush
4. Verify Drush using below command:
# drush status
Update Latest Release
# composer global update
Install A Specific Version Of Drush
If you want to install specific version of Drush follow the below command. For example Drush 7.1.0
# composer global require drush/drush:7.1.0
Alternate Way To Install For All Users using Composer
COMPOSER_HOME=/opt/drush COMPOSER_BIN_DIR=/usr/local/bin COMPOSER_VENDOR_DIR=/opt/drush/7 composer require drush/drush:7
Follow the below command to uninstall Drush
# composer global remove drush/drush