permission denied for composer in /usr/local/bin/

this might be the case if you have downloaded composer.phar directly ,

but not by running php composer-setup.php

make composer.phar executable with following command before moving it to /usr/local/bin/composer or after moving

sudo chmod 755 composer.phar

composer-setup.php will make this change for us by default

On Ubuntu server >= 16.04

FIRST

sudo rm /usr/local/bin/composer

AND

cd ~/.cache/composer
chmod 755 composer-temp.phar
sudo mv composer-temp.phar /usr/local/bin/composer

You should check the permissions of the directory /usr/local/bin/, not just the file within. The process has to write both, the file and into the directory which both must be granted.

Apart from that, a general hint: do not always set everything to 777. There is no reason for that and it makes your system vulnerable.

According to the line you posted in the last comment the directory is currently writeable only for the root user himself. That would explain the error you get. You should not make your own user account the owner, Linux systems are multi user environments. Instead think about one of these approaches:

• add the account that is meant to execute the composer to the group root (a user account can belong to several groups) and make the directory group writeable
• change the groups ownership of the directory to a group that account is a member of and make the directory group writeable
• use the sudo utility to install and update the composer utility

The last option is the typically chosen and preferred one. It leaves permissions as they are (conservative) and only uses raised privileges for system maintenance jobs like installation and upgrade.