Why do I get this error "Statement failed, SQLSTATE = 08001"?

When ever I try to connect with Firebird SQL I get this error,

firebird$ isql-fb 'db.FDB' -u SYSDBA -p 

Statement failed, SQLSTATE = 08001
I/O error during "open" operation for file "CAFW.FDB"
-Error while trying to open file
-Permission denied
Use CONNECT or CREATE DATABASE to specify a database

What can I do to resolve it, I've already ensured that my current user has read/write access to the file.

$ sudo chown firebird:firebird ./db.FDB
$ sudo chmod 770 ./db.FDB # should only need 660

$ ls -lah
-rw-rw---- 1 firebird firebird 53M May  2 17:13 CAFW.FDB

You need to add yourself in the Firebird group

sudo adduser `id -un` firebird

or always use localhost address in front of db if Server is Classic or SuperClassic

See the long thread here
http://firebird.1100200.n4.nabble.com/Fwd-Have-you-tried-firebird-2-5-SuperClassic-from-the-packages-td3053790.html

StackExchange Database Administrators Q#64403, answer score: 2