Some Useful PostgreSQL Command in below example->
Backup database-

pg_dump -U geekstuff erp -f mydb.sql

Restore database-

psql -U erp -d erp_devel -f mydb.sql

Change owner of database-

ALTER DATABASE erp_v1 OWNER TO openerp;

Describing table-

\d table_name;

Delete content of file without deleting the file

echo -n > FILE_NAME

Export output of query to a file-
1) Login to postgres

psql -d dbname1 -U user1

2.) Set the output to a file

\o /home/administrator/QueryOutput.txt

3) Fire the query

select * from company1 where salary > 50000;

4.) Check if table is locked-

select bl.pid as blocked_pid, a.usename as blocked_user,
kl.pid as blocking_pid, ka.usename as blocking_user, a.current_query as blocked_statement
from pg_catalog.pg_locks bl
join pg_catalog.pg_stat_activity a
on bl.pid = a.procpid
join pg_catalog.pg_locks kl
join pg_catalog.pg_stat_activity ka
on kl.pid = ka.procpid
on bl.transactionid = kl.transactionid and bl.pid != kl.pid
where not bl.granted;

5.)Finding 10 largest files in a given location

du -ah /path_to_folder | sort -n -r | head -n 10