Count the number of lines in a file without reading entire file into memory?

Reading the file a line at a time:

count = File.foreach(filename).inject(0) {|c, line| c+1}

or the Perl-ish

File.foreach(filename) {}
count = $.

or

count = 0
File.open(filename) {|f| count = f.read.count("\n")}

Will be slower than

count = %x{wc -l #{filename}}.split.first.to_i