The $y macro holds the name of the controlling terminal device, if there is one. The controlling terminal is determined by first calling ttyname(3) with the sendmail program's standard error output as an argument. If ttyname(3) returns the name of a terminal device (such as /dev/ttypa), sendmail strips everything up to and including the last / character and stores the result into $y.

$y is intended for use in debugging sendmail problems. It is not used internally by sendmail. In determining whether it can write to a user's terminal screen, sendmail calls ttyname(3) separately on its standard input, output, and error output without updating $y.

Note that the device name in $y depends on the implementation of ttyname(3). Under BSD Unix, all terminals are in /dev, whereas under other versions of Unix they can be in subdirectories such as /dev/ttys. Also note that $y is defined only if TTYNAME is defined (TTYNAME) when sendmail is compiled.

$y is transient. If it is defined in the configuration file or the command line, that definition will be ignored by sendmail. Finally, note that $y is set only when mail is being sent and, therefore, is of most value in headers.