To install Drush via Composer, i follow the instructions of drush website

1. Install Composer globally.

~# curl -sS | php

    All settings correct for using Composer

    Composer successfully installed to: /root/composer.phar
    Use it: php composer.phar

~# mv composer.phar /usr/local/bin/composer
~# bash -l
~# composer

2. Add composer’s bin directory to the system path by placing composer path on bash_profile (Mac OS users) or into your ~/.bashrc (Linux users).

    ~# echo "export PATH="$HOME/.composer/vendor/bin:$PATH"" >>  ~/.bash_profile


    ~# echo "export PATH="$HOME/.composer/vendor/bin:$PATH"" >> ~/.bashrc

3. Install latest stable Drush:

    ~# composer global require drush/drush.

        Changed current directory to /root/.composer
        Using version ^7.1 for drush/drush
        ./composer.json has been created
        Loading composer repositories with package information
        Updating dependencies (including require-dev)
          - Installing pear/console_table (v1.2.1)
            Downloading: 100%         
          - Installing symfony/polyfill-mbstring (v1.1.0)
            Downloading: 100%         
          - Installing symfony/var-dumper (v2.8.2)
            Downloading: Connecting...

4. Verify that Drush works:

    ~# drush status