To connect the PostgreSQL via Command line you have to install a PostgreSQL provided tool called "psql" if not installed already.

Then use the below command to connect to PostgreSQL database.

psql -h localhost -p 5432 -U postgres 

Explanation:

localhost: is the hostname of machine where PostgreSQL is installed.
5432 : is the default PostgreSQL port
postgres : is the username of to connect to DB

Solution for your issue:

FATAL: password authentication failed for user "testuser"

As error message specified, either user not exists on database or the password that you supplied to connect is incorrect. Re-verify your user and password, and try again.

All the best.

WSL2 assigns IP address to the Windows host dynamically and the IP addresses can change without even rebooting Windows (see Notes below). So to reliably connect we'll need to:

1. Allow Windows and Postgres to accept connections from the WSL2 IP address range (not allowed by default)
2. From WSL2, determine the Windows/Postgresql host's IP address (which is dynamic) when connecting via psql. We'll make this convenient via .bashrc and alias.

Unfortunately I couldn't find the exact specification for the WSL2 IP address range. From several tests/reboots it appears that WSL2 is assigning IP addresses primarily in range of 172.*.*.* but I have occasionally been assigned 192.*.*.* so we'll use these when configuring the firewall and Postgres.

Add Windows Firewall Inbound Port Rule for WSL2 IP Addresses:

1. Open Windows Defender Firewall with Advanced Security
2. Click New Rule...
3. Select Port for rule type
4. Select TCP and for Specific local ports enter 5432
5. Select Allow the connection. Connecting from WSL2 won't be secure so don't select the secure option
6. Select at least Public. Can select Domain and Private as well. I could only connect if Public was selected
7. Name the rule e.g. Postgres - connect from WSL2 and create it
8. Right click newly created rule and select Properties then click on the Scope tab
9. Under Remote IP address, select These IP addresses then click Add... and enter range 172.0.0.1 to 172.254.254.254
10. Repeat step 9 for IP address range 192.0.0.1 to 192.254.254.254
11. Click Apply then OK
12. Make sure rule is enabled

Configure Postgres to Accept Connections from WSL2 IP Addresses

Assuming a default install/setup of Postgresql for Windows the following files are located under C:\Program Files\PostgreSQL\$VERSION\data

Verify that postgresql.conf has following set:

listen_addresses = '*'

This should already be set to '*' so nothing do here.

Update pg_hba.conf to allow connections from WSL2 range e.g. for Postgresl 12:

# TYPE  DATABASE        USER            ADDRESS                 METHOD

# IPv4 local connections:
host    all             all             127.0.0.1/32            md5
host    all             all             172.0.0.0/8             md5
host    all             all             192.0.0.0/8             md5

For Postgresql 13+ you should use scram-sha-256 as the method.

Restart Postgres for changes to take effect. This can be done either from the Windows Services app or from cmd with Administrator privileges e.g. for Postgresql 12:

net stop postgresql-x64-12
net start postgresql-x64-12

WSL Shell Conveniences

In WSL, add following to your ~/.bashrc or similar:

# Add DNS entry for Windows host
if ! $(cat /etc/hosts | grep -q 'winhost'); then
  echo 'Adding DNS entry for Windows host in /etc/hosts'
  echo '\n# Windows host - added via ~/.bashhrc' | sudo tee -a /etc/hosts
  echo -e "$(grep nameserver /etc/resolv.conf | awk '{print $2, "   winhost"}')" | sudo tee -a /etc/hosts
fi

Then reload your .bashrc changes: source ~/.bashrc

Usage

psql -h winhost -p 5432 -U postgres

Notes:

• The IP address assigned to the Windows host by WSL2 is not the same as the IP address assigned to your physical Windows machine on your network. WSL2 uses vEthernet connections.
• You can inspect the vEthernet connections via Control Panel\Network and Internet\Network Connections
• Note that when looking at the IPv4 properties that the IP addresses will appear as if they are statically set but they aren't! Try rebooting and inspecting IPv4 properties again
• If one day you're unable to connect to Postgres, check that winhost is in the IP address range per firewall rules. Could be WSL has assigned an IP address that we weren't expecting!