#!/bin/sh
#
# shell_select_example
#
# Example of using "shell_select" in a co-process
#   https://antofthy.gitlab.io/software/#shell_select
#
# In this example we send commands to "bc" which can produce both normal output
# and errors, and handle them without lockups or problems.  basically showing
# how you can read from multiple sources of input simultaniously in shell.
#
# I could have rolled both stdout and stderr into the same file descriptor, and
# this is typically what is done.  But then I would loose information about
# what was real output, and what was error.
#
# Basically using "shell_select", a shell script can run multiple commands or
# network channels in the background and handle them all gracefully.
#
# NOTE: Due to buffering, "bc" will always accept input. As such that channel
# will always be 'ready' for writing.  Same goes for network channels unless
# you are sending something like more than 8 Mbytes of data!
#
# Anthony Thyssen  <Anthony.Thyssen@gmail.com>
#
echo "Starting a 'bc' Co-Process (using named pipes in bounre shell)"
echo "Note that  10 is 'bc' stdin,  11 is stdout,  and 12 is stderr".
mkfifo pipe.in pipe.out pipe.err

bc <pipe.in >pipe.out 2>pipe.err &

exec 10>pipe.in 11<pipe.out 12<pipe.err
rm -f pipe.in pipe.out pipe.err  # the fifo files are no longer needed


echo ""
echo "Report what file descriptors (fd's) are ready (poll)"
echo "shell_select -t 0 -w 10 -r 11,12"
ready=`shell_select -t 0 -w 10 -r 11,12`
echo "$ready"
eval "$ready"

echo "Only input (write to the BC command) will be ready."
echo "Actually it will always be ready, unless the large buffers overflow!"
echo "BC has not output anything, not even a prompt (no tty attached)"

echo ""
echo "Sending Request: a multiplication to 4 decimal places"
echo ">>>> scale=4; 1.2121*7.312"
echo >&10 "scale=4; 1.2121*7.312"

echo ""
echo "Wait for response (read) and report what fd's are now readable!"
echo "shell_select -r 11,12"
ready=`shell_select -r 11,12`
echo "$ready"
eval "$ready"

echo "There is output to read, so lets read it."

echo ""
while true; do
  case ",$rd_ready," in
    *,11,*) read <&11 result
            echo "OUTPUT => $result"
            ;;
    *,12,*) read <&12 error
            echo "ERROR => $error"
            ;;
    *)      break
            ;;
  esac
  eval `shell_select -t 0 -r 11,12`   # poll check for more output
done

echo ""
echo "Reporting what fd's are now ready! (poll)"
echo "shell_select -t 0 -w 10 -r 11,12"
ready=`shell_select -t 0 -w 10 -r 11,12`
echo "$ready"   # input only
eval "$ready"
echo "Again only input (write to the BC command) will be ready."

echo ""
echo "Send a invalid 'bc' command!"
echo ">>>> this is not valid 'bc' request."
echo >&10 "this is not valid 'bc' request."

echo ""
echo "Wait for response and report what fd's are now readable!"
echo "shell_select -r 11,12"
ready=`shell_select -r 11,12`
echo "$ready"
eval "$ready"

echo "Note that this time we have errors, but no normal output!"
echo "If we had just simply read output - we would have locked up!"
echo "This is the real point of using a \"shell_select\" command."

echo ""
echo "Reading Results (errors) from BC command..."
while true; do
  case ",$rd_ready," in
    *,11,*) read <&11 result
            echo "OUTPUT => $result"
            ;;
    *,12,*) read <&12 error
            echo "ERROR => $error"
            ;;
    *)      break
            ;;
  esac
  eval `shell_select -t 0 -r 11,12`  # poll check for more errors
done

echo ""
echo "What fd's are now ready? (poll)"
echo "shell_select -t 0 -w 10 -r 11,12"
ready=`shell_select -t 0 -w 10 -r 11,12`
echo "$ready"   # input only
eval "$ready"
echo "Only write to bc (input to command) is ready. -- All done!"

echo ""
echo "Close all channels and Wait for process to finish"
exec 10>&- 11<&- 12<&-
wait
echo "Final Exit status = $?"