I know it is not recommended but I use it often enough, so here you have it too: A one-liner to install composer in ubuntu.

As I said before, this is not the recommended way, so you do it on your own risk.

The following will make sure you have everything installed you need for composer to install and work, get the installer, execute it with the install directory being set to /usr/local/bin and renaming it to composer.

Ubuntu

sudo apt install curl php-cli php-mbstring php-xml git unzip \
    && php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');" \
    && sudo php composer-setup.php --install-dir=/usr/local/bin --filename=composer \
    && php -r "unlink('composer-setup.php');" \
    && composer --version

Fedora

sudo dnf install curl php-cli php-mbstring php-xml git unzip \
    && php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');" \
    && sudo php composer-setup.php --install-dir=/usr/local/bin --filename=composer \
    && php -r "unlink('composer-setup.php');" \
    && composer --version

If this works, you should be see the composer version that got installed at the end of the output.

Done! 👍