CS31 Lab 8 C Shell program

Makefile: A Makefile simplifies the process of compiling your program. We’ll look at these in more detail later in the course. You’re welcome to look over this one, but you shouldn’t need to edit it for this lab. If you are interested, take a look at Section 10 for more information about make and makefiles. make builds the cs31shell program

README.md: some notes to you

cs31shell.c: the file in which to implement your shell program

parsecmd.h: the header file for the parsecmd library (open this in an editor to view the comments that contain lot of documentation about using this library).

sleeper.c: a program that sleeps for some time, useful for testing your shell’s support for running commands in the background.

Implement and test incrementally (and run valgrind as you go).

Read the parsecmd.h file comments to see how to use the parse_cmd function. Use this function to create the argv array of strings from the command line string. This is the argv value passed to execvp.

The maximum length of a command line is defined in parsecmd.h. You can use the MAXLINE constant in your program.

Use the functions execvp(), waitpid(), and signal() for executing programs, waiting on children, and registering signal handlers. Note that the first argument to waitpid is the process id (PID) of the process you’d like to wait for, but if you pass in an argument of -1, it will wait for ANY reapable child process. This is useful inside your SIGCHLD handler, where you won’t know which child (or children) exited. You can pass it WNOHANG as the third parameter to prevent it from blocking when there are no children to reap.

You can call fflush(stdout); after any calls to printf to ensure the printf output is written immediately to the terminal. Otherwise, the C standard I/O library might buffer it for a short while.

circular queue: I suggest first implementing the Wednesday lab circular queue of ints program, and then once you get that working implement your circular array of strings in your shell program. If you get stuck on the circular queue part, start with a simpler version, where the first history list element is always in bucket 0 and the last in bucket MAXHIST-1, and then shift values over as you add a new command to the last bucket each time. Then, come back to this later and try to turn this into it a circular queue (which, for large MAXHIST values, is much more efficient than having to shift values over to maintain the queue ordering).

built-in commands: your shell should read in and parse built-in commands just like regular commands. I recommend checking for a built-in command after calling parse_cmd, which strips off any leading or trailing white space characters. Remember you can use strcmp to compare two string values.

Remember that you know how to define and use structs in C, and that you know how to use strings in C. See past weekly lab code, your lab solutions, and my C documentation.

strings: remember that you can compare individual char values directly using relational operators (e.g., ==, <), but you need to use the strcmp string library function to compare two string values:

  char str[20];
  strcpy(str, "hello");
  if(str[2] == 'x') {
    printf("x in bucket 2\n");
  }
  if(strcmp(str, "yo ho ho") == 0) {
    printf("str seems like a pirate\n");
  }

cleaning up zombie children. Your correct shell program should reap all exited children and not leave around zombie children after it exits. However, in its development stages, you may find that it creates zombie processes that it never reaps. You should check for, and kill, all zombies after running your shell program.

  ps
  233  pts/7   00:00:01  sleeper <zombie>

  kill -9 233       # kills the process with pid 233
  pkill -9 sleeper  # kills all sleeper processes

When in doubt about what your shell should do, try running the command in the bash shell (a standard system terminal) and see what it does.

Refer to your lecture notes and to the OS Chapter of the textbook. Both have example code that is very related to some of the main things your shell needs to do to run a command in the foreground and background.

String functions:

Process-related functions:

Shell-related functions: