A self-reminde ron how to install on Linux. Instruction taken from here.

curl -sS https://getcomposer.org/installer | php
mv composer.phar /usr/local/bin/composer

Note: If the above fails due to permissions, run the mv line again with sudo.

curl -sS https://getcomposer.org/installer | sudo php -- --install-dir=/usr/local/bin --filename=composer