The fundamental difference lies in their roles within the PostgreSQL ecosystem:

• psql: PostgreSQL's interactive terminal client (frontend)
• postgres: The actual database server executable (backend)

Here's how these commands normally behave:

# Connecting to an existing server
psql -U username -d dbname

# Starting a server instance (requires config)
postgres -D /usr/local/var/postgres

When installed via Homebrew on macOS:

# Typical Homebrew locations
/usr/local/bin/psql
/usr/local/bin/postgres
/usr/local/var/postgres # Default data directory

The error you're seeing with postgres indicates missing server configuration. Here's how to properly initialize and start:

# Initialize data directory
initdb /usr/local/var/postgres

# Start server with default config
postgres -D /usr/local/var/postgres

A complete local development setup might look like:

# Initialize data directory (first time only)
initdb /usr/local/var/postgres

# Start server in background
pg_ctl -D /usr/local/var/postgres -l logfile start

# Connect using psql
psql postgres

If you modified system paths (like removing /usr/bin/psql):

# Verify which binaries are being used
which -a psql
which -a postgres

# Check Homebrew links
brew --prefix postgresql
brew link --force postgresql

Key variables that affect behavior:

# For psql
export PGHOST=localhost
export PGPORT=5432

# For postgres
export PGDATA=/usr/local/var/postgres

In production environments, you'd typically use:

# Managed service startup
pg_ctl start
# or through systemd
systemctl start postgresql

While development often uses direct commands as shown earlier.

The psql and postgres commands serve fundamentally different purposes in PostgreSQL:

• psql: The PostgreSQL interactive terminal (client-side)
• postgres: The actual PostgreSQL server executable (server-side)

# Typical psql connection command:
psql -h localhost -U username -d database_name

# Typical postgres server startup:
postgres -D /usr/local/var/postgres

The different responses you're seeing are completely expected:

> psql
psql (14.5)
Type "help" for help.

ovatsug25=#  # This shows successful client connection

> postgres
postgres does not know where to find the server configuration file.
# This occurs because server needs explicit data directory

When installing via Homebrew on macOS, you'll typically find these binaries in /usr/local/bin. The commands have different runtime requirements:

# Verify installations:
brew list postgresql

# Common Homebrew paths:
/usr/local/Cellar/postgresql/{version}/bin/psql
/usr/local/Cellar/postgresql/{version}/bin/postgres

The server (postgres) needs explicit configuration, while the client (psql) can use defaults:

# Minimum server startup command:
postgres -D /usr/local/var/postgres

# Alternative using environment variable:
export PGDATA=/usr/local/var/postgres
postgres

After removing /usr/bin/psql, ensure your PATH prioritizes Homebrew binaries:

echo $PATH
# Should show /usr/local/bin before /usr/bin

# Verify which binary is being used:
type -a psql
type -a postgres

Common scenarios where you'd use each command:

# Using psql for database operations:
psql -c "SELECT version();"
psql -f backup_restore.sql

# Using postgres for server management:
postgres -D /data/directory -k /tmp
postgres --config-file=/path/to/postgresql.conf

When Rails connects but command line tools fail:

# Check running Postgres processes:
ps aux | grep postgres

# Verify socket location:
ls -la /tmp/.s.PGSQL.*

# Alternative connection string:
psql "host=/tmp user=postgres dbname=mydb"