\message (LaTeX2e unofficial reference manual (November 2018))

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#### 27.5.1 `\message`

Synopsis:

    \message{string}

Write `string` to the log file and the terminal.

Typically, LaTeX authors use `\typeout` (see \typeout). It allows you to use `\protect` on any fragile commands in string (see \protect). But `\typeout` always inserts a newline at the end of string while `\message` does not, so the latter can be useful.

With this example document body.

    before\message{One Two}\message{Three}\message{Four^^JI}
    \message{declare a thumb war.}After

under some circumstances (see below) LaTeX writes the following to both the terminal and the log file.

    One Two Three Four
    I declare a thumb war.

Note that `^^J` produces a newline. Also, in the output document, between ‘`before`’ and ‘`After`’ will be a single space (from the end of line following ‘`I}`’).

While `\message` allows you more control over formatting interaction lines, a gotcha is that LaTeX may mess up your formatting because it inserts line breaks depending on what it has already put out. Contrast this document body, where the ‘`Two`’ has moved, to the one given above.

    before\message{One}\message{Two Three}\message{Four^^JI}
    \message{declare a thumb war.}After

This can happen: when LaTeX is outputting the messages to the terminal, now the message with ‘`One`’ is shorter and it fits at the end of the output terminal line, and so LaTeX breaks the line between it and the ‘`Two Three`’. That line break appears also in the log file. This line break insertion can depend on, for instance, the length of the full path names of included files. So producing finely-formatted lines in a way that is portable is hard.

Unofficial LaTeX2e reference manual