This post shows how to use iteration and condition in fish shell.
For example, as to each file in current directory, let's check if 400KB < the file size and run some commands.
for x in * if test -f $x && test 800 -lt (du "$x" | cut -f1) some commands "$x" end end
Well, one-liner is also available.
for x in *; if test -f $x && test 800 -lt (du "$x" | cut -f1); somecommand "$x"; end; end;