\read ¶Synopsis:
\read number tomacro
Make the command macro contain the next line of input from text
stream number, as in \read5 to\data.
This opens the file email.tex for reading, puts the contents of
the first line into the command \email, and then closes the file.
\newread\recipientfile
\openin\recipientfile=email
\read\recipientfile to\email
\typeout{Email address: \email}
\closein\recipientfile
If number is outside the range from 0 to 15 or if no file
of that number is open, or if the file has ended, then \read
will take input from the terminal (or exit if interaction is not
allowed, e.g., \nonstopmode; see interaction modes).
(However, the natural way in LaTeX to take input from the terminal
is \typein (see \typein.)
To read an entire file as additional LaTeX source, use
\input (see \input) or \include (see \include & \includeonly).
A common reason to want to read from a data file is to do mail merges.
CTAN has a number of packages for that; one is datatool.