ack has a very useful feature for listing out the files that would be searched by the chosen --filetype option - also taking into account any ignored VCS files etc. This was a useful feature for piping into xargs or using to automatically generate a file listing to pass into cscope or ctags. It would be wonderful if ag could support the same feature.
-f Only print the files that would be searched, without actually doing any
searching. PATTERN must not be specified, or it will be taken as a path
Use -f for working with big codesets
Ack does more than search files. "ack -f --perl" will create a list of
all the Perl files in a tree, ideal for sending into xargs.