Guide to LaTeX (4th Edition) (Tools and Techniques

9.1.3 Monitor input and output
9.1. Processing parts of a document 211
There are times when it is desirable that L AT E X write a message to the
computer monitor during processing. This can be achieved with the
command
\typeout{message}
where message stands for the text that is to appear on the monitor. This
text is printed when the L AT E X processing reaches this command. It is also
written to the transcript file, but does not appear in the processed output.
If message contains a user-defined command, it will be interpreted
and its translation appears on the monitor. The same thing applies to
L AT E X commands. This could have dire consequences if the commands,
either user or L AT E X, are not really printable. To print the command name
literally, precede it with the command \protect.
The command
\typein[\com name]{message}
also writes the message text to the monitor, but then it waits for the
user to enter a line of text from the keyboard, terminated by typing the
〈return〉 key. If the optional argument \com name is missing, the line of
text is inserted directly into the processing. In this way, one could, for
example, reuse the same text for a letter for several addressees, entering
the name each time from the keyboard. Suppose the text contains
Dear \typein{Name:}\\ ...
then what appears on the monitor is:
✎
Name:
☞ At this point, one enters the name of the
recipient. If on successive processings
‘George’, ‘Fred’, and ‘Mary’ are entered, the
result will be a set of identical letters dif-
\@typein=
fering only in their salutations as ‘Dear
✍
✌ George’, ‘Dear Fred’, and ‘Dear Mary’.
If the \typein command contains the optional argument \com name,
this is treated the same as
\typeout{message} \newcommand{\com name}{entered definition}
In this case the definition is stored under the command name \com name
interactively and may be invoked and executed in the rest of the document
like any other L AT E X command.
With some experience in L AT E X methodology, it soon becomes obvious
that interactive processing with the \typein command can be very
practical. For example, if the preamble contains
\typein[\files]{Which files?}
\includeonly{\files}