Find wrong utf-8 chars in a large file

check-wrong-utf8-char.sh

# Assuming you have your locale set to UTF-8 (see locale output), this works well to recognize invalid UTF-8 sequences:

grep -axv '.*' file.txt

# Grep man page:
#    -a, --text: treats file as text, essential prevents grep to abort once finding an invalid byte sequence (not being utf8)
#    -v, --invert-match: inverts the output showing lines not matched
#    -x '.*' (--line-regexp): means to match a complete line consisting of any utf8 character.