Shell Syntax: How to correctly use \ to break lines?

If the statement would be correct without continuation, you need to use \. Therefore, the following works without a backslash, as you can't end a command with a &&:

echo 1 &&
echo 2

Here, you need the backslash:

echo 1 2 3 \
4

or

echo 1 \
&& echo 2

Otherwise, bash would execute the command right after processing the first line without waiting for the next one.

One of the scripting style guidelines I've encountered during my professional life at a huge IT company, obligated me to use no longer than 80 characters per line in a shellscript and indenting after breaking the line. Also, I had to break line before a pipe or && or ||. Like :

command1 \
  && command2 \
  || command3 \
  | command4

The goal was to have a clear readability.