Poll serial device at 2 Hz write/read using Bash, Screen, bc

screen_serial_poll.sh

#!/bin/bash

set -x  # debug

# This polls the windows serial device over screen(1) connection 
# with one command and one response.
# Clean the data point, print to stdout and log it to LOGFILE.
# Uses named pipe, unix fifo, attached to bash fd 4 to take the serial data.

if [[ "Darwin" != $(uname) ]]; then  # windows
    cd /home/joeg/Work/collector_data
    STTYDEV="/dev/ttyS$(mode.com | grep COM | sed -e 's/.*\([0-9]\).*/\1/' | head -n 1)"
    DATE=date
else  # macos
    cd ~/Work/collector_rev_c
    STTYDEV="$(ls /dev/tty.usb*)"
    DATE=gdate  # gnu coreutiles from brew to get %N
fi

NL=$(echo -ne '\015')  # darwin needs it, wsl linux can use \r or \n

DELAY=0.5  # seconds

CMD="gt2"
CMD="ffr"
SCREEN_NAME="SCOL"

FIFO=/tmp/fifo_s
LOGFILE="$($DATE +%Y-%m-%d_%T)_logfile"

function egress {
    set -x
    screen -S $SCREEN_NAME -p0 -X colon "quit$NL"
    rm -f $FIFO
}
trap egress EXIT

# this sequencing is important with the fifo and screen
if [[ ! -p $FIFO ]]; then
    mkfifo $FIFO
fi

screen -dmS $SCREEN_NAME $STTYDEV 115200  # headless -dm
screen -S $SCREEN_NAME -p0 -X colon "logfile $FIFO$NL"
screen -S $SCREEN_NAME -p0 -X log on
screen -S $SCREEN_NAME -p0 -X logfile flush 0

exec 4<>$FIFO  # fd 4 fifo, named pipe, used for temp buffer

$DATE "+%Y-%m-%d_%H:%M:%S.%3N"

# TODO: link -ls logfile_latest to the last $LOGFILE
# screen -S $SCREEN_NAME -p0 -X stuff "$CMD$NL"  # put one in the queue
while :; do
    # use \r for the command, \n for the log
    t1=$($DATE "+%S%3N")
    screen -S $SCREEN_NAME -p0 -X stuff "$CMD$NL"
    sleep 0.1
    read -ru 4 REPLY  # this line takes 433 ms
    t2=$($DATE "+%S%3N")
    echo $($DATE "+%T.%3N ") $(echo $REPLY | sed -e "s/$CMD$NL//") | tee -a $LOGFILE
    set -x
    dt=$(bc <<< "($t2-$t1) / 1")  # millis
    if [[ $dt -lt 0 ]]; then dt=60000+$dt; fi  # handle boundary condition for 60 second rollover
    ddt=$(bc <<< "($DELAY*1000 - 0.1*1000 - $dt) / 1")
    # if dt elapsed time is less than desired delay time, delay for what is left
    if [[ $ddt -gt 0 ]]; then
        sleep $(bc <<< "scale=3;$ddt/1000.0")
    fi
    set +x
done

# TODO: read can take more than 400 ms to complete, work around it with extra reading in fifo 
# or something else that reads one string from fifo up to the newline.

#
# $(echo -ne '\015')