diff --git a/src/expansion/expansion.c b/src/expansion/expansion.c
index dca13ec..407e2f7 100644
--- a/src/expansion/expansion.c
+++ b/src/expansion/expansion.c
@@ -86,10 +86,21 @@ static bool expand_var(char **str, size_t pos, const struct hash_map *vars)
     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 *value;
+        char rnd_str[10]; // max 5 digits + null terminator
+        if (strcmp(var_name, "RANDOM") == 0)
+        {
+            short rnd = short_random();
+            snprintf(rnd_str, 10, "%d", rnd);
+            value = rnd_str;
+        }
+        else
+        {
+            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);
diff --git a/src/utils/hash_map/hash_map.c b/src/utils/hash_map/hash_map.c
index 46ac9d1..3d77734 100644
--- a/src/utils/hash_map/hash_map.c
+++ b/src/utils/hash_map/hash_map.c
@@ -55,7 +55,7 @@ struct hash_map *hash_map_init(size_t size)
 bool hash_map_insert(struct hash_map *hash_map, const char *key, void *value,
                      bool *updated)
 {
-    if (hash_map == NULL || hash_map->size == 0 || key == NULL)
+    if (hash_map == NULL || hash_map->size == 0 || key == NULL || value == NULL)
         return false;
 
     size_t h = hash(key);
@@ -71,6 +71,7 @@ bool hash_map_insert(struct hash_map *hash_map, const char *key, void *value,
             if (strcmp(iter->key, key) == 0)
             {
                 // update
+                free(iter->value);
                 iter->value = value;
                 if (updated)
                     *updated = true;
@@ -96,16 +97,16 @@ bool hash_map_insert(struct hash_map *hash_map, const char *key, void *value,
     return true;
 }
 
-void hash_map_free(struct hash_map *hash_map)
+void hash_map_free(struct hash_map **hash_map)
 {
     struct pair_list *l;
     struct pair_list *prev;
 
-    if (hash_map)
+    if (hash_map != NULL && *hash_map != NULL)
     {
-        for (size_t i = 0; i < hash_map->size; i++)
+        for (size_t i = 0; i < (*hash_map)->size; i++)
         {
-            l = hash_map->data[i];
+            l = (*hash_map)->data[i];
             while (l != NULL)
             {
                 prev = l;
@@ -113,8 +114,8 @@ void hash_map_free(struct hash_map *hash_map)
                 destroy_pair_list(&prev);
             }
         }
-        free(hash_map->data);
-        free(hash_map);
+        free((*hash_map)->data);
+        free(*hash_map);
     }
 }
 
diff --git a/src/utils/hash_map/hash_map.h b/src/utils/hash_map/hash_map.h
index efdabc2..048a882 100644
--- a/src/utils/hash_map/hash_map.h
+++ b/src/utils/hash_map/hash_map.h
@@ -19,10 +19,23 @@ struct hash_map
 
 struct hash_map *hash_map_init(size_t size);
 
+/**
+ * @brief Inserts a key-value pair into the hash map. Key and value are expected
+ * to be on the heap and will be managed by the hash map. It means they are
+ * consumed by this function. If the key already exists, its value is updated
+ * and the key is not consumed so it must be freed by the caller.
+ *
+ * @param hash_map The hash map.
+ * @param key The key to insert.
+ * @param value The value to insert.
+ * @param updated If not NULL, set to true if the key was already present and
+ * updated, false if the key was newly inserted.
+ * @return true on success, false on failure.
+ */
 bool hash_map_insert(struct hash_map *hash_map, const char *key, void *value,
                      bool *updated);
 
-void hash_map_free(struct hash_map *hash_map);
+void hash_map_free(struct hash_map **hash_map);
 
 void hash_map_foreach(struct hash_map *hash_map,
                       void (*fn)(const char *, const void *));
diff --git a/src/utils/vars/vars.c b/src/utils/vars/vars.c
index 985e085..8d762da 100644
--- a/src/utils/vars/vars.c
+++ b/src/utils/vars/vars.c
@@ -2,8 +2,11 @@
 #include "vars.h"
 
 #include <stdbool.h>
+#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <time.h>
+#include <unistd.h>
 
 #include "../hash_map/hash_map.h"
 
@@ -14,6 +17,26 @@ struct hash_map *vars_init(void)
     return hash_map_init(VARS_INITIAL_SIZE);
 }
 
+void vars_default(struct hash_map *vars)
+{
+    set_var_copy(vars, "?", "0");
+    pid_t pid = getpid();
+    char pid_str[20];
+    snprintf(pid_str, sizeof(pid_str), "%d", pid);
+    set_var_copy(vars, "$", pid_str);
+}
+
+short short_random(void)
+{
+    static bool seeded = false;
+    if (!seeded)
+    {
+        srand((unsigned)time(NULL));
+        seeded = true;
+    }
+    return (short)(rand() & 0x7FFF); // force 16 bits positive
+}
+
 char *get_var(const struct hash_map *vars, const char *key)
 {
     return (char *)hash_map_get(vars, key);
@@ -27,14 +50,21 @@ char *get_var_or_env(const struct hash_map *vars, const char *key)
     return value;
 }
 
-bool set_var(struct hash_map *vars, const char *key, const char *value)
+bool set_var(struct hash_map *vars, const char *key, const char *value,
+             bool *updated)
 {
-    if (key == NULL || value == NULL)
-        return false;
-    return hash_map_insert(vars, key, (void *)value, NULL);
+    return hash_map_insert(vars, key, (void *)value, updated);
 }
 
 bool set_var_copy(struct hash_map *vars, const char *key, const char *value)
 {
-    return set_var(vars, strdup(key), strdup(value));
+    char *key_copy = strdup(key);
+    char *value_copy = strdup(value);
+    bool updated;
+    bool res = set_var(vars, key_copy, value_copy, &updated);
+    if (updated || !res)
+        free(key_copy);
+    if (!res)
+        free(value_copy);
+    return res;
 }
diff --git a/src/utils/vars/vars.h b/src/utils/vars/vars.h
index de1c4c6..85fb5ee 100644
--- a/src/utils/vars/vars.h
+++ b/src/utils/vars/vars.h
@@ -10,6 +10,16 @@
  */
 struct hash_map *vars_init(void);
 
+/**
+ * Set default variables.
+ */
+void vars_default(struct hash_map *vars);
+
+/**
+ * Generate a random short integer (16 bits positive [0-32767]).
+ */
+short short_random(void);
+
 /**
  * Get the value of a variable, NULL if not found.
  */
@@ -26,7 +36,8 @@ char *get_var_or_env(const struct hash_map *vars, const char *key);
  * the hash_map and need to be on the heap. Returns true on success, false on
  * failure.
  */
-bool set_var(struct hash_map *vars, const char *key, const char *value);
+bool set_var(struct hash_map *vars, const char *key, const char *value,
+             bool *updated);
 
 /**
  * Same as set_var, but makes copies of key and value so you don't have to worry
diff --git a/tests/unit/expansion/expand.c b/tests/unit/expansion/expand.c
index 1e25d7c..fd4295d 100644
--- a/tests/unit/expansion/expand.c
+++ b/tests/unit/expansion/expand.c
@@ -3,6 +3,7 @@
 #include <criterion/new/assert.h>
 #include <criterion/redirect.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 #include "../../../src/expansion/expansion.h"
 #include "../../../src/utils/ast/ast.h"
@@ -42,7 +43,7 @@ Test(expand, single_quotes_no_expansion)
     cr_assert_str_eq((char *)command2->command->data, "echo $VAR",
                      "Variable should not expand inside single quotes");
     ast_free(&ast);
-    hash_map_free(vars);
+    hash_map_free(&vars);
 }
 
 Test(expand, single_dollar)
@@ -61,7 +62,7 @@ Test(expand, single_dollar)
     cr_assert_str_eq((char *)command2->command->data, "echo $ sign",
                      "Variable should not expand inside single quotes");
     ast_free(&ast);
-    hash_map_free(vars);
+    hash_map_free(&vars);
 }
 
 Test(expand, empty_braces_no_expansion)
@@ -78,7 +79,7 @@ Test(expand, empty_braces_no_expansion)
     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);
+    hash_map_free(&vars);
 }
 
 Test(expand, basic_expansion)
@@ -97,7 +98,7 @@ Test(expand, basic_expansion)
     cr_assert_str_eq((char *)command2->command->data, "echo expanded",
                      "Variable should expand correctly");
     ast_free(&ast);
-    hash_map_free(vars);
+    hash_map_free(&vars);
 }
 
 Test(expand, multiple_expansion)
@@ -119,7 +120,7 @@ Test(expand, multiple_expansion)
                      "echo expanded values here",
                      "Multiple variables should expand correctly");
     ast_free(&ast);
-    hash_map_free(vars);
+    hash_map_free(&vars);
 }
 
 Test(expand, env_variable)
@@ -154,7 +155,7 @@ Test(expand, undefined_variable)
     cr_assert_str_eq((char *)command2->command->data, "echo ",
                      "Undefined variable should expand to empty string");
     ast_free(&ast);
-    hash_map_free(vars);
+    hash_map_free(&vars);
 }
 
 Test(expand, nested_expansion)
@@ -174,7 +175,7 @@ Test(expand, nested_expansion)
     cr_assert_str_eq((char *)command2->command->data, "echo expanded",
                      "Nested variable should expand correctly");
     ast_free(&ast);
-    hash_map_free(vars);
+    hash_map_free(&vars);
 }
 
 Test(expand, mixed_quotes_expansion)
@@ -196,7 +197,7 @@ Test(expand, mixed_quotes_expansion)
                      "Variable in double quotes should expand, while variable "
                      "in single quotes should not");
     ast_free(&ast);
-    hash_map_free(vars);
+    hash_map_free(&vars);
 }
 
 Test(expand, adjacent_variables)
@@ -216,5 +217,61 @@ Test(expand, adjacent_variables)
     cr_assert_str_eq((char *)command2->command->data, "echo helloworld",
                      "Adjacent variables should expand correctly");
     ast_free(&ast);
-    hash_map_free(vars);
+    hash_map_free(&vars);
+}
+
+Test(expand, random)
+{
+    char str[] = "$RANDOM";
+    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");
+    int rnd = atoi((char *)command2->command->data);
+    cr_assert(rnd >= 0 && rnd <= 32767,
+              "RANDOM variable should expand to a value between 0 and 32767");
+    ast_free(&ast);
+}
+
+Test(expand, pid)
+{
+    char str[] = "$$";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+    vars_default(vars);
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    int pid = atoi((char *)command2->command->data);
+    cr_assert(pid == getpid(),
+              "$$ variable should expand to the pid of the process");
+    ast_free(&ast);
+    hash_map_free(&vars);
+}
+
+Test(expand, default_last_exit_code)
+{
+    char str[] = "$?";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+    vars_default(vars);
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    int code = atoi((char *)command2->command->data);
+    cr_assert(code == 0,
+              "$? variable should expand to the last exit code (default 0)");
+    ast_free(&ast);
+    hash_map_free(&vars);
 }
diff --git a/tests/unit/utils/hash_map.c b/tests/unit/utils/hash_map.c
new file mode 100644
index 0000000..3c3bd04
--- /dev/null
+++ b/tests/unit/utils/hash_map.c
@@ -0,0 +1,217 @@
+#define _POSIX_C_SOURCE 200809L
+#include "../../../src/utils/hash_map/hash_map.h"
+
+#include <criterion/criterion.h>
+#include <criterion/new/assert.h>
+#include <criterion/redirect.h>
+#include <stdio.h>
+
+TestSuite(utils_hash_map);
+
+Test(utils_hash_map, init_free)
+{
+    struct hash_map *map = hash_map_init(10);
+    cr_expect_not_null(map);
+    cr_expect_eq(map->size, 10);
+    hash_map_free(&map);
+}
+
+Test(utils_hash_map, insert_basic)
+{
+    struct hash_map *map = hash_map_init(10);
+    cr_expect_not_null(map);
+    cr_expect_eq(map->size, 10);
+
+    bool updated = false;
+    bool res = hash_map_insert(map, strdup("key1"), strdup("value1"), &updated);
+    cr_expect_eq(res, true);
+    cr_expect_eq(updated, false);
+
+    hash_map_free(&map);
+}
+
+Test(utils_hash_map, insert_multiple)
+{
+    struct hash_map *map = hash_map_init(10);
+    cr_expect_not_null(map);
+    cr_expect_eq(map->size, 10);
+
+    bool updated = false;
+    bool res = hash_map_insert(map, strdup("key1"), strdup("value1"), &updated);
+    cr_expect_eq(res, true);
+    cr_expect_eq(updated, false);
+    res = hash_map_insert(map, strdup("key2"), strdup("value2"), &updated);
+    cr_expect_eq(res, true);
+    cr_expect_eq(updated, false);
+
+    hash_map_free(&map);
+}
+
+Test(utils_hash_map, insert_update)
+{
+    struct hash_map *map = hash_map_init(10);
+    cr_expect_not_null(map);
+    cr_expect_eq(map->size, 10);
+
+    bool updated = false;
+    bool res = hash_map_insert(map, strdup("key1"), strdup("value1"), &updated);
+    cr_expect_eq(res, true);
+    cr_expect_eq(updated, false);
+    res = hash_map_insert(map, "key1", strdup("value2"), &updated);
+    cr_expect_eq(res, true);
+    cr_expect_eq(updated, true);
+
+    hash_map_free(&map);
+}
+
+Test(utils_hash_map, insert_update_multiple)
+{
+    struct hash_map *map = hash_map_init(10);
+    cr_expect_not_null(map);
+    cr_expect_eq(map->size, 10);
+
+    bool updated = false;
+    bool res = hash_map_insert(map, strdup("key1"), strdup("value1"), &updated);
+    cr_expect_eq(res, true);
+    cr_expect_eq(updated, false);
+    res = hash_map_insert(map, strdup("key2"), strdup("value2"), &updated);
+    cr_expect_eq(res, true);
+    cr_expect_eq(updated, false);
+    res = hash_map_insert(map, "key1", strdup("value2"), &updated);
+    cr_expect_eq(res, true);
+    cr_expect_eq(updated, true);
+    res = hash_map_insert(map, "key1", strdup("value3"), &updated);
+    cr_expect_eq(res, true);
+    cr_expect_eq(updated, true);
+
+    hash_map_free(&map);
+}
+
+Test(utils_hash_map, get_basic)
+{
+    struct hash_map *map = hash_map_init(10);
+    cr_expect_not_null(map);
+    cr_expect_eq(map->size, 10);
+
+    bool updated = false;
+    bool res = hash_map_insert(map, strdup("key1"), strdup("value1"), &updated);
+    cr_expect_eq(res, true);
+    cr_expect_eq(updated, false);
+
+    char *value = (char *)hash_map_get(map, "key1");
+    cr_expect_str_eq(value, "value1");
+
+    hash_map_free(&map);
+}
+
+Test(utils_hash_map, get_after_update)
+{
+    struct hash_map *map = hash_map_init(10);
+    cr_expect_not_null(map);
+    cr_expect_eq(map->size, 10);
+
+    bool updated = false;
+    bool res = hash_map_insert(map, strdup("key1"), strdup("value1"), &updated);
+    cr_expect_eq(res, true);
+    cr_expect_eq(updated, false);
+
+    res = hash_map_insert(map, "key1", strdup("value2"), &updated);
+    cr_expect_eq(res, true);
+    cr_expect_eq(updated, true);
+
+    char *value = (char *)hash_map_get(map, "key1");
+    cr_expect_str_eq(value, "value2");
+
+    hash_map_free(&map);
+}
+
+Test(utils_hash_map, get_unknown_key)
+{
+    struct hash_map *map = hash_map_init(10);
+    cr_expect_not_null(map);
+    cr_expect_eq(map->size, 10);
+
+    char *value = (char *)hash_map_get(map, "unknown_key");
+    cr_expect_null(value);
+
+    hash_map_free(&map);
+}
+
+Test(utils_hash_map, delete_key)
+{
+    struct hash_map *map = hash_map_init(10);
+    cr_expect_not_null(map);
+    cr_expect_eq(map->size, 10);
+
+    bool updated = false;
+    bool res = hash_map_insert(map, strdup("key1"), strdup("value1"), &updated);
+    cr_expect_eq(res, true);
+    cr_expect_eq(updated, false);
+
+    res = hash_map_remove(map, "key1");
+    cr_expect_eq(res, true);
+
+    char *value = (char *)hash_map_get(map, "key1");
+    cr_expect_null(value);
+
+    hash_map_free(&map);
+}
+
+Test(utils_hash_map, delete_unknown_key)
+{
+    struct hash_map *map = hash_map_init(10);
+    cr_expect_not_null(map);
+    cr_expect_eq(map->size, 10);
+
+    bool res = hash_map_remove(map, "unknown_key");
+    cr_expect_eq(res, false);
+
+    hash_map_free(&map);
+}
+
+Test(utils_hash_map, free_nonnull_map)
+{
+    struct hash_map *map = hash_map_init(10);
+    cr_expect_not_null(map);
+    cr_expect_eq(map->size, 10);
+
+    bool res = hash_map_insert(map, strdup("key1"), strdup("value1"), NULL);
+    cr_expect_eq(res, true);
+    res = hash_map_insert(map, strdup("key2"), strdup("value2"), NULL);
+    cr_expect_eq(res, true);
+
+    hash_map_free(&map);
+}
+
+Test(utils_hash_map, free_null_map)
+{
+    hash_map_free(NULL);
+}
+
+static size_t count = 0;
+void foreach_fn(const char *key, const void *value)
+{
+    printf("Key: %s, Value: %s\n", key, (const char *)value);
+    count++;
+}
+
+Test(utils_hash_map, foreach, .init = cr_redirect_stdout)
+{
+    struct hash_map *map = hash_map_init(10);
+    cr_expect_not_null(map);
+    cr_expect_eq(map->size, 10);
+
+    bool res = hash_map_insert(map, strdup("key1"), strdup("value1"), NULL);
+    cr_expect_eq(res, true);
+    res = hash_map_insert(map, strdup("key2"), strdup("value2"), NULL);
+    cr_expect_eq(res, true);
+
+    count = 0;
+    hash_map_foreach(map, foreach_fn);
+    fflush(stdout);
+    cr_expect_eq(count, 2);
+    cr_expect_stdout_eq_str(
+        "Key: key2, Value: value2\nKey: key1, Value: value1\n");
+
+    hash_map_free(&map);
+}