The psql --list command can be used to display the Postgres databases. Something like this should be returned.

~]# psql --username johndoe --dbname db001 --list
                                 List of databases
   Name    |  Owner   | Encoding |  Collate   |   Ctype    |   Access privileges   
-----------+----------+----------+------------+------------+-----------------------
 db001     | postgres | UTF8     | en_US.utf8 | en_US.utf8 | 
 db002     | john.doe | UTF8     | en_US.utf8 | en_US.utf8 |

The psql command with the \dt (describe table) command can be used to list the tables in a database. Something like this should be returned.

~]# psql --username johndoe --dbname db001 --command "\dt"
                   List of relations
 Schema |             Name              | Type  | Owner 
--------+-------------------------------+-------+-------
 public | table001                      | table | johndoe
 public | table002                      | table | johndoe
 public | table003                      | table | johndo

The psql command with the \d <table name> command can be used to list the columns in a table.

psql --username johndoe --dbname mydb --command "\d mytable"

Something like this should be returned.

                                            Table "public.mytable"
        Column         |           Type           | Collation | Nullable |                       Default                       
-----------------------+--------------------------+-----------+----------+-----------------------------------------------------
 id                    | integer                  |           | not null | 
 name                  | varchar(30)              |           |          | 
 date_updated          | date                     |           |          | 

Here is the syntax to change a column type.

alter table <table name> alter column <column name> type <new column type>

For example.

alter table mytable alter column name type varchar(100)