Running gfix without a command (or an invalid command), or with the new -? switch in Firebird 2.5, results in the following screen of helpful information being displayed:
usage: gfix [options] <database>
plausible options are:
-activate activate shadow file for database usage
-attach shutdown new database attachments
-buffers set page buffers <n>
-commit commit transaction <tr / all>
-cache shutdown cache manager
-full v alidate record fragments (-v)
-force force database shutdown
-fetch_password fetch_password from file
-housekeeping set sweep interval <n>
-ignore ignore checksum errors
-kill kill all unavailable shadow files
-list show limbo transactions
-mend prepare corrupt database for backup
-mode read_only or read_write
-no_update read-only validation (-v)
-online database online <single / multi / normal>
-prompt prompt for commit/rollback (-l)
-password default password
-rollback rollback transaction <tr / all>
-sql_dialect set database dialect n
-sweep force garbage collection
-shut shutdown <full / single / multi>
-two_phase perform automated two-phase recovery
-tran shutdown transaction startup
-use use full or reserve space for versions
-user default user name
-validate validate database structure
-write write synchronously or asynchronously
-z print software version number
qualifiers show the major option in parenthesis