需要把 composer 拷贝到/usr/local/bin/，然后编辑.bash_profile 让终端支持 composer 命令。
curl -sS https://getcomposer.org/installer | php
sudo mv composer.phar /usr/local/bin/
Add this to your .bash_profile. It may be empty or non-existent, so go ahead and create it:
cd ~ nano ./bash_profile
alias composer="php /usr/local/bin/composer.phar"
Now, relaunch your terminal and you’ll be able to access Composer simply by calling composer.