Advanced Programming in the UNIX® Environment

ptg10805159

736 Pseudo Terminals Chapter 19

pty slowout > file.out &

theptyprocess is stopped immediately when the child tries to read from its standard

input (the terminal). The reason is that the job is a background job and gets job-control

stopped when it tries to access the terminal. If we redirect standardinput so thatpty

doesn’t try to read from the terminal, as in

pty slowout < /dev/null > file.out &

theptyprogram stops immediately because it reads an end of file on its standardinput

and terminates. The solution for this problem is the-ioption, which says to ignorean

end of file on the standardinput:

pty -i slowout < /dev/null > file.out &

This flag causes theptychild in Figure19.12 to exit when the end of file is encountered,

but the child doesn’t tell the parent to terminate. Instead, the parent continues copying

the PTY slave output to standardoutput (the filefile.outin the example).

scriptProgram

Using theptyprogram, we can implement thescript( 1 )program as the following

shell script:

#!/bin/sh

pty "${SHELL:-/bin/sh}" | tee typescript

Once we run this shell script, we can execute thepscommand to see all the process

relationships. Figure19.14 details these relationships.

login

shell sh tee

pty

parent

pty

child ksh ps

typescript

file

line

discipline

TTY masterPTY

line

discipline

PTY

slave

user

pipe

Figure 19.14 Arrangement of processes forscriptshell script