because while dealing with interactive programs one often come across numerous hidden traps which doesn’t usually happen with ordinary sh-scripts. Though fortunately or may be not, but most of these problems generally turn up within first five minutes of the work under the script. The symptoms typically look like that author can’t pass the authentication from the script.

now I'll tell you what really takes place when we start interactive applications from shell scripts

among these scripts there certainly will be those to cooperate with interactive applications such as telnet, ftp, su, password, ssh

In this framework, a stream is a chain of coroutines that pass messages between a program and a device driver (or between a pair of programs)

empty is an utility that provides an interface to execute and/or interact with processes under pseudo-terminal sessions (PTYs). This tool is definitely useful in programming of shell scripts designed to communicate with interactive programs like telnet, ssh, ftp, etc.