What is the function of
find subdir -lname 'test*' \! -newer somethinghere
Putting a backslash before a character escapes it, preventing it from being parsed as syntax. Thus,
\! is a terser equivalent to
'!' -- it ensures that
find is passed
! as an argument, without the shell interpreting it in any way.
Without either of these, if
[[ $- = *H* ]] (which is true in an interactive shell and by default), and if
histchars is unmodified from its default value,
! triggers history expansion. This is per the bash man page's QUOTING section:
When the command history expansion facilities are being used (see HISTORY EXPANSION below), the history expansion character, usually
!, must be quoted to prevent history expansion.