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.
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.
Your basic shell, called TEDShell, is
basically an interactive loop: it repeatedly prints a prompt TEDShell> (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 TEDShell.
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> ./TEDShell
TEDShell>_
At this point, TEDShell 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> ./TEDShell batch.txt
One difference between batch and interactive modes: in interactive mode, a
prompt is printed (TEDShell> ). 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).
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.
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.
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.
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 TEDShell 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 typesexit, your shell should simply call theexitsystem call with 0 as a parameter. It is an error to pass any arguments toexit. -
cd:cdalways take one argument (0 or >1 args should be signaled as an error). To change directories, use thechdir()system call with the argument supplied by the user; ifchdirfails, that is also an error. -
path: Thepathcommand takes 0 or more arguments, with each argument separated by whitespace from the others. A typical usage would be like this:TEDShell> path /bin /usr/bin, which would add/binand/usr/binto 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). Thepathcommand always overwrites the old path with the newly specified path.
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).
Your shell will also allow the user to launch parallel commands. This is accomplished with the ampersand operator as follows:
TEDShell> 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).
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).
Once you are done with writing the shell, you can check its functionality by
performaing various tests. A script TEDShell-tester is provided, all you need to do is to
run it by providing the command ./TEDShell-tester, which in turn apply various tests available
in tests folder. You may look into the folder various tests inputs and expected outputs. You can also run
a specific test by executing ./TEDShell-tester -t n, where n is a specific number of test.
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. Vscode has builtin graphical interface to manipulate git repo. 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.
This homework is adapted from OSTEP: Operating Systems Three Easy Parts by Remzi et al. at University of Wisconsin-Madison.