Simple Unix Shell

In this project, you'll build a simple Unix shell. The shell is the heart of
the command-line interface, and thus is central to the Unix/C programming
environment. Mastering use of the shell is necessary to become proficient in
this world; knowing how the shell itself is built is the focus of this
project.

There are three specific objectives to this assignment:

* To further familiarize yourself with the Linux programming environment.
* To learn how processes are created, destroyed, and managed.
* To gain exposure to the necessary functionality in shells.

   Overview

In this assignment, you will implement a *command line interpreter (CLI)* or,
as it is more commonly known, a *shell*. The shell should operate in this
basic way: when you type in a command (in response to its prompt), the shell
creates a child process that executes the command you entered and then prompts
for more user input when it has finished.

The shells you implement will be similar to, but simpler than, the one you run
every day in Unix. If you don't know what shell you are running, it's probably
`bash`. One thing you should do on your own time is learn more about your
shell, by reading the man pages or other online materials.

   Program Specifications

    Basic Shell: `wish`

Your basic shell, called `wish` (short for Wisconsin Shell, naturally), is
basically an interactive loop: it repeatedly prints a prompt `wish> ` (note
the space after the greater-than sign), parses the input, executes the command
specified on that line of input, and waits for the command to finish. This is
repeated until the user types `exit`.  The name of your final executable
should be `wish`.

The shell can be invoked with either no arguments or a single argument;
anything else is an error. Here is the no-argument way:

```
prompt> ./wish
wish> 
```

At this point, `wish` is running, and ready to accept commands. Type away!

The mode above is called *interactive* mode, and allows the user to type
commands directly. The shell also supports a *batch mode*, which instead reads
input from a batch file and executes commands from therein. Here is how you
run the shell with a batch file named `batch.txt`:

```
prompt> ./wish batch.txt
```

One difference between batch and interactive modes: in interactive mode, a
prompt is printed (`wish> `). In batch mode, no prompt should be printed.

You should structure your shell such that it creates a process for each new
command (the exception are *built-in commands*, discussed below).  Your basic
shell should be able to parse a command and run the program corresponding to
the command.  For example, if the user types `ls -la /tmp`, your shell should
run the program `/bin/ls` with the given arguments `-la` and `/tmp` (how does
the shell know to run `/bin/ls`? It's something called the shell **path**;
more on this below).

   Structure

    Basic Shell

The shell is very simple (conceptually): it runs in a while loop, repeatedly
asking for input to tell it what command to execute. It then executes that
command. The loop continues indefinitely, until the user types the built-in
command `exit`, at which point it exits. That's it!

For reading lines of input, you should use `getline()`. This allows you to
obtain arbitrarily long input lines with ease. Generally, the shell will be
run in *interactive mode*, where the user types a command (one at a time) and
the shell acts on it. However, your shell will also support *batch mode*, in
which the shell is given an input file of commands; in this case, the shell
should not read user input (from `stdin`) but rather from this file to get the
commands to execute.

In either mode, if you hit the end-of-file marker (EOF), you should call
`exit(0)` and exit gracefully. 

To parse the input line into constituent pieces, you might want to use
`strsep()`. Read the man page (carefully) for more details.

To execute commands, look into `fork()`, `exec()`, and `wait()/waitpid()`.
See the man pages for these functions, and also read the relevant [book
chapter](http://www.ostep.org/cpu-api.pdf) for a brief overview.

You will note that there are a variety of commands in the `exec` family; for
this project, you must use `execv`. You should **not** use the `system()`
library function call to run a command.  Remember that if `execv()` is
successful, it will not return; if it does return, there was an error (e.g.,
the command does not exist). The most challenging part is getting the
arguments correctly specified. 

    Paths

In our example above, the user typed `ls` but the shell knew to execute the
program `/bin/ls`. How does your shell know this?

It turns out that the user must specify a **path** variable to describe the
set of directories to search for executables; the set of directories that
comprise the path are sometimes called the *search path* of the shell. The
path variable contains the list of all directories to search, in order, when
the user types a command. 

**Important:** Note that the shell itself does not *implement* `ls` or other
commands (except built-ins). All it does is find those executables in one of
the directories specified by `path` and create a new process to run them.

To check if a particular file exists in a directory and is executable,
consider the `access()` system call. For example, when the user types `ls`,
and path is set to include both `/bin` and `/usr/bin`, try `access("/bin/ls",
X_OK)`. If that fails, try "/usr/bin/ls". If that fails too, it is an error.

Your initial shell path should contain one directory: `/bin`

Note: Most shells allow you to specify a binary specifically without using a
search path, using either **absolute paths** or **relative paths**. For
example, a user could type the **absolute path** `/bin/ls` and execute the
`ls` binary without a search path being needed. A user could also specify a
**relative path** which starts with the current working directory and
specifies the executable directly, e.g., `./main`. In this project, you **do
not** have to worry about these features.

    Built-in Commands

Whenever your shell accepts a command, it should check whether the command is
a **built-in command** or not. If it is, it should not be executed like other
programs. Instead, your shell will invoke your implementation of the built-in
command. For example, to implement the `exit` built-in command, you simply
call `exit(0);` in your wish source code, which then will exit the shell.

In this project, you should implement `exit`, `cd`, and `path` as built-in
commands.

* `exit`: When the user types `exit`, your shell should simply call the `exit`
  system call with 0 as a parameter. It is an error to pass any arguments to
  `exit`. 

* `cd`: `cd` always take one argument (0 or >1 args should be signaled as an
error). To change directories, use the `chdir()` system call with the argument
supplied by the user; if `chdir` fails, that is also an error.

* `path`: The `path` command takes 0 or more arguments, with each argument
  separated by whitespace from the others. A typical usage would be like this:
  `wish> path /bin /usr/bin`, which would add `/bin` and `/usr/bin` to the
  search path of the shell. If the user sets path to be empty, then the shell
  should not be able to run any programs (except built-in commands). The
  `path` command always overwrites the old path with the newly specified
  path. 

    Redirection

Many times, a shell user prefers to send the output of a program to a file
rather than to the screen. Usually, a shell provides this nice feature with
the `>` character. Formally this is named as redirection of standard
output. To make your shell users happy, your shell should also include this
feature, but with a slight twist (explained below).

For example, if a user types `ls -la /tmp > output`, nothing should be printed
on the screen. Instead, the standard output of the `ls` program should be
rerouted to the file `output`. In addition, the standard error output of
the program should be rerouted to the file `output` (the twist is that this
is a little different than standard redirection).

If the `output` file exists before you run your program, you should simple
overwrite it (after truncating it).  

The exact format of redirection is a command (and possibly some arguments)
followed by the redirection symbol followed by a filename. Multiple
redirection operators or multiple files to the right of the redirection sign
are errors.

Note: don't worry about redirection for built-in commands (e.g., we will
not test what happens when you type `path /bin > file`).

    Parallel Commands

Your shell will also allow the user to launch parallel commands. This is
accomplished with the ampersand operator as follows:

```
wish> cmd1 & cmd2 args1 args2 & cmd3 args1
```

In this case, instead of running `cmd1` and then waiting for it to finish,
your shell should run `cmd1`, `cmd2`, and `cmd3` (each with whatever arguments
the user has passed to it) in parallel, *before* waiting for any of them to
complete. 

Then, after starting all such processes, you must make sure to use `wait()`
(or `waitpid`) to wait for them to complete. After all processes are done,
return control to the user as usual (or, if in batch mode, move on to the next
line).


    Program Errors

**The one and only error message.** You should print this one and only error
message whenever you encounter an error of any type:

```
    char error_message[30] = "An error has occurred\n";
    write(STDERR_FILENO, error_message, strlen(error_message)); 
```

The error message should be printed to stderr (standard error), as shown
above. 

After most errors, your shell simply *continue processing* after
printing the one and only error message. However, if the shell is
invoked with more than one file, or if the shell is passed a bad batch
file, it should exit by calling `exit(1)`.

There is a difference between errors that your shell catches and those that
the program catches. Your shell should catch all the syntax errors specified
in this project page. If the syntax of the command looks perfect, you simply
run the specified program. If there are any program-related errors (e.g.,
invalid arguments to `ls` when you run it, for example), the shell does not
have to worry about that (rather, the program will print its own error
messages and exit).


    Miscellaneous Hints

Remember to get the **basic functionality** of your shell working before
worrying about all of the error conditions and end cases. For example, first
get a single command running (probably first a command with no arguments, such
as `ls`). 

Next, add built-in commands. Then, try working on redirection. Finally, think
about parallel commands. Each of these requires a little more effort on
parsing, but each should not be too hard to implement.

At some point, you should make sure your code is robust to white space of
various kinds, including spaces (` `) and tabs (`\t`). In general, the user
should be able to put variable amounts of white space before and after
commands, arguments, and various operators; however, the operators
(redirection and parallel commands) do not require whitespace.

Check the return codes of all system calls from the very beginning of your
work. This will often catch errors in how you are invoking these new system
calls. It's also just good programming sense.

Beat up your own code! You are the best (and in this case, the only) tester of
this code. Throw lots of different inputs at it and make sure the shell
behaves well. Good code comes through testing; you must run many different
tests to make sure things work as desired. Don't be gentle -- other users
certainly won't be. 

Finally, keep versions of your code. More advanced programmers will use a
source control system such as git. Minimally, when you get a piece of
functionality working, make a copy of your .c file (perhaps a subdirectory
with a version number, such as v1, v2, etc.). By keeping older, working
versions around, you can comfortably work on adding new functionality, safe in
the knowledge you can always go back to an older, working version if need be.