Advanced Programming in the UNIX® Environment

ptg10805159

720 Pseudo Terminals Chapter 19

terminal

device driver PTY master PTY slave

terminal

line discipline

terminal

line discipline

kernel

scriptprocess

script

file

fork

exec

login

shell

(sleeping)

forkexec shell

stdout

stderr

stdin

user at a

terminal

Figure 19.5 Thescriptprogram

After developing the generalptyprogram in Section 19.5, we’ll see that a trivial

shell script turns it into a version of thescriptprogram.

expectProgram

Pseudo terminals can be used to drive interactive programs in noninteractive modes.

Numerous programs arehard-wired to requireaterminal to run. One example is the

passwd( 1 )command, which requires that the user enter a password in response to a

prompt.

Rather than modify all the interactive programs to support a batch mode of

operation, a better solution is to provide a way to drive any interactive program from a

script. Theexpectprogram[Libes 1990, 1991, 1994]provides a way to do this. It uses

pseudo terminals to run other programs, similar to theptyprogram in Section 19.5.

But expectalso provides a programming language to examine the output of the

program being run to make decisions about what to send the program as input. When

an interactive program is being run from a script, we can’t just copy everything from the

script to the program, and vice versa. Instead, we have to send the program some

input, look at its output, and decide what to send it next.