On this page:

https://www.phpbb.com/support/docs/en/3.1/kb/article/transferring-your-board-to-a-new-host-or-domain/

These two lines may not work on some linux systems:

gunzip < NAME_OF-FILE.GZ | mysql -u USERNAME -p PASSWORD --default-character-set=utf8 DATABASE_NAME

and

mysql -u USERNAME -p PASSWORD --default-character-set=utf8 DATABASE_NAME < NAME_OF-FILE

In many cases you will want to leave the PASSWORD out and then enter it at the prompt.  Some hosts have disabled it due to security implications of having the password in the command, so the commands should be:

gunzip < NAME_OF-FILE.GZ | mysql -u USERNAME -p --default-character-set=utf8 DATABASE_NAME

<then enter password at the prompt>

and

mysql -u USERNAME -p --default-character-set=utf8 DATABASE_NAME < NAME_OF-FILE

<then enter password at the prompt>