Testing: unit & shellcheck

Some minor fixes

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "shell-ci-build"]
+	path = shell-ci-build
+	url = https://github.com/caarlos0/shell-ci-build.git

.travis.yml

@@ -0,0 +1,19 @@
+language: bash
+
+sudo: required
+
+os:
+    - linux
+    - osx
+
+install:
+    -  ./shell-ci-build/install.sh
+
+before_script:
+    - curl -L "http://downloads.sourceforge.net/shunit2/shunit2-2.0.3.tgz" | tar zx
+    - chmod +x $(pwd)/shunit2-2.0.3/src/shell/shunit2
+
+script:
+    - export SHUNIT2=$(pwd)/shunit2-2.0.3/src/shell/shunit2
+    - make test
+    - ./shell-ci-build.sh

shell-ci-build.sh

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+#!/usr/bin/env bash
+set -eo pipefail
+source ./shell-ci-build/build.sh
+check "./check_nagios_latency"

test/unit_tests.sh

@@ -0,0 +1,47 @@
+#!/bin/sh
+
+if [ -z "${SHUNIT2}" ] ; then
+    cat <<EOF
+To be able to run the unit test you need a copy of shUnit2
+You can download it from http://shunit2.googlecode.com/
+
+Once downloaded please set the SHUNIT2 variable with the location
+of the 'shunit2' script
+EOF
+    exit 1
+fi
+
+if [ ! -x "${SHUNIT2}" ] ; then
+    echo "Error: the specified shUnit2 script (${SHUNIT2}) is not an executable file"
+    exit 1
+fi
+
+SCRIPT=../check_nagios_latency
+if [ ! -r "${SCRIPT}" ] ; then
+    echo "Error: the script to test (${SCRIPT}) is not a readable file"
+fi
+
+# constants
+
+NAGIOS_OK=0
+NAGIOS_CRITICAL=1
+NAGIOS_WARNING=2
+NAGIOS_UNKNOWN=3
+
+testUsage() {
+    ${SCRIPT} > /dev/null 2>&1
+    EXIT_CODE=$?
+    assertEquals "wrong exit code" ${NAGIOS_UNKNOWN} "${EXIT_CODE}"
+}    
+
+# the script will exit without executing main
+export SOURCE_ONLY='test'
+
+# source the script.
+. ${SCRIPT}
+
+unset SOURCE_ONLY
+
+# run shUnit: it will execute all the tests in this file
+# (e.g., functions beginning with 'test'
+. ${SHUNIT2}