initdb is the command for creating a new database cluster, which performs the creation of the directory in which to store the databases, creation of the shared catalog tables, the creation of the template databases template0 and template1, and the creation of the database named postgres.

Because the database server process must be able to access the database files, initdb must be run with the privileges of the database administrator user. However, because the Linux root user cannot run database processes, the database file cannot be owned by root, and initdb cannot be run as root.

initdb has the ability to create a new directory for the database cluster, but on Linux and so on, you might not have write permission for that directory. To work around this problem, it is common to create a database cluster directory with root privileges, then use the chown command to change the owner of the directory to the database administrator user, and then run initdb. However, the contents of this directory must be empty (If it is not empty, an error occurs when initdb runs.).

initdb only creates the directory and database files for the database cluster and does not start the server processes for the database. Therefore, running this command is not enough to enable the connection to the database. You must run a separate command to start the server process, or configure the server to start automatically at system startup.

So the correct answers are A, D, and E.