How to specify the libpq correct connection string URL for Unix domain socket connection in the shell?

Question

https://www.postgresql.org/docs/current/libpq-example.html
compile the 1st example source code.

gcc test.c -L/usr/local/pgsql15/lib -I/usr/local/pgsql15/include -lpq

Cannot figure out the connection string. All the following failed.

./a.out postgresql:///test15?host=/tmp&port=5415
./a.out postgresql://test15?host=/tmp:5415
./a.out postgresql://test15?host=/tmp&port='5415'
./a.out postgresql://test15?host=/tmp:'5415'
./a.out postgresql://test15?host=/tmp&port=/tmp
./a.out postgresql:///tmp:5415/test15?

The error is almost similar:

connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such file or directory
        Is the server running locally and accepting connections on that socket?

[1]+  Exit 1                  ./a.out postgresql:///test15?host=/tmp

cat /usr/local/pgsql15/data_42091266/postgresql.conf | rg unix return

unix_socket_directories = '/tmp'        # comma-separated list of directories
#unix_socket_group = ''                 # (change requires restart)
unix_socket_permissions = 0777          # begin with 0 to use octal notation

Answer 1

Thanks to jjanes
The following two way works.

./a.out postgresql:///test15?host=/tmp\&port=5415

./a.out "postgresql:///test15?host=/tmp&port=5415"

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.htmlhttps://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html

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Answer 2

In addition to the postgresql:// and socket:// schemes addressed by others, you can also use the form:

"host=localhost port=5432  user=user1 dbname=db1 password=user1pass"

It should be passed as a single argument to the rest of the psql command. Working example below.

The port is optional if it is the system default (5432). If the socket is not in the system's default path to the socket directory (on many systems, /var/run/postgres, then you can specify the path instead of localhost.

In .pgpass, the hostname should match, and in the pg_hba.conf file, you need a line such as:

local all all scram-sha-256

Working example:

read -s DBPASS   # input password without echo
connstring="host=localhost user=testuser dbname=mydb password=$DBPASS"
psql -Atc 'select 1' "$connstring"

NOTE: THE ABOVE EXAMPLE IS INSECURE ON MULTI-USER SYSTEMS. When you do it this way, the command arguments and the password are visible to anyone else on the system who can run the "ps" command or view the "/proc" filesystem (Linux). The same goes (usually) for setting environment variables and/or using containers. Instead, save the password in .pgpass (make sure its not world-readable) or use peer authentication with ident maps (pg_hba, pg_ident configs).