This script uses the Swarm client to auto-connect to Jenkins master who has the Swarm Plugin installed.

swarm_auto_connect.sh

#!/bin/bash

#  -------------------------------------------------------------
# |	Extract password and username from passed parameters 	|
#  -------------------------------------------------------------
while getopts ":u:p:" opt; do
  case $opt in
    u) username="$OPTARG"
    ;;
    p) password="$OPTARG"
    ;;
    \?) echo "Invalid option -$OPTARG" 
		echo "Supported options: -u <username> -p <password>">&2
		exit 1
    ;;
  esac
done

#  -------------------------------------------------------------
# |	Safety net in case user doesn't pass any credentials	|
#  -------------------------------------------------------------
if [ -z $username ] || [ -z $password ] ; then
	echo 'Please provide -u <username> _and_ -p <password>'
	exit 1
fi

#  ---------------------------------------------------------------------
# |	Extract which of the listed tools are installed on the machine	|
#  ---------------------------------------------------------------------

commands=( "python3" "rbenv" "xcodebuild" "android" "pyenv" "ruby" )
declare -a labels
for command in "${commands[@]}"; do
	location=$(command -v $command)
	if [ $? -eq 0 ]; then
		tool="$command=$location"
		labels+=($command)
	fi
done
echo "Labeling our agent for the following tools: ${labels[*]}"

#  -----------------------------------------------------------------------------
# |	Amount of workers = 1/4th of the cores. Memory = 512*workers	 	|
#  -----------------------------------------------------------------------------
cores=`getconf _NPROCESSORS_ONLN`
workers=$(($cores/4>=1?$cores/4:1))
let memory="$workers * 512"

#  -----------------------------------------------------------------------------
# |	Download swarm client if not already present in current folder  	|
#  -----------------------------------------------------------------------------
client_file_name="swarm-client.jar"
version_client="3.3"
if [ ! -f $client_file_name ]; then
	echo 'Swarm client not found. Downloading it!'
	curl -o "$client_file_name" -JL "https://repo.jenkins-ci.org/releases/org/jenkins-ci/plugins/swarm-client/${version_client}/swarm-client-${version_client}.jar"
fi

#  -----------------------------------------------------------------------------
# |	Use a consistent hostname when connecting to the swarm master   	|
#  -----------------------------------------------------------------------------
if [ ! -f hostname.txt ]; then
	host_name="$HOSTNAME"
	echo "$host_name" > hostname.txt
else
	host_name=$(cat hostname.txt)
fi

#  -----------------------------------------------------------------------------
# |	Launch the actual client with the parameters we determined      	|
#  -----------------------------------------------------------------------------
export PASSWORD=${password} 
java -Xms${memory}m -Xmx${memory}m -jar "$client_file_name" -deleteExistingClients -labels "${labels[*]}" -executors $workers -name "$host_name" -username "$username" -passwordEnvVariable "PASSWORD"