Unable to write keys.dev.pub to: /home/ubuntu/.composer

try

sudo apt-get install composer

if you have installed composer using sudo, or with a user different to the owner of .composer folder, as mentioned by @PhoneixS

Check if this is the case, by running:

ls -al ~/.composer  

which may return

ls -al .composer 
total 16
drwxr-xr-x  2 root root 4096 May  1 15:04 .
drwxr-xr-x 53 user-name  user-group  4096 May  1 15:07 ..
-rw-r--r--  1 root root  799 May  1 15:04 keys.dev.pub
-rw-r--r--  1 root root  799 May  1 15:04 keys.tags.pub

Then:

• if the owner of .composer is the root user, running the command with sudo, as fallow sudo php composer-setup.php ,will fix the issue
• if the owner is root but you don't want to have to run composer as root, then you can change the owner-user:owner-group of .composer to the current user as follow: sudo chown -R owner-user:owner-group ~/.composer
• if the owner of .composer is the a different user than user-name, then you can run the command as that specific user, using su

It can happen when you have a previous installation of composer not completely removed or if you are running the installer with a user different to the owner of .composer folder.

You can rename the old folder (to have a backup) and then try again:

mv ~/.composer ~/.composer.old

This is less risky than executing something unknown with sudo.

While installing the Composer in your Project Directory you may Get this error. Im Using the Ubuntu 16,While installing i faced the Same Issue

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All settings correct for using Composer Unable to write keys.dev.pub to: /home/ubuntu/.composer

Step 1: just go to your Project Directory and run a Command

sudo su

Step 2: then Try to install the Composer using The Following Command

curl -sS https://getcomposer.org/installer | php