Show the current value of a configuration parameter or show the current values for all parameters.

SHOW parameter | ALL
For example:
premdb=> show search_path;
 public, allpremdb
(1 row)
premdb=# show yb_server_version;
(1 row)
SHOW ALL includes a description of each parameter.
yellowbrick=# show all;
                name                 |             setting              |                                description                                 
 application_name                    | ybsql                            | Sets the application name to be reported in statistics and logs.
 block_size                          | 32768                            | Shows the size of a disk block.
 bytea_output                        | hex                              | Sets the output format for bytea.
 client_encoding                     | UTF8                             | Sets the client's character set encoding.
 client_min_messages                 | warning                          | Sets the message levels that are sent to the client.
 DateStyle                           | ISO, MDY                         | Sets the display format for date and time values.
 enable_alternative_round            | on                               | Translates round to round_var which allows for a non const second argument
 enable_silent_coerce                | off                              | Enables silent and lossy coerces.
 extra_float_digits                  | 0                                | Sets the number of digits displayed for floating-point values.
 idle_in_transaction_session_timeout | 0                                | Sets the maximum allowed duration of any idling transaction.
 idle_session_timeout                | 0                                | Sets the maximum allowed session duration after any statement.
 lc_collate                          | C                                | Shows the collation order locale.
 lc_ctype                            | C                                | Shows the character classification and case conversion locale.
 lc_time                             | C                                | Sets the locale for formatting date and time values.
 listen_addresses                    | *                                | Sets the host name or IP address(es) to listen to.
 log_timezone                        | UTC                              | Sets the time zone to use in log messages.
 max_connections                     | 2300                             | Sets the maximum number of concurrent connections.
 max_index_keys                      | 32                               | Shows the maximum number of index keys.
 max_user_connections                | 2000                             | Sets the maximum number of concurrent user connections.
 max_wal_size                        | 16GB                             | Sets the WAL size that triggers a checkpoint.
 search_path                         | "$user", public                  | Sets the schema search order for names that are not schema-qualified.
 server_encoding                     | LATIN9                           | Sets the server (database) character set encoding.
 ssl                                 | on                               | Enables SSL connections.
 ssl_ca_dir                          | /mnt/ybdata/certs/trust-database | Location of the SSL certificate authority directory.
 ssl_cert_file                       | /mnt/ybdata/certs/ybd.crt        | Location of the SSL server certificate file.
 ssl_ciphers                         | ECDHE:!3DES:!DES:!NULL:!RC4      | Sets the list of allowed SSL ciphers.
 ssl_key_file                        | /mnt/ybdata/certs/ybd.key        | Location of the SSL server private key file.
 ssl_prefer_server_ciphers           | on                               | Give priority to server ciphersuite order.
 statement_timeout                   | 0                                | Sets the maximum allowed duration in milliseconds of any statement.
 tcp_keepalives_count                | 10                               | Maximum number of TCP keepalive retransmits.
 tcp_keepalives_idle                 | 10                               | Time between issuing TCP keepalives.
 tcp_keepalives_interval             | 1                                | Time between TCP keepalive retransmits.
 TimeZone                            | UTC                              | Sets the time zone for displaying and interpreting time stamps.
 transaction_isolation               | read committed                   | Sets the current transaction's isolation level.
 wal_block_size                      | 8192                             | Shows the block size in the write ahead log.
 wal_level                           | minimal                          | Set the level of information written to the WAL.
 yb_last_execid                      |                                  | Shows the execution-id of the last backend query run in this session.
 yb_server_version                   | 5.4.0                            | Shows the Yellowbrick Database server version.
 yb_server_version_num               | 50400                            | Shows the Yellowbrick Database server version as an integer.
 ybd_query_tags                      |                                  | Sets the optional query tags to be stored in query history.
(40 rows)

For more details about each parameter, see Configuration Parameters.