ack [OPTIONS]... PATTERN [FILES OR DIRECTORIES]
PATTERN in each source file in the tree from the current directory on down. If any files or directories are specified, then only those files and directories are checked.
ack may also search
STDIN, but only if no file or directory arguments are specified, or if one of them is
Default switches may be specified in the
ACK_OPTIONS environment variable or an
.ackrc file. If you want no dependency on the environment, turn it off with
ack -i select