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+---
+date: 2023-11-28
+description: Walkthrough of Shell Lab for CSCI 2400 Computer Systems
+tags: C, CSCI2400, UNIX
+draft: true 
+---
+
+# Shell Lab
+
+## Introduction
+
+
+...
+
+
+We will be solving this lab trace by trace.
+
+## trace01
+
+We don't have to do anything here. The pre-existing code skeleton already passes this.
+
+## trace 02
+
+```
+#
+# trace02.txt - Process builtin quit command.
+#
+quit
+WAIT
+```
+
+From the lab handout, we know that we need to implement this in the `builtin_cmd` 
+
+```
+// builtin_cmd: Recognizes and interprets the built-in commands: quit, fg, bg, and jobs. [25 lines]
+
+/////////////////////////////////////////////////////////////////////////////
+//
+// builtin_cmd - If the user has typed a built-in command then execute
+// it immediately. The command name would be in argv[0] and
+// is a C string. We've cast this to a C++ string type to simplify
+// string comparisons; however, the do_bgfg routine will need 
+// to use the argv array as well to look for a job number.
+//
+int builtin_cmd(char **argv) 
+{
+  string cmd(argv[0]);
+  return 0;     /* not a builtin command */
+}
+```
+
+We can simply add an if statement and check if the passes string is equal to `quit`
+
+```
+int builtin_cmd(char **argv) 
+{
+  string cmd(argv[0]);
+  if (cmd == "quit") {
+    exit(0);
+  }
+  return 0;     /* not a builtin command */
+}
+```
+
+Now, we also need to make sure we are checking if the passes command is a builtin command. So, looking at `eval`
+
+```
+/////////////////////////////////////////////////////////////////////////////
+//
+// eval - Evaluate the command line that the user has just typed in
+// 
+// If the user has requested a built-in command (quit, jobs, bg or fg)
+// then execute it immediately. Otherwise, fork a child process and
+// run the job in the context of the child. If the job is running in
+// the foreground, wait for it to terminate and then return.  Note:
+// each child process must have a unique process group ID so that our
+// background children don't receive SIGINT (SIGTSTP) from the kernel
+// when we type ctrl-c (ctrl-z) at the keyboard.
+//
+void eval(char *cmdline) 
+{
+  /* Parse command line */
+  //
+  // The 'argv' vector is filled in by the parseline
+  // routine below. It provides the arguments needed
+  // for the execve() routine, which you'll need to
+  // use below to launch a process.
+  //
+  char *argv[MAXARGS];
+
+  //
+  // The 'bg' variable is TRUE if the job should run
+  // in background mode or FALSE if it should run in FG
+  //
+  int bg = parseline(cmdline, argv); 
+  if (argv[0] == NULL)  
+    return;   /* ignore empty lines */
+
+  return;
+}
+```
+
+We simply add a check for `builtin_cmd`
+
+```
+void eval(char *cmdline) 
+{
+  /* Parse command line */
+  //
+  // The 'argv' vector is filled in by the parseline
+  // routine below. It provides the arguments needed
+  // for the execve() routine, which you'll need to
+  // use below to launch a process.
+  //
+  char *argv[MAXARGS];
+
+  //
+  // The 'bg' variable is TRUE if the job should run
+  // in background mode or FALSE if it should run in FG
+  //
+  int bg = parseline(cmdline, argv); 
+
+  if (argv[0] == NULL)  
+    return;   /* ignore empty lines */
+
+  // First check if it is a builtin command
+  if (!builtin_cmd(argv)) {
+    return;
+  }
+
+  return;
+}
+```
+
+## Trace 03
+
+```
+#
+# trace03.txt - Run a foreground job.
+#
+/bin/echo tsh> quit
+quit
+```
+
+