initdb creates a new PostgreSQL database cluster. A database cluster is a collection of databases that are managed by a single server instance.
Creating a database cluster consists of creating the directories in which the database data will live, generating the shared catalog tables (tables that belong to the whole cluster rather than to any particular database), and creating the template1 and postgres databases. When you later create a new database, everything in the template1 database is copied. (Therefore, anything installed in template1 is automatically copied into each database created later.) The postgres database is a default database meant for use by users, utilities and third party applications.
Although initdb will attempt to create the specified data directory, it might not have permission if the parent directory of the desired data directory is root-owned. To initialize in such a setup, create an empty data directory as root, then use chown to assign ownership of that directory to the database user account, then su to become the database user to run initdb.
initdb must be run as the user that will own the server process, because the server needs to have access to the files and directories that initdb creates. Since the server may not be run as root, you must not run initdb as root either. (It will in fact refuse to do so.)
initdb initializes the database cluster's default locale and character set encoding. The collation order (LC_COLLATE) and character set classes (LC_CTYPE, e.g. upper, lower, digit) are fixed for all databases and can not be changed. Collation orders other than C or POSIX also have a performance penalty. For these reasons it is important to choose the right locale when running initdb. The remaining locale categories can be changed later when the server is started. All server locale values (lc_*) can be displayed via SHOW ALL. More details can be found in Section 21.1.
The character set encoding can be set separately for a database when it is created. initdb determines the encoding for the template1 database, which will serve as the default for all other databases. To alter the default encoding use the --encoding option. More details can be found in Section 21.2.
This option specifies the authentication method for local users used in pg_hba.conf. Do not use trust unless you trust all local users on your system. Trust is the default for ease of installation.
This option specifies the directory where the database cluster should be stored. This is the only information required by initdb, but you can avoid writing it by setting the PGDATA environment variable, which can be convenient since the database server (postmaster) can find the database directory later by the same variable.
Selects the encoding of the template database. This will also be the default encoding of any database you create later, unless you override it there. The default is derived from the locale, or SQL_ASCII if that does not work. The character sets supported by the PostgreSQL server are described in Section 21.2.1.
Sets the default locale for the database cluster. If this option is not specified, the locale is inherited from the environment that initdb runs in. Locale support is described in Section 21.1.
Like --locale, but only sets the locale in the specified category.
Selects the user name of the database superuser. This defaults to the name of the effective user running initdb. It is really not important what the superuser's name is, but one might choose to keep the customary name postgres, even if the operating system user's name is different.
Makes initdb prompt for a password to give the database superuser. If you don't plan on using password authentication, this is not important. Otherwise you won't be able to use password authentication until you have a password set up.
Makes initdb read the database superuser's password from a file. The first line of the file is taken as the password.
Other, less commonly used, parameters are also available:
Print debugging output from the bootstrap backend and a few other messages of lesser interest for the general public. The bootstrap backend is the program initdb uses to create the catalog tables. This option generates a tremendous amount of extremely boring output.
Specifies where initdb should find its input files to initialize the database cluster. This is normally not necessary. You will be told if you need to specify their location explicitly.
By default, when initdb determines that an error prevented it from completely creating the database cluster, it removes any files it may have created before discovering that it can't finish the job. This option inhibits tidying-up and is thus useful for debugging.
Specifies the directory where the database cluster is to be stored; may be overridden using the -D option.