diff --git a/shlab/tsh.c b/shlab/tsh.c
index ee14e94..0a6bf8d 100644
--- a/shlab/tsh.c
+++ b/shlab/tsh.c
@@ -169,6 +169,8 @@ void eval(char *cmdline)
     char buf[MAXLINE]; // command will be parsed and modified?
     int bg;            // whether it runs in background
     pid_t pid;
+    sigset_t mask;
+    sigemptyset(&mask);
     // preprocess cmd line
     strcpy(buf, cmdline);
     bg = parseline(buf, argv); // convert the command into argv
@@ -179,26 +181,29 @@ void eval(char *cmdline)
     // run external command
     if (!builtin_cmd(argv)) // built-in command is done in `builtin_cmd`
     {
-        if ((pid = fork()) == 0) // this is child
+        sigaddset(&mask, SIGCHLD);
+        sigprocmask(SIG_BLOCK, &mask, NULL); // 5. block SIGCHLD
+        if ((pid = fork()) == 0)             // this is child
         {
+            sigprocmask(SIG_UNBLOCK, &mask, NULL);  // 5. unblock SIGCHLD
+            setpgid(0, 0);                          // put the child process (0=current) into a new process group (0=current)
             if (execve(argv[0], argv, environ) < 0) // execute command failed
             {
                 printf("%s: Command not found\n", argv[0]);
                 exit(0); // here only child exited
             }
         }
-        if (!bg) // run the process in foreground:
+        addjob(jobs, pid, bg ? BG : FG, cmdline); // add the job to job list.
+        // When bg=1, state=2; bg=0, state=1. this way it's just elegant
+        sigprocmask(SIG_UNBLOCK, &mask, NULL); // 5. unblock SIGCHLD
+        if (!bg)                               // run the process in foreground:
         // wait for foreground job to terminate
         {
-            int status;
-            if (waitpid(pid, &status, 0) < 0) // if return -1, then waiting failed
-            {
-                unix_error("waitfg: waitpid error");
-            }
+            waitfg(pid); // the waiting stuff should be done in `waitfg`
         }
         else
         {
-            printf("%d %s", pid, cmdline);
+            printf("[%d] (%d) %s", pid2jid(pid), pid, cmdline); // background job information
         }
     }
     return;
@@ -271,6 +276,11 @@ int builtin_cmd(char **argv)
     {
         exit(0);
     }
+    if (strcmp(argv[0], "jobs") == 0) // t5: process jobs command
+    {
+        listjobs(jobs);
+        return 1; // this IS a builtin command, return 1 to notify
+    }
     return 0;     /* not a builtin command */
 }
 
@@ -287,6 +297,10 @@ void do_bgfg(char **argv)
  */
 void waitfg(pid_t pid)
 {
+    while (pid == fgpid(jobs))
+    {
+        sleep(0);
+    }
     return;
 }
 
@@ -303,6 +317,15 @@ void waitfg(pid_t pid)
  */
 void sigchld_handler(int sig) 
 {
+    pid_t pid;
+    int status;
+    while((pid=waitpid(-1,&status,WNOHANG|WUNTRACED))>0) // check if a child has become zombie, without wait
+    {
+        if(WIFEXITED(status))
+        {
+            deletejob(jobs,pid); // remove pid from job list
+        }
+    }
     return;
 }