Option '-a' is not valid in ack 2
This is because we now have -k/--known-types which makes it only select files
of known types, rather than any text file (which is the behavior of ack 1.x).
You may have options in a .ackrc, or in the ACKRC_OPTIONS environment variable.
Try using the --dump flag.
What's missing is an explicit "You don't need -a, ack 1.x users."
https://gist.github.com/leto/eb928259ee9ba3662552 shows a frustrated Duke Leto not realizing that "ack1 -a foo" is equivalent to "ack2 foo". The warning message for -a right now says:
What's missing is an explicit "You don't need -a, ack 1.x users."