Changelog:

Your Task

  1. Download the skeleton code last updated 2019-12-26, which contains:
    • a starter Makefile which has a target to build a binary called msh
    • a source file main.cc which implements a prompt that prints a “Not implemented” error on any command but “exit”.
  2. Modify msh to be a Unix-like shell that supports running commands as described below, that:
    • prompts for commands using > (greater than, followed by space)
    • support running simple commands (e.g. /bin/cat foo.txt bar.txt)
    • support input redirection from a file (e.g. commands like /usr/bin/gcc -E - < somefile.txt)
    • support output redirection to a file (e.g. commands like /usr/bin/gcc -E file.cc > somefile.txt)
    • support pipelines of multiple commands (e.g. commands like /bin/cat foo.txt | /bin/grep bar | /bin/grep baz or /bin/cat foo | /bin/grep baz > ouptut.txt)
    • supports the builtin command exit
    • outputs the exit status of each command as described below
    • prints out error messages to stderr (e.g. via std::cerr) as described below

    The sections below attempt to precisely describe the syntax of commands and the procedure to be followed when running commands.

    We strongly recommend creating partially working versions such as described below. A full solution will be around 200 lines of code.

    You may use additional source files, rename source files, switch from C++ to C, etc. so long as you modify the Makefile appropriately and still produce an executable called msh.

  3. For the checkpoint, you must be able to run simple commands with no arguments (like /bin/something) and wait for them to finish before executing the next command and implement at least three of the following:

    • printing the exit status of each command (like /bin/foo exit status: 4) or printing that a command terminated other than by exiting
    • running commands with one or more arguments (/bin/something argument1 argument2)
    • running commands with an input redirection (/bin/something < input.txt)
    • running commands with an output redirection (/bin/something > output.txt)
    • running a pipeline of two commands (/bin/first-program | /bin/second-program)
    • running a pipeline of three or more commands (/bin/first-program | /bin/second-program | /bin/third-program)

    For the checkpoint, we do not care how your program behaves from commands that you do not implement. For example, if you do not implement pieplines of two or more commands, you may reject all commands with a | token, or interpret the | as a normal argument, or something else.

  4. We intend to test the shell you submit on a Linux enviornment similar to the course VM or to the department machines (e.g. portal.cs.virginia.edu). (If you using the department machines, run module load gcc-7.1.0 before compiling to use a recent version of GCC.) Please make sure your submitted shell will build and work in this environment.

    To aid you in testing your final submission we have supplied a test in shell_test.py which can be run via make test.

  5. Prepare a .tar.gz file like the one built using make archive in the given Makefile. This should have all your source files and a Makefile which can produce an msh binary. It should not contain any object files or a pre-built msh executable.

  6. Submit the .tar.gz file on the submission site for the final submission or for the checkpoint submission

Specification

Shell language

Shell commands are lines which consist of a sequence of whitespace-seperated tokens. Whitespace characters are space (' ' in C or C++), form feed ('\f'), newline ('\n'), carriage return ('\r'), horizontal tab ('\t'), vertical tab ('\v').

Each line has a maximum length of 100 characters. We do not care what your shell does with longer input lines. This limit implies certain limits on the number of arguments, number of commands in a pipeline, etc. that may simplify your implementation.

Each token is either

Each line of inputs is a pipeline, which consists of a series of a commands (possibly just one) seperated by | tokens.

Each well-formed command consists of a series of (in any order):

Any command which has a < or > operator not followed by a word token is malformed. Any command which has no word which is not part of a redirection operation is malformed. Any command which has more than one input redirection operation or more than one output redirection operation is malformed.

As a special case, you may optionally accept and execute no programs for a line containing no tokens, or you may treat it as a malformed command.

Running commands

To run a pipeline, the shell runs each command in the pipeline, then waits for all the commands to terminate. If any command in the pipeline is malformed, you may instead print an error and not execute any command in the pipeline.

To run a command, the shell:

Both redirection operations above must occur after connecting file descriptors for pipelines, so running a command like foo > test.txt | bar should result in foo’s standard output being the file test.txt and bar’s input being a pipe that immediately indicates end-of-file.

You may assume that enough file descriptors are available to run any one pipeline, even if you were to attempt to create all pipes and open all files necessary to run the pipeline all at once. However, you must close file descriptors between commands, so that you do not run out of file descriptors after running many commands that use pipes and/or redirection. You should also not exec programs with extra file descriptors open so that they have the full number of file descriptors available to them.

Any errors that occur executing or preparing to execute a command should result in error messages as described below. If an error occurs while preparing to execute a command, then the executable should not be run. However, it is okay if you fork() and print an exit status in this case; this means that it is permissible to detect errors after fork()ing and exit with an error code.

Outputting exit statues

After running each command in the pipeline and before prompting for the next pipeline, the shell should wait for all commands in the pipeline and output their exit statuses. To output their exit statuses, for each command in the order the commands appeared in the pipeline, the shell should print out to stdout information about how the command terminated on its own line:

Built-in command

Your shell should support the built-in command exit. When this command is run your shell should terminate normally (with exit status 0). You should not run a program called exit, even if one exists.

Handling errors

Your shell should print out all error messages to stderr (file descriptor 2).

You must use the following error messages:

You must also print out an error message (but you may choose what text to output) if:

If a command results in an error and you detect this error after successfully fork()ing, then in addition to printing out the error, you may also print out an exit status message for the program (as described above, most likely indicating exit status 255), even though it was not executed successfully.

If one command in a pipeline results in an error, you must print out at least one error message, but it is okay if other commands in the pipeline are executed even though some error messages are printed out (e.g. it’s okay if something_that_does_not_exist | something_real prints an error about something_that_does_not_exist after starting something_real). It is also acceptable for you to execute none of the commands in the pipeline in this case.

If multiple errors could occur while executing a command, then you may print out messages for any or all of the possible errors. For example, when running the command foo < input.txt > output.txt, if opening both input.txt and output.txt would fail, then you may output an error message about opening input.txt failing, about opening output.txt failing, or both.

Example shell session

  1. When your shell is complete, a typical session might look like:

    > /bin/ls
main.cc  main.o  Makefile  msh  msh.cc  msh.h  shell_test.py  test
/bin/ls exit status: 0
> /bin/cat Makefile | /bin/grep msh:
msh: main.o
/bin/cat exit status: 0
/bin/grep exit status: 0
> /usr/bin/wc < Makefile > wc.out
/usr/bin/wc exit status: 0
> /bin/cat wc.out | /bin/sed -e s/^/wc:/g > wc.filtered.out
/bin/cat exit status: 0
/bin/sed exit status: 0
> /bin/cat wc.filtered.out does-not-exist
wc: 19  47 362
/bin/cat: does-not-exist: No such file or directory
/bin/cat exit status: 1
> exit

    Where text after a > prompt is the input to the shell.

Hints

Testing tool

  1. We have supplied a shell_test.py program, which you can run by running make test. This will often produce a lot of output, so you might try redirecting its output to a file like with make test >test-output.txt.

    We strongly advise against making changes and just looking at how it effects make test’s count of passed tests, except when you are very close to finish. Some tests may be looking for things which you could pass “by accident” (e.g. not producing excess output in some circumstances, which both a very incomplete or buggy but complete implementation mgiht do), so you must look at what tests in particular you are failing.

    Note that we may use different tests when we grade your submission. The supplied tests are intended to help you test your final submission and not to substitute for doing your own testing.

    The supplied Makefile builds with AddressSanitizer to help you detect memory errors, such as out-of-bounds accesses and memory leaks (at the cost of making your shell run a bit slower). You should expect any submission with memory errors not to get full credit (even if you disable AddressSanitizer in the Makefile you submit).

    The testing tool includes some “fork fails” tests which arranges for calls to fork() to return an error by setting a limit on the number of processes that can be created before running your shell. There are two issues you may encounter related to this setting:

    • LeakSanitizer, the memory leak detector that is part of AddressSanitizer, does not work when it is not possible to use fork(). You may see an error message from LeakSanitizer because of this, but it should not prevent the rest of the test from running properly.

    • The Windows Subsystem for Linux (WSL) apparently does not support the limit on the number of processes, so the test will not work properly on it. If you are concerned about this test, you may wish to try your shell on another system (such as via SSH’ing into portal.cs.virginia.edu (password reset; run module load gcc-7.1.0 before compiling to use a recent C compiler) or using a Linux VM) instead.

A possible order of operations

(This roughly matches how I implemented my reference solution.)

  1. Implement and test parsing commands into whitespace-separated tokens. Collect the tokens into an array or vector to make future steps easier.

  2. Implement and test running commands without pipelines or redirection. In this case the list of tokens you have will be exactly the arguments to pass to the command.

  3. Add support for redirection.

  4. Add support for pipelines.

Parsing

  1. In C++, you can read a full line of input with std::getline. In C, you can read a full line of input with fgets.

  2. In C++, one way to divide a line of input into tokens is by using std::istringstream like

    std::istringstream s(the_string);
while (s >> token) {
    processToken(token);
}

  3. In C, one way to divide a line of input into tokens is by using strsep like

    char *p = the_string;
char *token;
while ((token = strsep(&p, " \t\v\f\r\n")) != NULL) {
    ...
}

    Note that strsep changes the string passed to it.

  4. My reference implementation creates a class to represent each command in a pipeline (|-seperated list of things to run), and a class to represent the pipeline as a whole. I first collect each command line into a vector of tokens, then iterate through that vector to create and update command objects.

  5. Our specification does not require redirection operations to appear in any particular place in commands. This means, for example, that

    foo bar < input.txt

    and

    foo < input.txt bar

    and

    < input.txt foo bar

    are all equivalent.

Converting vectors of strings to arrays of char* for execv

  1. You can convert a std::string to nul-terminated const char * (like execv expects, except for const-ness, see next item) using its .c_str() method:

    std::string example = "something"
const char *example_as_char_ptr = example.c_str();

    but be careful:

    • The return value of c_str() is pointer to the string it comes from. It is only valid as long as the string it comes from is still in scope and is not moved. (This means, for example, that calling .c_str() on a string in a vector and then resizing the vector is not safe.)

  2. For no good reason, execv() expects an array of nul-terminated char * instead of an array of const char * for its argv array. You can convert from const char * using char * using a cast such as:

    (char *) some_string.c_str()

    or

    const_cast<char*>(some_string.c_str())

    which is safe as long as you don’t try to modifying the string through the resulting char*.

    (This casting is not needed for the path argument.)

  3. Given a vector<char*> v, you can get a pointer to the underlying array using something like

    v.data()

    or

    &v[0]

    Provided that the last element of the vector is a NULL pointer, this should be suitable to pass to execv.

Running commands

  1. Pseudocode for running commands is as follows:

    for each command in the line {
    pid = fork();
    if (pid == 0) {
        do redirection stuff
        execv ( command, args );
        oops, print out a message about exec failing and exit
    } else {
        store pid somewhere
    }
}
for each command in the line {
    waitpid(stored pid, &status);
    check return code placed in status;
}

  2. To implement redirection, probably the most useful function is dup2, which can replace stdin (file descriptor 0) or stdout (file descriptor 1) with another file you have opened. When redirecting to a file, you will most commonly use open() to open the file, call dup2 to replace stdin or stdout with a copy of the newly opened file descriptor, then close() the original file descriptor. This occurs typically would be done just before the call to execve as in the pseudocode above.

  3. To implement pipelines, the typical technique is to call pipe to obtain a connected pair of file descriptors, use dup2 to assign these to stdout or stdin, and close the original file descriptor just before calling execve.

  4. To convert from a const char** or array of const char*s to the type expected by execv, you can use a cast like (char**) pointer_to_first_element.

Printing Error Messages

  1. In C++, one way to print to stderr is using cerr, which works like cout:

    #include <iostream>
...
std::cerr << "Some message.\n";

  2. In C or C++, one way to print to stderr is using fprintf:

    #include <stdio.h>
...
fprintf(stderr, "Some message.\n");

Close-on-exec

  1. As an alternative to manually close()ing file descriptors before calling exec*() in a child process, you can set them as “close-on-exec” using the fcntl library call as in this example code from the POSIX documentation:

    #include <unistd.h>
#include <fcntl.h>
...
int flags = fcntl(fd, F_GETFD);
if (flags == -1)
    /* Handle error */;
flags |= FD_CLOEXEC;
if (fcntl(fd, F_SETFD, flags) == -1)
    /* Handle error */;"

    Setting this flag won’t prevent you from having to close to the file descriptor in the parent process that does not call an exec function (assuming it is opened before forking).

    You may (or may not) find that this simpler than inserting manually close calls before every exec.

Common problems

My shell hangs

  1. If pipelines hang, then a likely cause is neglecting to close the ends of the pipes the parent process.

    (reads on the pipe will not indicate end-of-file until all of the write ends of the pipe are closed.)

My shell stops working after I run too many commands

  1. A likely cause is running out of file descriptors by failing to close all file descriptors.

Sometimes execv fails with Bad address

  1. execv needs a NULL pointer at the end of the array of arguments. If you did not explicitly supply one, then execv may try to read past the end of your array and read a bogus pointer.

General sources for documentation

  1. All the Unix functions have “manpages” (short for manual pages) retrieved with the man command. For example, to get documentation on the pipe() function, you can run, from within a Linux enviornment, run

    man pipe

    The man command retrieves documentation both for commands and C functions. If both a command and a function exist with the same name, man by default shows information about the command. For example, there is a command called write and a function called write. Running

    man write

    gives only the documentation about command. There are two ways to get the documentation about the function. One is to run

    man -a write

    which shows the documentation for all things named “write” — it will show the documentation for the command and then for the function. Alternately, the documentation retrieved by man is divided into “sections”. You can get a list of all the entries for a word like “write” along with their sections by running

    man -k write

    On my system this shows

    write (1)            - send a message to another user
write (2)            - write to a file descriptor
write (1posix)       - write to another user
write (3posix)       - write on a file

    The text in parenthesis are the section numbers/names. For example, you can access the write entry in section 2 using

    man 2 write

    Generally, Linux divides its documentation into sections as follows:

    *  section 1: commands intended for normal users
*  section 1posix: commands, but shows the POSIX standard's description (things that should be the same on all Unix-like OSs) rather than Linux-specific information about a command
*  section 2: "low-level" functions that usually wrap a specific system call
*  section 3: other functions
*  section 3posix: functions, but shows the POSIX standard's description (things that should be the same on all Unix-like OSs) rather than Linux-specific information about a function
*  section 8: commands intended for system adminstrators