diff --git a/chapter_5/exercise_5_10/expr.c b/chapter_5/exercise_5_10/expr.c
index a8de7e1..3e1eed5 100644
--- a/chapter_5/exercise_5_10/expr.c
+++ b/chapter_5/exercise_5_10/expr.c
@@ -13,6 +13,7 @@ void push(float element);
 
 int main(int argc, char *argv[]) {
     char Error = 0;
+    int op_count = 0;
 
     // Using int instead of size_t for loop variable to match signed argc
     // This prevents sign comparison warnings
@@ -29,19 +30,23 @@ int main(int argc, char *argv[]) {
                 char op = *argv[i];
                 switch (op) {
                 case '+':
+                    op_count++; 
                     push(number1 + number2);
                     break;
 
                 case '-':
+                    op_count++;
                     push(number1 - number2);
                     break;
 
                 case '*': // This char might require to be escaped when passed
                           // as an argument.
+                    op_count++;
                     push(number1 * number2);
                     break;
 
                 case '/':
+                    op_count++;
                     if (number2 == 0) {
                         Error = 4;
                     } else {
@@ -87,15 +92,20 @@ int main(int argc, char *argv[]) {
 
         return EXIT_FAILURE;
     }
-
-    printf("result: %.3f", pop());
+    
+    if (op_count == 0 && argc > 2) {
+        printf("Error: Operator is missing!\n");
+        return EXIT_FAILURE;
+    } else {
+        printf("result: %.3f", pop());
+    }
 
     return EXIT_SUCCESS;
 }
 
 float pop(void) {
     if (stack_pointer > 0) {
-        return stack[stack_pointer--];
+        return stack[--stack_pointer];
     }
 
     printf("Error: the stack is empty.\n");
@@ -104,7 +114,7 @@ float pop(void) {
 
 void push(float element) {
     if (stack_pointer < STACK_SIZE) {
-        stack[++stack_pointer] = element;
+        stack[stack_pointer++] = element;
     } else {
         printf("Error: the stack is full.\n");
     }
diff --git a/chapter_5/exercise_5_10/expr_test.sh b/chapter_5/exercise_5_10/expr_test.sh
new file mode 100755
index 0000000..dc15e28
--- /dev/null
+++ b/chapter_5/exercise_5_10/expr_test.sh
@@ -0,0 +1,87 @@
+#!/bin/bash
+
+# check if file is present and executable
+if [ ! -f expr ]; then
+    echo "File 'expr' found!"
+    exit 1
+fi
+
+if [ ! -x expr ]; then
+    echo "File 'expr' not executable!"
+    exit 1
+fi
+
+echo "=== Testing expr calculator ==="
+
+# Test 1: Simple addition
+T=1
+result=$(./expr 2 3 +)
+expected="result: 5.000"
+if [[ "$result" == "$expected" ]]; then
+    echo "Test $T PASS: 2 + 3 = 5"
+else
+    echo "Test $T FAIL: Expected '$expected', got '$result'"
+fi
+
+# Test 2: Commutativity of addition
+T=2
+result=$(./expr 3 2 +)
+expected="result: 5.000"
+if [[ "$result" == "$expected" ]]; then
+    echo "Test $T PASS: 3 + 2 = 5"
+else
+    echo "Test $T FAIL: Expected '$expected', got '$result'"
+fi
+
+# Test 3: Subtraction
+T=3
+result=$(./expr 10 3 -)
+expected="result: 7.000"
+if [[ "$result" == "$expected" ]]; then
+    echo "Test $T PASS: 10 - 3 = 7"
+else
+    echo "Test $T FAIL: Expected '$expected', got '$result'"
+fi
+
+# Test 4: Complex expression
+T=4
+# ------------------------------------
+# Expected:
+# 5 --> [5]
+# 1 --> [5, 1]
+# 2 --> [5, 1, 2]
+# + --> (1 + 2 = 3) --> [5, 3]
+# 4 --> [5, 3, 4]
+# * --> (3 * 4 = 12) --> [5, 12]
+# + --> (5 + 12 = 17) --> [17]
+# 3 --> [17, 3]
+# - --> (17 - 3 = 14) --> [14]
+# ------------------------------------
+result=$(./expr 5 1 2 + 4 \* + 3 -)
+expected="result: 14.000"
+if [[ "$result" == "$expected" ]]; then
+    echo "Test $T PASS: Complex RPN expression"
+else
+    echo "Test $T FAIL: Expected '$expected', got '$result'"
+fi
+
+# Test 5: Division by zero
+T=5
+result=$(./expr 5 0 / 2>&1)
+if [[ "$result" == *"division by zero"* ]]; then
+    echo "Test $T PASS: Division by zero detected"
+else
+    echo "Test $T FAIL: Should detect division by zero"
+fi
+
+# Test 6: Missing operator
+T=6
+result=$(./expr 5 1)
+expected="Error: Operator is missing!"
+if [[ "$result" == "$expected" ]]; then
+    echo "Test $T PASS: Missing operator"
+else
+    echo "Test $T FAIL: Expected '$expected', got '$result'"
+fi
+
+echo "=== Tests Complete ==="
\ No newline at end of file