head.body.date.txt

head_body_date.txt

for f in *.*.*.txt; do i="${f%.txt}"; echo mv -i -- "$f" "${i//./_}.txt"; done

for f in *.*.*.txt; do i="${f%.txt}"; mv -i -- "$f" "${i//./_}.txt"; done

for f in *.*.*.*; do pre="${f%.*}"; suf="${f##*.}";
                     echo mv -i -- "$f" "${pre//./_}.${suf}"; done

for f in *.*.*.*; do pre="${f%.*}"; suf="${f##*.}";
                     mv -i -- "$f" "${pre//./_}.${suf}"; done

for f in *.*; do pre="${f%.*}"; suf="${f##*.}";
                 mv -i -- "$f" "${pre//./_}.${suf}"; done

$ rename -n 's/.[^.]+$(*SKIP)(*F)|./_/g' head.body.date.txt
rename(head.body.date.txt, head_body_date.txt)

$ rename -n 's/.(?![^.]+$)/_/g' head.body.date.txt
rename(head.body.date.txt, head_body_date.txt)

rename 's/.(?=[^.]*.)/_/g' *.txt

old=head.body.date.txt oldb=${old%.*} olde=${old##*.} ;
mv -- "$old" "${oldb//./_}.${olde}"

mv "head.body.date.txt" "$(echo head.body.date.txt | rev | sed 's/./_/2g' | rev)"

for file in *.txt; do mv "$file" "$(echo $file | rev | sed 's/./_/2g' | rev)"; done

mv head.body.date.txt head_body_date.txt