preconfigure an empty password for mysql via debconf-set-selections

I am setting up a bash script to automate the building of a LAMP environment.

I am using debconf-set-selections to set options before installing mysql, phpmyadmin, etc ...

It works mainly great. But the problem is that I have to set an empty password for mysql and it still asks for the password during installation even with the lines typed before :

echo "mysql-server-5.5  mysql-server/root_password  password" | debconf-set-selections
echo "mysql-server-5.5  mysql-server/root_password_again    password" | debconf-set-selections

Solution

The recommended approach for setting an empty password during mysql-server's package installation process using debconf-set-selections depends on the package version in hand.

MySQL 5.7

Although it's possible to configure the password to be empty, the installation process will automatically enable the authentication plugin auth_socket in such a case. Hence, the password-less login shall be conditioned on using UNIX socket based authentication (e.g. requiring the login to be made from the user account root on the local machine).
- For the package provided by the MySQL APT repository:
```
sudo debconf-set-selections <<< "mysql-community-server mysql-community-server/root-pass password "
sudo debconf-set-selections <<< "mysql-community-server mysql-community-server/re-root-pass password "
```
- For the package provided by the Ubuntu package repositories:
```
sudo debconf-set-selections <<< "mysql-server mysql-server/root-password password "
sudo debconf-set-selections <<< "mysql-server mysql-server/root-password_again password "
```

MySQL 5.5 and MySQL 5.6

The solution suggested by this answer should work, with a minor addition that confirms the empty password:

sudo debconf-set-selections <<< "mysql-server mysql-server/root_password password \"''\""
sudo debconf-set-selections <<< "mysql-server mysql-server/root_password_again password \"''\""

A fallback solution

If all approaches fail, it's possible to circumvent the issue and implicitly leave the password blank by preventing debconf from asking the user for a password altogether. Unfortunately, this applies to all questions, rather than just the password-related ones.

Here are two methods that achieve the desired result:

Set debconf's frontend to noninteractive:

DEBIAN_FRONTEND=noninteractive apt-get install [-y] [-q] mysql-server

Set debconf's priority level to critical:
```
DEBIAN_PRIORITY=critical apt-get install [-y] [-q] mysql-server
```
This method is probably preferable as it permits the display of critical messages.