This script will keep a jenkins slave always running.

start_jenkins_slave.sh

#!/bin/bash

## Set here the Jenkins URL to enable this slave
# and its secret
export JENKINS_URL=''
export SECRET=''
export JENKINS_IP=''

function test_master_conn() {
  ping -c 4 "$JENKINS_IP" &> /dev/null
  return $?
}

function test_google_conn() {
  ping -c 4 www.google.com.br &> /dev/null
  return $?
}

function start_slave() {
  echo "Process [ FAIL ]"
  echo "Trying to start the slave process..."
  java -jar slave.jar -jnlpUrl "$JENKINS_URL" -secret "$SECRET" \
    &> /dev/null &
  sleep 5
  disown
  if is_running; then
    success "Slave process started successfully!"
  else
    fail "Failure starting the slave process."
  fi
  return $?
}

function is_running() {
  ps -ef |
  grep -v grep |
  grep -q 'java -jar slave.jar'
  return $?
}

function fail() {
  echo "$@"
  echo "Trying again in 60 seconds..."
  sleep 60
  return 1
}

function success() {
  echo "$@"
  return 0
}

while true; do
  test_google_conn && success "Internet [  OK  ]" || fail "Internet [ FAIL ]"
  test_master_conn && success "Master [  OK  ]" || fail "Master [ FAIL ]"
  is_running && success "Process [  OK  ]" || start_slave
  sleep 60
  echo '------------------------------------------------------------'
done