The postinstall script of the 5.5.33 CentOS 6 bundle checks to see if the data directory exists, and if not, create it and run mysql_install_db:

  if [ ! -e $datadir ]; then

    # Create data directory

    mkdir -p $datadir/{mysql,test}

    # Initiate databases

    /usr/bin/mysql_install_db --rpm --user=mysql

  fi

This prevents mysql_install_db from running if $datadir exists but is empty. An example of this situation is when $datadir is a separate partition, or perhaps someone wanted to do a fresh install and does "rm -rf /var/lib/mysql/*".

I believe checking for $datadir/mysql would be a more accurate check, since that must exist or something really bad has happened.

Thanks,
-will