mostly POSIX-compliant wait-for-it.sh; works for alpine

wait-for-it.sh

#!/bin/sh
#   Use this script to test if a given TCP host/port are available

cmdname=$(basename "$0")

echoerr() { if [ "$QUIET" -ne 1 ]; then echo "$@" 1>&2; fi }

usage()
{
    cat << USAGE >&2
Usage:
    $cmdname host:port [-s] [-t timeout] [-- command args]
    -h HOST | --host=HOST       Host or IP under test
    -p PORT | --port=PORT       TCP port under test
                                Alternatively, you specify the host and port as host:port
    -s | --strict               Only execute subcommand if the test succeeds
    -q | --quiet                Don't output any status messages
    -t TIMEOUT | --timeout=TIMEOUT
                                Timeout in seconds, zero for no timeout
    -- COMMAND ARGS             Execute command with args after the test finishes
USAGE
    exit 1
}

wait_for()
{
    if [ "$TIMEOUT" -gt 0 ]; then
        echoerr "$cmdname: waiting $TIMEOUT seconds for $HOST:$PORT"
    else
        echoerr "$cmdname: waiting for $HOST:$PORT without a timeout"
    fi
    start_ts=$(date +%s)
    while :
    do
        if command -v nc > /dev/null ; then
            nc -z "$HOST" "$PORT"
            result=$?
        else
            bash -c "(echo > /dev/tcp/$HOST/$PORT) >/dev/null 2>&1"
            result=$?
        fi
        if [ $result -eq 0 ]; then
            end_ts=$(date +%s)
            echoerr "$cmdname: $HOST:$PORT is available after $((end_ts - start_ts)) seconds"
            break
        fi
        sleep 1
    done
    return $result
}

wait_for_wrapper()
{
    # In order to support SIGINT during timeout: http://unix.stackexchange.com/a/57692
    if [ "$QUIET" -eq 1 ]; then
        timeout "$BUSYTIMEFLAG" "$TIMEOUT" "$0" --quiet --child --host="$HOST" --port="$PORT" --timeout="$TIMEOUT" &
    else
        timeout "$BUSYTIMEFLAG" "$TIMEOUT" "$0" --child --host="$HOST" --port="$PORT" --timeout="$TIMEOUT" &
    fi
    PID=$!
    trap 'kill -INT -$PID' INT
    wait $PID
    RESULT=$?
    if [ $RESULT -ne 0 ]; then
        echoerr "$cmdname: timeout occurred after waiting $TIMEOUT seconds for $HOST:$PORT"
    fi
    return $RESULT
}

# process arguments
while [ $# -gt 0 ]
do
    case "$1" in
        *:* )
        HOST=${1%:*}
        PORT=${1##*:}
        shift 1
        ;;
        --child)
        CHILD=1
        shift 1
        ;;
        -q | --quiet)
        QUIET=1
        shift 1
        ;;
        -s | --strict)
        STRICT=1
        shift 1
        ;;
        -h)
        HOST="$2"
        if [ "$HOST" = "" ]; then break; fi
        shift 2
        ;;
        --host=*)
        HOST="${1#*=}"
        shift 1
        ;;
        -p)
        PORT="$2"
        if [ "$PORT" = "" ]; then break; fi
        shift 2
        ;;
        --port=*)
        PORT="${1#*=}"
        shift 1
        ;;
        -t)
        TIMEOUT="$2"
        if [ "$TIMEOUT" = "" ]; then break; fi
        shift 2
        ;;
        --timeout=*)
        TIMEOUT="${1#*=}"
        shift 1
        ;;
        --)
        shift
        CLI="$*"
        break
        ;;
        --help)
        usage
        ;;
        *)
        echoerr "Unknown argument: $1"
        usage
        ;;
    esac
done

if [ "$SKIP_WAITING" != "" ]; then
    HOST="localhost"
    PORT="8000"
fi

if [ "$HOST" = "" ] || [ "$PORT" = "" ]; then
    echoerr "Error: you need to provide a host and port to test."
    usage
fi

TIMEOUT=${TIMEOUT:-15}
STRICT=${STRICT:-0}
CHILD=${CHILD:-0}
QUIET=${QUIET:-0}

TIMEOUT_PATH=$(realpath "$(command -v timeout)")
if echo "$TIMEOUT_PATH" | grep -q busybox ; then
        BUSYTIMEFLAG="-t"
else
        BUSYTIMEFLAG=""
fi

# are we skipping this entire business?
if [ "$SKIP_WAITING" != "" ]; then
    RESULT=0
else
    if [ "$CHILD" -gt 0 ]; then
        wait_for
        RESULT=$?
        exit $RESULT
    else
        if [ "$TIMEOUT" -gt 0 ]; then
            wait_for_wrapper
            RESULT=$?
        else
            wait_for
            RESULT=$?
        fi
    fi
fi

if [ "$CLI" != "" ]; then
    if [ "$RESULT" -ne 0 ] && [ "$STRICT" -eq 1 ]; then
        echoerr "$cmdname: strict mode, refusing to execute subprocess"
        exit $RESULT
    fi
    exec "$@"
else
    exit $RESULT
fi