Install the latest version of Composer on Ubuntu
If you have previously installed composer with command...
sudo apt-get install composer
...it is recommended to uninstall it first. Run the command:
sudo apt-get purge --auto-remove composer
Let's install the latest version now. Go to whatever directory you like and follow the instructions for Command-line installation on the official Composer website: https://getcomposer.org/download/
After you run the four commands (copy, verify, install, remove), you should end up with a single file named composer.phar.
All you have to do now is move this file to the appropriate directory with a command:
sudo mv composer.phar /usr/local/bin/composer
Try running this command anywhere...
composer -V
...and you should see the latest version of your freshly installed Composer.
Running composer might display this error message (where [user] is your username):
Cannot create cache directory /home/[user]/.composer/cache/repo/https---packagist.org/, or directory is not writable.
Proceeding without cache
Cannot create cache directory /home/[user]/.composer/cache/files/, or directory is not writable.
Proceeding without cache
Set the permissions with these commands (where [user] is your username):
cd ~
sudo chown [user]:[user] .composer -R