There is an easy way to generate a Symfony compliant password hash from the command line. Assuming you’re using the bcrypt algorithm (the preferred choice according to Symfony’s security best practices), the default cost (13) and you have PHP >= 5.5 installed, just run the following command:

php -r "echo password_hash('ThePassword', PASSWORD_BCRYPT, ['cost' => 13]) . PHP_EOL;"

 It will output something like: $2y$13$7mBTrD0lgdgBxt1.YbdvOOeSOrPUYOBfeC1Ra2osPs9lpCHdplw1m

You can directly use this value in your app/config/security.yml  file:

security:
    firewalls:
        secured_area:
            pattern:    ^/
            anonymous: ~
            http_basic:
                realm: "Secured Demo Area"

    access_control:
        - { path: ^/admin, roles: ROLE_ADMIN }

    providers:
        in_memory:
             memory:
                users:
                    admin: { password: "$2y$13$7mBTrD0lgdgBxt1.YbdvOOeSOrPUYOBfeC1Ra2osPs9lpCHdplw1m", roles: 'ROLE_ADMIN' }

    encoders:
        Symfony\Component\Security\Core\User\User: bcrypt

Thanks to Sarah Khalil, a built-in Symfony command will be available in a next release (and that command will support all installed algorithms).