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@@ -39,130 +39,147 @@ should be `wish`:
```
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
```
You should structure your shell such that it creates a new process for each
new command (note that there are a few exceptions to this, which we discuss
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`.
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).
You might be wondering how the shell knows to run `/bin/ls` (which means the
program binary `ls` is found in the directory `/bin`) when you type `ls`. The
shells knows this thanks to a **path** variable that the user sets. The path
variable contains the list of all directories to search, in order, when the
user types a command. We'll learn more about how to deal with the path below.
## Structure
**Important:** Note that the shell itself does not *implement* `ls` or really
many other commands at all (it does implement a few, called *built-ins*,
described further below). All it does is find those executables in one of the
directories specified by `path` and create a new process to run them. More on
this below.
### 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.
To parse the input line into constituent pieces, you might want to use
`strtok()`. 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.
## Built-in Commands
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.
### 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 C program.
call `exit(0);` in your wish source code, which then will exit the shell.
So far, you have added your own `exit` built-in command. Most Unix shells have
many others such as `cd`, `pwd`, etc. In this project, you should implement
`exit`, `cd`, `pwd`, and `path`.
In this project, you should implement `exit`, `cd`, `pwd`, and `path` as
built-in commands.
The formats for `exit`, `cd`, and `pwd` are:
* `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`.
```
[optional-space]exit[optional-space]
[optional-space]pwd[optional-space]
[optional-space]cd[optional-space]
[optional-space]cd[oneOrMoreSpace]dir[optional-space]
```
* `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.
When you run `cd` (without arguments), your shell should change the working
directory to the path stored in the $HOME environment variable. Use the call
`getenv("HOME")` in your `wish` source code to obtain this value.
* `pwd`: When a user types `pwd`, your shell should call getcwd() and show the
result. It is an error to pass any arguments to `pwd`.
You do not have to support tilde (~). Although in a typical Unix shell you
could go to a user's directory by typing `cd ~username`, in this project you
do not have to deal with tilde. You should treat it like a common character,
i.e., you should just pass the whole word (e.g. "~username") to chdir(), and
chdir will return 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).
Basically, when a user types `pwd`, you simply call getcwd(), and show the
result. When a user changes the current working directory (e.g. \"cd
somepath\"), you simply call chdir(). Hence, if you run your shell, and then
run pwd, it should look like this:
### Redirection
```
% cd
% pwd
/afs/cs.wisc.edu/u/m/j/username
% echo $PWD
/u/m/j/username
% ./wish
wish> pwd
/afs/cs.wisc.edu/u/m/j/username
```
The format of the `path` built-in command is:
```
[optionalSpace]path[oneOrMoreSpace]dir[optionalSpace] (and possibly more directories, space separated)
```
A typical usage would be like this:
```
wish> path /bin /usr/bin
```
By doing this, your shell will know to look in `/bin` and `/usr/bin`
when a user types a command, to see if it can find the proper binary to
execute. If the user sets path to be empty, then the shell should not be able
to run any programs unless XXX (but built-in commands, such as path, should
still work).
## Redirection
Many times, a shell user prefers to send the output of his/her 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
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 `output.out` file. In addition, the standard error output of
the file should be rerouted to the file `output.err` (the twist is that this
rerouted to the file `output`. In addition, the standard error output of
the file should be rerouted to the file `output` (the twist is that this
is a little different than standard redirection).
If the `output.out` or `output.err` files already exists before you run your
program, you should simple overwrite them (after truncating). If the output
file is not specified (e.g., the user types `ls >` without a file), you should
print an error message and not run the program `ls`.
If the `output` file exists before you run your program, you should simple
overwrite them (after truncating it).
Here are some redirections that should **not** work:
```
ls > out1 out2
ls > out1 out2 out3
ls > out1 > out2
```
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
### Parallel Commands
Your shell will also allow the user to launch 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).
## Program Errors
### 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:
@@ -172,8 +189,8 @@ message whenever you encounter an error of any type:
write(STDERR_FILENO, error_message, strlen(error_message));
```
The error message should be printed to stderr (standard error). Also,
do not add whitespaces or tabs or extra error messages.
The error message should be printed to stderr (standard error), as shown
above.
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
@@ -183,178 +200,32 @@ invalid arguments to `ls` when you run it, for example), let the program
prints its specific error messages in any manner it desires (e.g., could be
stdout or stderr).
## White Spaces
The `>` operator will be separated by spaces. Valid input may include the
following:
```
wish> ls
wish> ls > a
wish> ls > a
```
But not this (it is ok if this works, it just doesn't have to):
```
wish> ls>a
```
## Defensive Programming and Error Messages
Defensive programming is good for you, so do it! It is also required. Your
program should check all parameters, error-codes, etc. before it trusts
them. In general, there should be no circumstances in which your C program
will core dump, hang indefinitely, or prematurely terminate. Therefore, your
program must respond to all input in a reasonable manner; by "reasonable",
we mean print the error message (as specified in the next paragraph) and
either continue processing or exit, depending upon the situation.
Since your code will be graded with automated testing, 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));
```
For this project, the error message should be printed to **stderr**. Also, do
not attempt to add whitespaces or tabs or extra error messages.
You should consider the following situations as errors; in each case, your
shell should print the error message to stderr and exit gracefully:
* An incorrect number of command line arguments to your shell program.
For the following situation, you should print the error message to
stderr and continue processing:
* A command does not exist or cannot be executed.
* A very long command line (over 128 bytes).
Your shell should also be able to handle the following scenarios below, which
are *not errors.*
* An empty command line.
* Multiple white spaces on a command line.
## Hints
Writing your shell in a simple manner is a matter of finding the relevant
library routines and calling them properly. To simplify things for you in
this assignment, we will suggest a few library routines you may want to use to
make your coding easier. You are free to use these routines if you want or to
disregard our suggestions. To find information on these library routines, look
at the manual pages.]
### Basic Shell
**Parsing:** For reading lines of input, once again check out `getline()`. To
open a file and get a handle with type `FILE *`, look into `fopen()`. Be sure
to check the return code of these routines for errors! You may find the
`strtok()` routine useful for parsing the command line (i.e., for extracting
the arguments within a command separated by whitespaces).
**Executing Commands:** Look into `fork`, `exec`, and `wait/waitpid`. See the
man pages for these functions, and also read [book chapter](http://www.ostep.org/cpu-api.pdf).
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. The first argument specifies the program that
should be executed, with the full path specified; this is
straight-forward. The second argument, `char *argv[]` matches those
that the program sees in its function prototype:
```c
int main(int argc, char *argv[]);
```
Note that this argument is an array of strings, or an array of
pointers to characters. For example, if you invoke a program with:
```
foo 205 535
```
Assuming that you find `foo` in directory `/bin` (or elsewhere in the defined
path), then argv[0] = "/bin/foo", argv[1] = "205" and argv[2] = "535".
Important: the list of arguments must be terminated with a NULL pointer; in
our example, this means argv[3] = NULL. We strongly recommend that you
carefully check that you are constructing this array correctly!
### Built-in Commands
For the `exit` built-in command, you should simply call `exit()` from within
your source code. The corresponding shell process will exit, and the parent
(i.e. your shell) will be notified.
For managing the current working directory, you should use `getenv(),
`chdir()`, and `getcwd()`. The `getenv()` call is useful when you want to go
to your HOME directory. The `getcwd()` call is useful to know the current
working directory, i.e., if a user types `pwd`, you simply call `getcwd()` and
use those results. Finally, `chdir` is useful for moving to different
directories. For more information on these topics, read the man pages or the
Advanced Unix Programming book (Chapters 4 and 7) or look around online.
### Redirection
Redirection is relatively easy to implement. For example, to redirect standard
output to a file, just use `close()` on stdout, and then `open()` on a
file. More on this below.
With a file descriptor, you can perform read and write to a file. Maybe in
your life so far, you have only used `fopen()`, `fread()`, and `fwrite()` for
reading and writing to a file. Unfortunately, these functions work on `FILE
*`, which is more of a C library support; the file descriptors are hidden.
To work on a file descriptor, you should use `open()`, `read()`, and `write()`
system calls. These functions perform their work by using file descriptors.
To understand more about file I/O and file descriptors you can read the
Advanced Unix Programming book (Chapter 3) (specifically, 3.2 to 3.5, 3.7,
3.8, and 3.12), or just read the man pages. Before reading forward, at this
point, you should become more familiar file descriptors.
The idea of redirection is to make the stdout descriptor point to your output
file descriptor. First of all, let's understand the STDOUT_FILENO file
descriptor. When a command `ls -la /tmp` runs, the `ls` program prints its
output to the screen. But obviously, the ls program does not know what a
screen is. All it knows is that the screen is basically pointed by the
STDOUT_FILENO file descriptor. In other words, you could rewrite
`printf("hi");` in this way: `write(STDOUT_FILENO, "hi", 2);`.
To check if a particular file exists in a directory, use the `stat()` system
call. For example, when the user types `ls`, and path is set to include both
`/bin` and `/usr/bin`, try `stat("/bin/ls")`. If that fails, try
`stat("/usr/bin/ls")`. If that fails too, print the **only error message**.
### 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`). Then try adding more arguments.
as `ls`).
Next, try working on multiple commands. Make sure that you are correctly
handling all of the cases where there is miscellaneous white space around
commands or missing commands. Next, add built-in commands. Finally, add
redirection support.
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.
We strongly recommend that you 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. And, it's just good
programming sense.
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.
Beat up your own code! You are the best (and in this case, the
only) tester of this code. Throw lots of junk at it and make sure the
shell behaves well. Good code comes through testing -- you must run
all sorts of different tests to make sure things work as
desired. Don't be gentle -- other users certainly won't be. Break it
now so we don't have to break it later.
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 junk at it and make sure the shell behaves well. Good
code comes through testing -- you must run all sorts of different tests to
make sure things work as desired. Don't be gentle -- other users certainly
won't be. Break it now so we don't have to break it later.
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