Add solution for directories_comparison exercise (#241)

* add solution

* Update question language

* Update question language

* Print "No difference" if no difference between dirs

Co-authored-by: Luke LeVasseur <llevasseur@flexport.com>

exercises/shell/solutions/directories_comparison.md

@@ -0,0 +1,69 @@
+## Directories Comparison
+
+### Objectives
+
+1. You are given two directories as arguments and the output should be any difference between the two directories
+
+### Solution
+
+```
+#!/usr/bin/env bash
+
+
+help () {
+  echo "Usage: compare <filename1> <filename2>"
+  echo
+}
+
+validate_args() {
+  # Ensure that 2 arguments are passed
+  if [ $# != 2 ]
+  then
+    help
+    exit 1
+  fi
+
+  i=1
+  for dir in "$@"
+  do
+      # Validate existence of directories
+      if [ ! -d "$dir" ]
+      then
+        echo "Directory $dir does not exist"
+        exit 1
+      fi
+      echo "Directory $i: $dir"
+      i=$((i + 1))
+  done
+  echo
+}
+
+compare() {
+  echo "Comparing directories..."
+  echo
+  diff -r "$1" "$2"
+  
+  if [ $? -eq 0 ]
+  then
+  	echo "No difference"
+  fi
+  
+  exit 0
+}
+
+while getopts ":h" option; do
+   case $option in
+      h) # display Help
+         help
+         exit 0;;
+      \?) # invalid option
+         echo "Error: Invalid option"
+         exit 1;;
+   esac
+done
+
+validate_args "$@"
+compare "$1" "$2"
+
+
+```