The following script, uses
cut to remove the last character from each line in a pipe.
rev utility reverses lines character-wise.
cut removes sections from each of line.
It is a very simple script where we reverse the line once, remove the first character (which was the last one in the original form of the line) and finally we reverse the line back with the last character missing.
echo -e "hi\nHI" | rev | cut -c 2- | rev; # Will produce: h H