When setting up PostgreSQL on a Linux system (particularly RHEL/CentOS using yum), you might encounter this error during initialization:

Data directory is not empty! [FAILED]

This typically happens when either:

• The data directory (/var/lib/pgsql/data) already contains files from a previous installation
• The PostgreSQL service was partially initialized before
• Another process created files in the directory

Here's the most straightforward fix:

sudo service postgresql stop
sudo rm -rf /var/lib/pgsql/data/*
sudo service postgresql initdb
sudo service postgresql start

If you want to preserve existing data (in case this isn't a fresh install), try:

sudo mv /var/lib/pgsql/data /var/lib/pgsql/data.old
sudo service postgresql initdb
sudo service postgresql start

Your pg_hba.conf modifications are correct for development environments, but note they reduce security by:

# WARNING: These settings disable password authentication for local connections
local   all         all                               trust
host    all         all         127.0.0.1/32          trust

For repeatable installations, consider this shell script:

#!/bin/bash
# PostgreSQL auto-installer for RHEL/CentOS
sudo yum install -y postgresql postgresql-server
sudo systemctl stop postgresql
sudo rm -rf /var/lib/pgsql/data
sudo postgresql-setup initdb
sudo sed -i 's/ident/trust/g' /var/lib/pgsql/data/pg_hba.conf
sudo systemctl start postgresql
sudo systemctl enable postgresql

If the error persists, check:

# Verify directory ownership
ls -ld /var/lib/pgsql/data

# Check for running processes
ps aux | grep postgres

# Verify SELinux context
ls -Z /var/lib/pgsql/data

For production systems, always:

• Maintain proper backups before modifying data directories
• Use md5 or scram-sha-256 authentication instead of trust
• Consider using separate data directories for major version upgrades

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When setting up PostgreSQL on RHEL/CentOS systems using yum, you might encounter this common initialization error. The system attempts to create a new database cluster but finds existing files in the designated data directory (/var/lib/pgsql/data by default).

Several scenarios can trigger this:

1. Previous incomplete installation
2. Manual configuration files created before initdb
3. Package manager leaving residual files
4. Aborted previous initialization attempt

First, stop PostgreSQL if running:

sudo service postgresql stop

Then properly clean the data directory (make absolutely sure you're targeting the correct path):

sudo rm -rf /var/lib/pgsql/data/*
sudo rm -rf /var/lib/pgsql/data/.*  # Removes hidden files too

After cleaning, follow this sequence:

sudo postgresql-setup initdb
sudo systemctl enable postgresql
sudo systemctl start postgresql

Check the initialization logs:

journalctl -u postgresql --no-pager -n 50

Validate directory ownership:

ls -ld /var/lib/pgsql/data/
# Should show postgres:postgres ownership

For more control, specify a new location:

sudo mkdir /pgdata
sudo chown postgres:postgres /pgdata
sudo -u postgres initdb -D /pgdata

After successful initdb, modify pg_hba.conf as needed (your original modifications were correct):

# TYPE  DATABASE    USER        ADDRESS         METHOD
local   all         all                         trust
host    all         all         127.0.0.1/32    trust
host    all         all         ::1/128         trust
host    all         all         0.0.0.0/0       md5

Remember to reload after changes:

sudo systemctl reload postgresql

• Always check directory contents before initialization
• Consider using pg_ctl instead of service wrappers for more control
• Document your PostgreSQL deployment steps