Re: Simple Shell in C++

Thu, 14 Apr 2011 20:23:00 -0700

Posted a 'reply on the LINUX programming forum.

I did but if you go to ucr, I'm not going to give you code. :)

Re: implementing software raid 5 in c

Mon, 25 Oct 2010 14:51:16 -0700

Posted a 'reply on the LINUX programming forum.

http://en.wikipedia.org/wiki/Mdadm
/thread

Re: help choosing a language

Mon, 25 Oct 2010 14:47:00 -0700

Posted a 'reply on the LINUX programming forum.

I would recommend c++, it will still teach you the c side of things, and still let you do system programing in linux, but you will also have the object oriented features to use, which I find to be helpful and easier to use for general purpose programming.

Re: Simple Shell in C++

Mon, 25 Oct 2010 14:44:12 -0700

Posted a 'reply on the LINUX programming forum.

I found that my problem was that I was secretly seg faulting in the child, which was causing the child to silently fail. The code works if you substitute the strcpy() with "arglist[x] = strdup(tok[x].c_str());" which actually allocates memory. The pipes are only needed for commands like "ps aux | grep bragr" in which the output of one child needs to be piped to the input of another.

Simple Shell in C++

Tue, 19 Oct 2010 14:02:53 -0700

Posted a 'new message on the LINUX programming forum.

I am new to linux programming, and to get my bearings, I have been writing some small programs. I have already written a ls clone that will do ls, ls -l, and ls -lR. My current project is to write a simple shell. My understanding on how to do this is to loop the following: Get some input from the user and parse it into an array of cstrings, fork the program, have the child exec the user's command. I tried this, but I found that I lost i/o with the child after the exec, which make sense considering what it does. I did some reading online, I found that I need to use pipes to connect the child stdout and stdin to the parents before the exec. I did a few experiments with the pipes, but they all either crashed or didn't work, so I'm clearly missing something.

Is my understanding of what I need to do correct? Am I missing some small, but important detail? Is it even possible to connect out pipe to the parent's stdout so the child's output is automatically output to the console, or do I need to manually read the pipe and output its contents. Can you give me some examples of using pipes in this context, preferably in C++? Thanks in advance.

My code so far:

#include <iostream>
#include <vector>
#include <string>
#include <sstream>
#include <cstdlib>
#include <cstring>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <errno.h>

const unsigned int MAX_ARGS = 128;

using namespace std;

// Execute a parsed command line returning the command's exit code
int doit(const vector<string>& tok)
{
    if (!tok.size() || tok[0] == "") return 0;
    // If user entered cd, change to specified dir. If no dir specified, change to the users home dir
    else if (tok[0] == "cd")
    {
        if (tok.size() > 1) chdir(tok[1].c_str());
        else chdir(getenv("HOME"));
        return 0;
    }

    //Else execute the command specified by the user
    if (pid_t kidpid = fork())
    {
        //Parent
        int status = 0;
        if (tok.back().at(tok.back().size() - 1) != '&')
        {
            waitpid(kidpid, &status, 0);

            #ifdef _INSPECT_EXIT_STATUS
                cout << "exit status=" << status << endl;
            #endif //_INSPECT_EXIT_STATUS
        }
        return status;
    }
    //Child - execute the command
    char* arglist[MAX_ARGS];
    for (unsigned int x = 0; x < tok.size() && x < MAX_ARGS - 1; x++)
        strcpy(arglist[x], tok[x].c_str());
    arglist[tok.size()] = NULL;

    execvp(tok[0].c_str(), arglist);

    //Program will never reach here unless execvp failed
    cerr << "execpv failed: " << strerror(errno) << endl;
    exit(errno);
}

int main(int argc, char* argv[], char* envp[])
{
    while (!cin.eof())
    {
        cout << "? ";
        string temp;
        getline(cin, temp);
        if (temp == "exit") break;

        vector<string> v;
        //Break string into separate strings on whitespace
        {
            stringstream foo(temp);
            string s;
            while (foo >> s)
            {
                if (s[0]=='~') s = getenv("HOME") + s.substr(1);
                v.push_back(s);
            }
        }
        doit(v);
    }
    cout << "exit" << endl;
    return 0;
}

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Re: Simple Shell in C++

Re: implementing software raid 5 in c

Re: help choosing a language

Re: Simple Shell in C++

Simple Shell in C++