Shell Programming and Scripting

#!/bin/ksh
## this script is designed to be a run as a coprocess,
## yet selectively pass some of its output back to the
## co-process invoker--which is what usually occurs with co-processes--
## yet have the output of other commands directed directly to user's
## terminal, and not sent as input to the calling program

## put commands whose output is meant to go to caller here;
## this is default behavior for a co-process, before I do any redirection
echo "This message from me, the co-porcess, is meant to go to caller, as my stdout is automatically sent to my caller."

## save coproc's default stdout to temp file descriptor for later retrieval,
## and redirect coproc's stdout to user's terminal;
## the user's termial is determined by using tty command in caller, and saving its value in
## a paramter that is passed to coprocess, here TTY
exec 3>&1 >$TTY

### Put the commands meant for the terminal, not the caller, here:

echo
echo "This is directly to terminal from the co-process."
echo "The command tty in the co-process yilds: \"$(tty)\". This is of no use"
echo "But I inherited variable \$TTY from my caller, and its value here is: $TTY"

## redirect coproc's output back to starting default, which is to caller, and close the temporary fd we created
exec >&3 3>&-

## again, commands whose output is meant to go to caller
echo "This is 2nd output meant to go to calling program."

## coproc's does its fancy logic, and yields a value that the caller cares about
## here this fancy logic is mimicked by using $RANDOM
RESULT_OF_COPROC_LOGIC=$RANDOM
echo "$RESULT_OF_COPROC_LOGIC"

#!/bin/ksh
# this program *executes* another script--i.e., as a child subprocess, not as a
# sourced program, but yet is able to retrieve a value from this child
# process.  Does this by creating the child process as a co-process

# determine which device file is user's terminal, save this in a
# variable, and pass this variable to all child processes by exporting it
# to the environment
export TTY=$(tty)
echo "The command tty in the caller yields: $(tty)"

# execute the script as a co-process; here I am executing it by piping its contents
# into a shell as its stdin, meant to mimick what could be done when script
# is on network (of course in this test my script is actuaally on filesystem,
# so I could have executed it simply by calling its name)
# (If script were on the network, the "cat" would here be ftp or wget)
cat ./coproc | ksh |&
#  ./coproc |&   # executes script from filesystem; should be identical to previous

# use read -p to read coproc's output
read -p
echo -e "\nFirst ouput of coproc is: $REPLY"
read -p
echo -e "\nThis is the 2nd read from coproc: $REPLY"

read -p NEEDED_VALUE
echo -e "\The real reason I executed coproc was so that it can do a bunch of stuff; but I also needed to get a value from it."
echo "I can store that value in a variabel to use it. That value is: $NEEDED_VALUE"