Using tr to replace by "\ " but only gets "\" symbol as an output

tr only does character by character translation. Every input character is mapped to exactly one output character. Because your search set contains a single character (';'), tr can only replace it with one character. In echo 'Hello;World' | tr ';' '\\ ' , the replacement set ('\\ ') has two characters (backslash + space). tr takes the first one (\) to translate ;, so you get Hello\World.

Ideally you should use sed like echo 'Hello;World' | sed 's/;/\\ /g' or printf like printf '%s\n' 'Hello;World' | sed 's/;/\\ /g'