diff --git a/hw4/include/store.h b/hw4/include/store.h
new file mode 100644
index 0000000..275fabd
--- /dev/null
+++ b/hw4/include/store.h
@@ -0,0 +1,16 @@
+typedef struct store_data
+{
+    char *name;
+    char *value;
+    struct store_data *prev;
+    struct store_data *next;
+} store_data;
+
+store_data *head;
+
+void store_init();
+store_data* store_get_data(char* var);
+void store_remove_data(store_data* data);
+void store_add_data(store_data* data);
+void store_fini();
+char* store_strcpy(char *str);
\ No newline at end of file
diff --git a/hw4/src/jobs.c b/hw4/src/jobs.c
index 8be3158..19ec671 100644
--- a/hw4/src/jobs.c
+++ b/hw4/src/jobs.c
@@ -11,6 +11,7 @@
 
 #include "mush.h"
 #include "debug.h"
+#include "store.h"
 
 /*
  * This is the "jobs" module for Mush.
@@ -47,8 +48,8 @@
  * @return 0 if initialization is successful, otherwise -1.
  */
 int jobs_init(void) {
-    // TO BE IMPLEMENTED
-    abort();
+    store_init();
+    return 0;
 }
 
 /**
@@ -62,8 +63,8 @@ int jobs_init(void) {
  * @return 0 if finalization is completely successful, otherwise -1.
  */
 int jobs_fini(void) {
-    // TO BE IMPLEMENTED
-    abort();
+    store_fini();
+    return 0;
 }
 
 /**
diff --git a/hw4/src/main.c b/hw4/src/main.c
index 9178838..ca5b15a 100644
--- a/hw4/src/main.c
+++ b/hw4/src/main.c
@@ -4,6 +4,7 @@
 #include <errno.h>
 
 #include "mush.h"
+#include "store.h"
 
 int main(int argc, char *argv[]) {
     jobs_init();
diff --git a/hw4/src/store.c b/hw4/src/store.c
index db9da8b..aca225e 100644
--- a/hw4/src/store.c
+++ b/hw4/src/store.c
@@ -1,6 +1,7 @@
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
+#include "store.h"
 
 /*
  * This is the "data store" module for Mush.
@@ -29,8 +30,10 @@
  * otherwise NULL.
  */
 char *store_get_string(char *var) {
-    // TO BE IMPLEMENTED
-    abort();
+    store_data *data = store_get_data(var);
+    if (data)
+        return data->value;
+    return NULL;
 }
 
 /**
@@ -47,8 +50,29 @@ char *store_get_string(char *var) {
  * otherwise 0 is returned.
  */
 int store_get_int(char *var, long *valp) {
-    // TO BE IMPLEMENTED
-    abort();
+    store_data *data = store_get_data(var);
+    if (data) {
+        // is NULL
+        char *ptr = data->value;
+        if (!ptr)
+            return -1;
+
+        // find whether negative or not
+        int sign = 1;
+        if (*ptr == '-')
+            sign = -1, ptr ++;
+
+        long ans = 0;
+        while (*ptr)
+        {
+            // cannot be interpreted as an integer
+            if(*ptr > '9' || *ptr < '0')
+                return -1;
+            ans = ans * 10 +(*(ptr++) - '0');
+        }
+        *valp = ans * sign;
+    }
+    return -1;
 }
 
 /**
@@ -67,8 +91,33 @@ int store_get_int(char *var, long *valp) {
  * un-set.
  */
 int store_set_string(char *var, char *val) {
-    // TO BE IMPLEMENTED
-    abort();
+    store_data* data = store_get_data(var);
+    // if want to unset a value that does not exist
+    if (!data && !val)
+        return -1;
+
+    // unset is val is NULL
+    if (!val)
+    {
+        store_remove_data(data);
+        return 0;
+    }
+    
+    // set the data to val if data exists
+    if (data)
+    {
+        char *tmp = data->value;
+        data->value = store_strcpy(val);
+        free(tmp);
+        return 0;
+    }
+    
+    // create a new data with the value and insert it
+    data = malloc(sizeof(store_data));
+    data->name = store_strcpy(var);
+    data->value = store_strcpy(val);
+    store_add_data(data);
+    return 0;
 }
 
 /**
@@ -84,8 +133,53 @@ int store_set_string(char *var, char *val) {
  * @param  val  The value to set.
  */
 int store_set_int(char *var, long val) {
-    // TO BE IMPLEMENTED
-    abort();
+    // if val is 0, set var to 0
+    if (val == 0)
+        return store_set_string(var, "0");
+    
+    char *str, *tmp, *ptr;
+    tmp = malloc(sizeof(char));
+    int is_negative = 0, size = 0;
+
+    // check whether val is negative
+    if (val < 0)
+        is_negative = 1;
+    
+    // get a string with reversed direction
+    while (val)
+    {
+        int remainder = val % 10;
+        if (is_negative)
+            remainder *= -1;
+        tmp[size] = ('0' + remainder);
+        tmp = realloc(tmp, (++size + 1) * sizeof(char));
+        val /= 10;
+    }
+
+    // alloc memory to str
+    if (is_negative) {
+        str = malloc((size+2)*sizeof(char));
+        ptr = str + 1;
+        str[0] = '-';
+    }
+    else {
+        str = malloc((size+1)*sizeof(char));
+        ptr = str;
+    }
+
+    // reverse the string back
+    for (size_t i = 0; i < size; i++)
+        ptr[i] = tmp[size - i - 1];
+    
+    ptr[size] = '\0';
+
+    // put it in data
+    int return_val = store_set_string(var, str);
+
+    free(str);
+    free(tmp);
+    
+    return return_val;
 }
 
 /**
@@ -97,6 +191,75 @@ int store_set_int(char *var, long val) {
  * @param f  The stream to which the store contents are to be printed.
  */
 void store_show(FILE *f) {
-    // TO BE IMPLEMENTED
-    abort();
+    store_data* ptr = head->next;
+
+    fprintf(f, "{");
+    while (ptr != head)
+    {
+        fprintf(f, "%s=%s", ptr->name, ptr->value);
+        ptr = ptr->next;
+        if ((ptr != head))
+            fprintf(f, ", ");
+        
+    }
+    fprintf(f, "}\n");
 }
+
+void store_init() {
+    head = malloc(sizeof(store_data));
+    head->name = NULL;
+    head->value = NULL;
+    head->next = head;
+    head->prev = head;
+}
+
+store_data* store_get_data(char* var) {
+    store_data* ptr = head->next;
+
+    while (ptr != head)
+    {
+        if (strcmp(ptr->name, var) == 0)
+            return ptr;
+
+        ptr = ptr->next;
+    }
+
+    return NULL;
+}
+
+void store_remove_data(store_data* data) {
+    data->prev->next = data->next;
+    data->next->prev = data->prev;
+    if (data->name)
+        free(data->name);
+    if (data->value)
+        free(data->value);
+    free(data);
+}
+
+void store_add_data(store_data* data) {
+    data->next = head->next;
+    data->prev = head;
+    data->next->prev = data;
+    data->prev->next = data;
+}
+
+void store_fini() {
+    while (head->next != head)
+        store_remove_data(head->next);
+
+    free(head);
+    head = NULL;
+}
+
+char* store_strcpy(char *str) {
+    size_t size = 0;
+    char *result = malloc(sizeof(char));
+    while (*str)
+    {
+        result[size] = *(str++);
+        result = realloc(result, (++size + 1) * sizeof(char));
+    }
+    result[size] = '\0';
+    return result;
+}
\ No newline at end of file