diff --git a/shlab/tsh.c b/shlab/tsh.c
index 8ad94d1..f25f82d 100644
--- a/shlab/tsh.c
+++ b/shlab/tsh.c
@@ -281,7 +281,12 @@ int builtin_cmd(char **argv)
         listjobs(jobs);
         return 1; // this IS a builtin command, return 1 to notify
     }
-    return 0;     /* not a builtin command */
+    if (strcmp(argv[0], "bg") == 0 || strcmp(argv[0], "fg") == 0) // judge bg & fg
+    {
+        do_bgfg(argv);
+        return 1;
+    }
+    return 0; /* not a builtin command */
 }
 
 /* 
@@ -289,6 +294,59 @@ int builtin_cmd(char **argv)
  */
 void do_bgfg(char **argv) 
 {
+    char *id = argv[1], *end; // JID or PID
+    struct job_t *job;
+    int numid;
+    // extract job or process
+    if (id == NULL) // not specified
+    {
+        printf("%s command requires PID or %%jobid argument\n", argv[0]);
+        return;
+    }
+    if (id[0] == '%') // this is a job
+    {
+        id++;                         // point to the number position
+        numid = strtol(id, &end, 10); // convert id char[] to integer
+        if (*end != '\0')             // contains non-digit numbers
+        {
+            printf("%s: argument must be a PID or %%jobid\n", argv[0]);
+            return;
+        }
+        job = getjobjid(jobs, numid); // try to get job
+        if (job == NULL)
+        {
+            printf("%%%d: No such job\n", numid);
+            return;
+        }
+    }
+    else // this is a process
+    {
+        numid = strtol(id, &end, 10);
+        if (*end != '\0')
+        {
+            printf("%s: argument must be a PID or %%jobid\n", argv[0]);
+            return;
+        }
+        job = getjobpid(jobs, numid); // try to get proc
+        if (job == NULL)
+        {
+            printf("(%d): No such process\n", atoi(id));
+            return;
+        }
+    }
+    // send signal
+    kill(-(job->pid), SIGCONT);
+    // process according to bg/fg
+    if (strcmp(argv[0], "fg") == 0) // foreground
+    {
+        job->state = FG;
+        waitfg(job->pid);
+    }
+    else // background
+    {
+        job->state = BG;
+        printf("[%d] (%d) %s", job->jid, job->pid, job->cmdline);
+    }
     return;
 }