Alternative Parser for `osh`

PA1 comes with ugly broken parser. This is an alternative implementation you can use.

Just mention you've used this library by Yutaka Tsutano instead in your README.txt.

1 Usage

Read main.cpp (usage) and command.hpp (data structure).

1.1 Compiling & Running on CSE

You can run the following four commands to compile and run on cse.unl.edu:

git clone https://github.com/ytsutano/osh-parser.git
cd osh-parser
make
./osh

1.2 Parsing

parse_command_string() returns a vector of parsed commands (defined in command.hpp) from a line of text input from the terminal.

// 1. Get a line from the terminal.
std::string input_line = "echo hello world | sort | uniq > test.txt";

// 2. Parse.
std::vector<shell_command> shell_commands = parse_command_string(input_line);

This is basically all you need to do for parsing. You just need to execute the commands listed in shell_commands vector to implement the assignment.

1.3 Printing the Parsed Commands

It is helpful to print the parsed commands when implementing the execution part of the shell. You can do so with the following code:

std::cout << "-------------------------\n";
for (const auto& cmd : shell_commands) {
    std::cout << cmd;
    std::cout << "-------------------------\n";
}

The result of parsing echo hello world | sort | uniq > test.txt:

-------------------------
cmd: echo
arg: hello
arg: world
cin_file:
cin_mode: term
cout_file:
cout_mode: pipe
next_mode: always
-------------------------
cmd: sort
cin_file:
cin_mode: pipe
cout_file:
cout_mode: pipe
next_mode: always
-------------------------
cmd: uniq
cin_file:
cin_mode: pipe
cout_file: test.txt
cout_mode: file
next_mode: always
-------------------------

2 Hints

2.1 Using `execvp()` with C++ Types

Our shell_command struct is defined as follows:

struct shell_command {
    std::string cmd;
    std::vector<std::string> args;

    // ...
};

At some point, you need to replace the current process with a new process image based on cmd and args by calling execvp(). Because execvp() takes

const char* and
a null-terminated array of C strings

instead of

std::string and
std::vector<std::string>,

we need to do a simple conversion. You can use the following helper function to pass cmd and args directly:

int exec(const std::string& cmd, const std::vector<std::string>& args)
{
    // Make an ugly C-style args array.
    std::vector<char*> c_args = {const_cast<char*>(cmd.c_str())};
    for (const auto& a : args) {
        c_args.push_back(const_cast<char*>(a.c_str()));
    }
    c_args.push_back(nullptr);

    return execvp(cmd.c_str(), c_args.data());
}

ihowell/osh-parser

Alternative Parser for osh