A simple queue system written in bash

urlqueue

#!/usr/bin/env bash
# Parially copied from https://stackoverflow.com/questions/37318999/multiprocess-queue-in-bash
# Requires: nc, tee, grep, and bash
#
# Example usage:
#   urlqueue -s
#   urlqueue https://www.youtube.com/watch?v=J5cnM1ODU0Y
#   urlqueue -q -p | while read -r URL    # read until end of list
#   do
#     youtube-dl $URL
#   done

# or

#   while true           # run forever, waiting for something to download
#   do
#      URL=$(urlqueue -q -p)
#      if [ -z "$URL" ]; then
#        sleep 30
#      else
#        youtube-dl $URL
#      fi
#   done

addit() {
    echo "$URL"|tee -a "$LOGFILE" >> /dev/tcp/127.0.0.1/4444
    exit 0
}

popit() {
    TIMEOUT=$(( 3 * 10 )) # 3 seconds
    (: < /dev/tcp/localhost/4444) 2>/dev/null || error "URL queue server not running."
    while ! { read -r a < /dev/tcp/localhost/4445; } 2>/dev/null
    do
        sleep .1
        DURATION=$(( DURATION + 1 ))
        [ $DURATION -ge $TIMEOUT ] && error "Error: Timeout."
    done
    [ -z "$a" ] && error "Error: Received no data"
    echo "$a"
    exit 0
}

startqueserver() {
    nc -k -l 4444 | while read -r a
    do
        echo "$a" | nc -l 4445
    done
}

killqueueserver() {
    # TODO: Plan on saving the pid of the subprocess to a file
    #       Not seeing how to get the pid of both nc processes right now
    #       $$, $!, $BASHPID, and cut -d' ' -f4 < /proc/self/stat aren't doing it
    #       Might have to move startqueserver into its own file to make it easier to kill
    #       Or maybe have it watch for a touched file to shutdown on its own
    #       Or just rewrite everything in python.
    #       Or look for an existing python script that works far better than mine ever could.
    #       For now I will be rude and killall
    killall -9 nc
    exit 0
}

error () {
    [ -z "$SILENT" ] && echo "$1" 1>&2
    exit 1
}

usage() {
    printf "Usage: %s [ -q ] [ -s | -k | -p | URL ]
       URL    Add URL to the queue
        -s    Start URL queue server
        -k    Kill URL queue server
        -p    Pop 1 item off the queue
        -q    Quiet or silent mode\n" "$(basename "$0")"
    exit 1
}

ARG="$1"
LOGHOME=$HOME/.local/share/urlqueue
mkdir -p "$LOGHOME"
LOGFILE=$LOGHOME/url.log

[ -z "$ARG" ] && usage

[[ "$ARG" == "-q" ]] && {  # I know this doesn't work if you do -p -q because of its position
    SILENT=1
    shift
    ARG=$1
}
[[ "$ARG" == "-s" ]] && {
    (: < /dev/tcp/localhost/4444) 2>/dev/null && error "URL queue server already running."
    startqueserver &
    exit 0
}
[[ "$ARG" == "-k" ]] && killqueueserver
[[ "$ARG" == "-p" ]] && popit

URL=$ARG
grep -E "^$URL$" "$LOGFILE" >/dev/null 2>&1 || addit

error "URL previously queued."