Rewrite haproxy configuration and reload service.

haproxy-servers.cfg

# Edit this file then run `update-haproxy haproxy-servers.cfg`.
# Alternatively, use a shell pipeline to build server list `... | update-haproxy`.
server demo1 127.0.0.1:8001 check cookie demo1 weight 100
server demo2 127.0.0.1:8002 check cookie demo2 weight 100
server demo3 127.0.0.1:8003 check cookie demo3 weight 0
server demo4 127.0.0.1:8004 check cookie demo3 weight 0

haproxy.cfg

global
    log 127.0.0.1 local0 notice
    chroot /var/lib/haproxy
    maxconn 10000
    user haproxy
    group haproxy
    daemon

defaults
    log global
    mode http
    option httplog
    option dontlognull
    timeout connect 3s
    timeout client 60s
    timeout server 60s
    retries 3
    option redispatch
    errorfile 400 /etc/haproxy/errors/400.http
    errorfile 403 /etc/haproxy/errors/403.http
    errorfile 408 /etc/haproxy/errors/408.http
    errorfile 500 /etc/haproxy/errors/500.http
    errorfile 502 /etc/haproxy/errors/502.http
    errorfile 503 /etc/haproxy/errors/503.http
    errorfile 504 /etc/haproxy/errors/504.http

listen demo 0.0.0.0:80
    mode http
    stats enable
    stats uri /__demo__?stats
    stats realm Authentication\ Required
    stats auth ops:ov9fifVEMmeDXLFLocGjAhXC
    balance roundrobin
    option httpclose
    option forwardfor
    cookie demo insert

update-haproxy

#!/bin/bash
# Rewrite haproxy configuration and reload service.
#
# Provide a file with haproxy config server lines, or stdin:
#
#     usage: update-haproxy [FILE_WITH_SERVER_LINES]
#
# NOTE: Assumes pre-loaded base haproxy.cfg is ready to be appended
#       with indented server lines, with 4-spaces of indentation.
#       File with server lines should not be indented.
#
# Uses 'haproxy.cfg' file next to 'update-haproxy' as pre-loaded base.

# Exit immediately if a command error or non-zero return occurs.
set -e

# The name of this executable.
PROG="$( basename "${BASH_SOURCE[0]}" )"

# Directory containing this executable.
DIR="$( cd -P "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"

# Path to production haproxy.cfg.
HAPROXY_CFG=${HAPROXY_CFG:-/etc/haproxy/haproxy.cfg}

# Command to reload haproxy.
RELOAD_HAPROXY=${RELOAD_HAPROXY:-service haproxy reload}

# Base configuration for concatenation with given servers.
BASE_CFG=$DIR/haproxy.cfg

if [ ! -e $BASE_CFG ]; then
    echo "$PROG: missing $BASE_CFG; where did it go?" >&2
    exit 2
fi

SERVER_CFG=$1
shift

# Use stdin if no server configuration file is given.
if [ -z "$SERVER_CFG" ]; then
    SERVER_CFG=-
fi

(
    cat $BASE_CFG
    cat $SERVER_CFG |\
    while read -r line; do
        echo "    $line"
    done
) > $HAPROXY_CFG

$RELOAD_HAPROXY