The most direct way to verify PostgreSQL's port is by examining the active network connections:

sudo netstat -plunt | grep postgres
# Sample output:
# tcp        0      0 127.0.0.1:5432          0.0.0.0:*               LISTEN      1234/postgres

Alternatively, use the modern ss command:

sudo ss -tulpn | grep postgres

PostgreSQL stores its port configuration in postgresql.conf:

grep ^port /etc/postgresql/8.4/main/postgresql.conf
# Expected output:
# port = 5432             # (change requires restart)

When already connected to PostgreSQL, run this SQL query:

SELECT name, setting FROM pg_settings WHERE name = 'port';

The service status output contains port information:

sudo service postgresql-8.4 status
# Look for lines containing "port" or "listening"

Examine the running postgres process for port arguments:

ps aux | grep postgres | grep -- -p
# The -p flag shows the port if specified

Here's a bash script to automatically detect the port:

#!/bin/bash
PORT=$(sudo -u postgres psql -t -c "SELECT setting FROM pg_settings WHERE name='port';" | xargs)
[ -z "$PORT" ] && PORT=$(grep ^port /etc/postgresql/*/main/postgresql.conf | awk '{print $3}')
echo "PostgreSQL is running on port: ${PORT:-5432}"

If you suspect the port is already in use:

sudo lsof -i :5432
# Or for Ubuntu's default tool:
sudo netstat -tulnp | grep :5432

Remember that changing the port requires a PostgreSQL service restart:

sudo service postgresql restart

---

While PostgreSQL defaults to port 5432, production environments often use custom ports. As a database administrator, you'll need several reliable ways to verify the actual port your PostgreSQL instance is using.

The simplest way to check the port is through the PostgreSQL client:

psql -h localhost -p 5432 -U postgres -c "SHOW port;"

This will output the current port number directly from the database engine.

On Linux systems, you can inspect the running Postgres process:

ps aux | grep postgres

Look for the process listing that includes the "-p" parameter followed by the port number.

The configuration file contains the definitive port setting:

grep ^port /etc/postgresql/8.4/main/postgresql.conf

For Ubuntu 9.10 with PostgreSQL 8.4, the default path would be as shown above. Modern installations might use:

grep ^port /etc/postgresql/14/main/postgresql.conf

Check active network connections using netstat or ss:

sudo netstat -plnt | grep postgres
# Or using ss:
sudo ss -ltnp | grep postgres

Systemd services reveal port information:

sudo systemctl status postgresql

Look for the "Listening" line that shows the port number.

For automation purposes, you might use this Python script:

import psycopg2

try:
    conn = psycopg2.connect(
        host="localhost",
        port=5432,
        user="postgres"
    )
    cur = conn.cursor()
    cur.execute("SHOW port;")
    print(f"PostgreSQL port: {cur.fetchone()[0]}")
except Exception as e:
    print(f"Connection failed: {e}")