Connecting using alias:
sudo isql -u SYSDBA -p 'YOUR_PASSWORD' YOUR_DB_ALIAS
Connecting using local DB path
sudo isql -u SYSDBA -p 'YOUR_PASSWORD' '/path/to/db/file.fdb'
Connecting to remote DB:
isql 192.168.0.195:YOUR_DB_ALIAS -user sysdba -password 'YOUR_PASSWORD'