diff --git a/src/expansion/expansion.c b/src/expansion/expansion.c
index 81867a1..dca13ec 100644
--- a/src/expansion/expansion.c
+++ b/src/expansion/expansion.c
@@ -6,6 +6,9 @@
 #include <string.h>
 
 #include "../utils/ast/ast.h"
+#include "../utils/hash_map/hash_map.h"
+#include "../utils/string_utils/string_utils.h"
+#include "../utils/vars/vars.h"
 
 static bool is_var_start_char(char c)
 {
@@ -77,14 +80,39 @@ size_t parse_var_name(char *str, char **res)
     return i;
 }
 
-struct ast_command *expand(struct ast_command *command)
+static bool expand_var(char **str, size_t pos, const struct hash_map *vars)
+{
+    char *var_name = NULL;
+    size_t r = parse_var_name(*str + pos, &var_name);
+    if (r > 0 && var_name != NULL)
+    {
+        char *value = get_var_or_env(vars, var_name);
+        if (value == NULL)
+            // Undefined variable: expand to empty string
+            value = "";
+
+        char *p = insert_into(*str, value, pos, r);
+        free(var_name);
+        if (p == NULL)
+        {
+            // error: insertion failed
+            return false;
+        }
+        *str = p;
+        return true;
+    }
+    return false;
+}
+
+struct ast_command *expand(struct ast_command *command,
+                           const struct hash_map *vars)
 {
     if (command == NULL)
         return NULL;
 
-    bool in_quotes = false;
     char *str;
     size_t len;
+    bool in_quotes;
     struct list *l = command->command;
 
     while (l != NULL)
@@ -93,28 +121,35 @@ struct ast_command *expand(struct ast_command *command)
         str = (char *)l->data;
         len = strlen(str);
 
-        for (size_t i = 0; str[i] != '\0'; i++)
+        for (size_t i = 0; str[i] != 0; i++)
         {
-            if (in_quotes)
-            {
-                continue; // do nothing
-            }
-            else if (str[i] == '\'')
+            if (str[i] == '\'')
             {
                 // remove quote
                 in_quotes = !in_quotes;
-                memmove(&str[i], &str[i + 1], strlen(&str[i + 1]) + 1);
+                memmove(str + i, str + i + 1, strlen(str + i + 1) + 1);
                 i--;
             }
+            else if (in_quotes)
+            {
+                continue; // do nothing
+            }
             else if (str[i] == '$' && str[i + 1] != 0 && !isspace(str[i + 1]))
             {
                 // variable expansion
+                bool r = expand_var(&str, i, vars);
+                if (r == false || str == NULL)
+                    return NULL;
+
+                i--; // -1 because loop will increment i
             }
         }
 
         if (in_quotes)
         {
             // error: quote not closed
+            fprintf(stderr, "Error: quote not closed in string: %s\n", str);
+            return NULL;
         }
 
         if (len != strlen(str))
@@ -123,6 +158,7 @@ struct ast_command *expand(struct ast_command *command)
             if (new_str == NULL)
             {
                 // error: realloc fail
+                return NULL;
             }
             l->data = new_str;
         }
diff --git a/src/expansion/expansion.h b/src/expansion/expansion.h
index 756e75e..7211ae3 100644
--- a/src/expansion/expansion.h
+++ b/src/expansion/expansion.h
@@ -3,6 +3,8 @@
 
 #include <stddef.h>
 
+#include "../utils/hash_map/hash_map.h"
+
 /**
  * Parse a variable from a string starting with '$'.
  * @param str The input string starting with '$'. It must start with '$'.
@@ -12,4 +14,13 @@
  */
 size_t parse_var_name(char *str, char **res);
 
+/**
+ * Expand variables in an AST command using the provided variable map.
+ * @param command The AST command to expand.
+ * @param vars The hash map containing variables.
+ * @return A new AST command with variables expanded, or NULL on error.
+ */
+struct ast_command *expand(struct ast_command *command,
+                           const struct hash_map *vars);
+
 #endif /* ! EXPANSION_H */
diff --git a/tests/unit/expansion/expand.c b/tests/unit/expansion/expand.c
new file mode 100644
index 0000000..1e25d7c
--- /dev/null
+++ b/tests/unit/expansion/expand.c
@@ -0,0 +1,220 @@
+#define _POSIX_C_SOURCE 200809L
+#include <criterion/criterion.h>
+#include <criterion/new/assert.h>
+#include <criterion/redirect.h>
+#include <stdlib.h>
+
+#include "../../../src/expansion/expansion.h"
+#include "../../../src/utils/ast/ast.h"
+#include "../../../src/utils/hash_map/hash_map.h"
+#include "../../../src/utils/vars/vars.h"
+
+TestSuite(expand);
+
+Test(expand, no_expansion)
+{
+    char str[] = "echo something";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct ast_command *command2 = expand(ast_command, NULL);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data, "echo something",
+                     "String without variables should remain unchanged");
+    ast_free(&ast);
+}
+
+Test(expand, single_quotes_no_expansion)
+{
+    char str[] = "echo '$VAR'";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+    set_var_copy(vars, "VAR", "expanded");
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data, "echo $VAR",
+                     "Variable should not expand inside single quotes");
+    ast_free(&ast);
+    hash_map_free(vars);
+}
+
+Test(expand, single_dollar)
+{
+    char str[] = "echo $ sign";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+    set_var_copy(vars, "VAR", "expanded");
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data, "echo $ sign",
+                     "Variable should not expand inside single quotes");
+    ast_free(&ast);
+    hash_map_free(vars);
+}
+
+Test(expand, empty_braces_no_expansion)
+{
+    char str[] = "echo ${}";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+    set_var_copy(vars, "VAR", "expanded");
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_null(command2, "Expansion should fail on empty braces");
+    ast_free(&ast);
+    hash_map_free(vars);
+}
+
+Test(expand, basic_expansion)
+{
+    char str[] = "echo $VAR";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+    set_var_copy(vars, "VAR", "expanded");
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data, "echo expanded",
+                     "Variable should expand correctly");
+    ast_free(&ast);
+    hash_map_free(vars);
+}
+
+Test(expand, multiple_expansion)
+{
+    char str[] = "echo $VAR1 $VAR2 ${VAR3}";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+    set_var_copy(vars, "VAR1", "expanded");
+    set_var_copy(vars, "VAR2", "values");
+    set_var_copy(vars, "VAR3", "here");
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data,
+                     "echo expanded values here",
+                     "Multiple variables should expand correctly");
+    ast_free(&ast);
+    hash_map_free(vars);
+}
+
+Test(expand, env_variable)
+{
+    char str[] = "echo $MY_ENV_VAR";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    setenv("MY_ENV_VAR", "environment", 0);
+
+    struct ast_command *command2 = expand(ast_command, NULL);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data, "echo environment",
+                     "Environment variable should expand correctly");
+    ast_free(&ast);
+}
+
+Test(expand, undefined_variable)
+{
+    char str[] = "echo $UNDEFINED";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data, "echo ",
+                     "Undefined variable should expand to empty string");
+    ast_free(&ast);
+    hash_map_free(vars);
+}
+
+Test(expand, nested_expansion)
+{
+    char str[] = "echo $B";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+    set_var_copy(vars, "A", "expanded");
+    set_var_copy(vars, "B", "$A");
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data, "echo expanded",
+                     "Nested variable should expand correctly");
+    ast_free(&ast);
+    hash_map_free(vars);
+}
+
+Test(expand, mixed_quotes_expansion)
+{
+    char str[] = "echo \"$VAR1 and '$VAR2'\"";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+    set_var_copy(vars, "VAR1", "expanded");
+    set_var_copy(vars, "VAR2", "not_expanded");
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data,
+                     "echo \"expanded and $VAR2\"",
+                     "Variable in double quotes should expand, while variable "
+                     "in single quotes should not");
+    ast_free(&ast);
+    hash_map_free(vars);
+}
+
+Test(expand, adjacent_variables)
+{
+    char str[] = "echo $VAR1$VAR2";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+    set_var_copy(vars, "VAR1", "hello");
+    set_var_copy(vars, "VAR2", "world");
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data, "echo helloworld",
+                     "Adjacent variables should expand correctly");
+    ast_free(&ast);
+    hash_map_free(vars);
+}
diff --git a/tests/unit/expansion/parse_var.c b/tests/unit/expansion/parse_var.c
index ce4c11a..27a4b94 100644
--- a/tests/unit/expansion/parse_var.c
+++ b/tests/unit/expansion/parse_var.c
@@ -6,11 +6,6 @@
 
 TestSuite(parse_var_name);
 
-// char *input = "$MY$VAR";
-// char *input = "$MY$VAR$";
-// char *input = "$MY$VAR${}";
-// char *input = "$MY$VAR${1}";
-
 Test(parse_var_name, basic_variable)
 {
     char *input = "$MY_VAR";