Stupid shell tricks

1 bash POST multiplex.md

One-liner to accept incoming http POST data and then relay it to some number of POST endpoints.

while true; do printf 'HTTP/1.0 200 OK\r\nContent-Length: 0\r\n' | { nc -q1 -l 8080; echo; } | sed '0,/^[[:space:]]*$/d' | tee >(http POST http://localhost:8081/path1) >(http POST http://localhost:8081/path2); done

With some linebreaks so it's readable:

while true
do
  printf 'HTTP/1.0 200 OK\r\nContent-Length: 0\r\n' \
  | { nc -q1 -l 8080; echo; } \
  | sed '0,/^[[:space:]]*$/d' \
  | tee \
    >(http POST http://localhost:8081/path1) \
    >(http POST http://localhost:8081/path2)
done

• Data being resubmitted is logged to STDOUT along with the response from the relay targets
• I just used another netcat instance as the web server, because lazy; who knows how this works with real problems
• This uses httpie because web developers have convinced me that's the right way to do things
• There is basically no error checking at all, so there's a strong chance this will explode your house, computers, and things that actually need it to run

2 sketchy bash web server.sh

#!/bin/bash

DELAY=5

function kid {
    local count=0
    # wait roughly half the time between polling loops because nyquist
    # "roughly" because bash doesn't do floating point :/
    local webdelay=$(( ( DELAY / 2 ) ))
    # just exit if the nc dies
    # note that this only works if everything else here is a builtin
    trap "exit" CHLD INT

    coproc nc -lk 8080
    echo "child spawned $COPROC_PID" >&2
    while true
    do
        echo "loop" >&2

        # wait a quarter second for new data; should be more than enough
        read -t 0.25
        if [[ $REPLY == +([0-9]) ]]
        then
            : $(( count += REPLY ))
            echo "count is now $count" >&2
        fi

        # check for a web client.
        read -t $webdelay -u ${COPROC[0]}
        if [[ -n $REPLY ]]
        then
            {
                if [[ $REPLY == @(GET /metrics*(/)*) ]]
                then
                    msg="count is $count"
                    printf "HTTP/1.1 200 OK\r\n"
                else
                    msg="unsupported"
                    printf "HTTP/1.1 400 not metrics\r\n"
                fi
                printf "Content-Type: text/plain\r\n"
                printf "Content-Length: ${#msg}\r\n"
                printf "\r\n"
                printf "$msg"
            } >&${COPROC[1]}
            while read -t $webdelay -u ${COPROC[0]}
            do
                [[ -z $REPLY ]] && break
            done
            echo "I served a client!" >&2
        fi
    done
}

coproc kid
trap "kill $COPROC_PID && exit" EXIT INT
echo "Process $$ spawned $COPROC_PID"

while sleep $DELAY
do
    printf "3\n" >&${COPROC[1]}
done
echo "and now we're done"

The "sketchy web server" doesn't like getting hit many times in a row. This is fine, because it's a terrible idea to write a web server this way.