ack alterou alguns comportamentos básicos desde a versão 2.0. A mudança mais importante é que, por padrão, o ack 2 procura em todos os arquivos não binários.
Do site :
GENERAL CHANGES
When no selectors are specified, ack 1.x only searches through files that it can map to a file type. ack 2.x, by constrast, will search through every regular, non-binary file that is not explicitly ignored via --ignore-file or --ignore-dir. This is similar to the behavior of the -a/--all option in ack 1.x.
REMOVED OPTIONS
Because of the change in default search behavior, the -a/--all and -u/--unrestricted options have been removed. In addition, the -k/--known-types option was added to cause ack to behave with the default search behavior of ack 1.x.