How to print EOL in \message?

Question

I'm trying to do this:

\documentclass{article}
\begin{document}
\message{Hello \\ world!}
\end{document}

However, the \\ is not printed as an end-of-line. I'm getting this in the console:

Hello \\ world!

What's the right syntax here?

Answer 1

This version works (I am assuming that priting end-of-line means for you going to the next line):

\documentclass{article}
\begin{document}
\message{Hello ^^J world!}
\end{document}

(^^J means newline character).

Answer 2

The (otherwise good) answer by Przemysław Scherwentke glosses over several details.

You cannot use \\ to separate lines in \message, because \\ is a typesetting instruction.

There is a general mechanism in TeX to produce new lines in \write or \message instructions, namely to use a character whose character code (the ASCII code) is the current value of the internal integer \newlinechar.

In the TeXbook we find (page 228)

The ⟨token list⟩ of a \write ought to be rather short, since it makes one line of output. Some implementations of TeX are unable to write long lines; if you want to write a lot of stuff, just give several \write commands. Alternatively, you can set TeX's \newlinechar parameter to the ASCII code number of some character that you want to stand for ”begin a new line”; then TeX will begin a new line whenever it would ordinarily output that character to a file. For example, one way to output two lines to the terminal in a single \write command is to say

\newlinechar=`\^^J
\immediate\write16{Two^^Jlines.}

It should be noted that plain TeX doesn't set a value for \newlinechar, so it keeps the default value −1.

To the contrary, LaTeX has always set \newlinechar=`\^^J, so you can always do something like

\documentclass{article}
\begin{document}
\message{Hello^^Jworld!}
\end{document}

Note that spaces around ^^J are preserved in the \message. This produces new lines in \message, \write and \typeout instructions.