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install composer 2 ubuntu

Teniendo composer instaldo, correr 
composer self-update --2

install composer v2 ubuntu

Purge existing composer v1

sudo apt purge composer

Download composer-setup v2

curl -sS https://getcomposer.org/installer -o composer-setup.php

Use php to install Composer
sudo php composer-setup.php --install-dir=/usr/local/bin --filename=composer

Check Composer Version
composer -V