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:
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 to learn more about your
shell, by reading the man pages or other online materials.
Your basic shell, called
smash (short for Super Madison Shell, naturally), is
basically an interactive loop: it repeatedly prints a prompt
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
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> ./smash smash>
At this point,
smash 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
prompt> ./smash batch.txt
One difference between batch and interactive modes: in interactive mode, a
prompt is printed (
smash> ). 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
/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
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
See the man pages for these functions, and also read the relevant book
chapter 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
library function call to run a command. Remember that if
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.
Clarification: Generally argv for programs is the program name instead of path
In our example above, the user typed
ls but the shell knew to execute the
/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,
access() system call. For example, when the user types
and path is set to include both
/bin(assuming empty path list at first,
/bin is added, then
/usr/bin is added), try
X_OK). If that fails, try
/bin/ls. If that fails too, it is an error.
Your initial shell path should contain one directory:
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
exit(0); in your smash source code, which then will exit the shell.
In this project, you should implement
path as built-in
exit: When the user types
exit, your shell should simply call the
system call with 0 as a parameter. It is an error to pass any arguments to
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 command takes 1 or more arguments, with each argument
separated by whitespace from the others. Three options are supported:
clear. Clarification: Invalid arguments should be an error.
addaccepts 1 path. Your shell should append it to the beginning of the path list. For example,
path add /usr/binresults in the path list containing
/bin(notice the order here). Your shell should not report an error if an invalid path is added. It should kindly accept it.
removeaccepts 1 path. It searches through the current path list and removes the corresponding one. If the path cannot be found, this is an error.
cleartakes no additional argument. It simply removes everything from the path list. If the user sets path to be empty, then the shell should not be able to run any programs (except built-in commands).
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
> 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). However, if the program
cannot be found (i.e., mistyped
pdd), an error should be reported,
but not to be redirected to
output file exists before you run your program, you should simply
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. Redirection without a command is also not allowed - an error should be printed out, instead of being redirected.
path /bin > file).
stderr. In other words, if a process writes to both, the output could be jumbled up. (This is okay!)
Your shell will also allow the user to launch parallel commands. This is accomplished with the ampersand operator as follows:
smash> cmd1 & cmd2 args1 args2 & cmd3 args1
In this case, instead of running
cmd1 and then waiting for it to finish,
your shell should run
cmd3 (each with whatever arguments
the user has passed to it) in parallel, before waiting for any of them to
Then, after starting all such processes, you must make sure to use
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
cd foo & ls -al).
cmd1 > output & cmd 2).
What if a shell user would like to type in multiple commands in a single line? Sometimes, they might turn in a long list of commands, prepare some popcorns and, wait until all of them to finish. This is supported by semicolons:
smash> cmd1 & cmd2 args1 args2 ; cmd3 args1
Here, your shell runs
cmd 1 and
cmd 2 in parallel, like specified above,
waits until they complete, and executes
cmd 3 afterwards.
ls ; cd foo ; ls
cmd1 > output ; cmd 2).
; cmd &).
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 = "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 file that doesn’t exist, it
should exit by calling
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).
For parallel and multiple commands, syntax errors (e.g.,
ls; > output)
or invalid programs names (e.g., a mistyped
lss) should prevent
the entire line from executing.
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
Next, add built-in commands. Then, try working on redirection. Finally, think about parallel and multiple commands. Each of these requires a little more effort on parsing, but each should not be too hard to implement. It is recommended that you separate the process of parsing and execution - parse first, look for syntax errors (if any), and then finally execute the commands.
At some point, you should make sure your code is robust to white space of
various kinds, including spaces (
\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, parallel commands, and multiple commands) do not require whitespace.
For example, your shell should accept commands like:
smash> ls& pwd>output ;cd /usr
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.
To test your Unix Shell:
It is possible to implement the shell in a single
.c file, so your handin
~cs537-1/handin/LOGIN/p2a should only contain these two files:
smash.cthat contains your source code
Use the README file for any explanation you feel would be helpful to someone trying to understand the structure of your code.
We will compile your code via the
gcc compiler with flags
gcc -Wall -Werror smash.c -o smash
And finally remember to do the quiz on Canvas as well!