使用C ++在Linux中从环境变量中查找文件的路径名

Question

So i am trying to create a Command Line Interpretor in Ubuntu using C/C++ It is part of our lab task for college The basic feature of the CLI is to take input from the user , parse it to find the command and its arguments , then look up the pathname for the command from the Environment Variable
I have been able to parse the String , get the command and its arguments , I have also been able to read to the Path Environment Variables to get the directories of all the paths. Now i must look through these directories to find where the file (command) lies , and then return the complete path so it can be sent to execve for execution in the child process i have to create a lookup function which takes the arguments array (the 0th index position contains the command name ) and the array of directories Here is an outline of the function that has been provided to us

// Search the directories identified by the dir argument to see
// if the argv[0] (the filename) appears there. Allocate a new
// string, place the full path name in it, then return the string.
//
char* lookupPath(char **argv, char **dir){
char* result;
char pName[MAX_PATH_LEN];
// check if is already an absolute path name
if( *argv[0] == '/' ) .....
// look in PATH directories, use access() to see if the
// file is in the dir
for( i = 0 ; i < MAX_PATHS ; i++ ) .....
// File name not found in any path variable
fprintf(stderr, "%s: command not found\n", argv[0]);
return NULL;
}

here argv is the array of arguments (0th index contains the command and the following index contains arguments) and dir[] contains an array of all the directories

Now how em i suppose to traverse trough the directories to find the full path for the give command ?

Answer 1

This more than I probably should have provided but there are enough things left you need to figure out that it isn't a slam dunk.

#include <unistd.h>

typedef std::vector<std::string> pathArray;

std::string lookupPath(const std::string pgm, pathArray &paths)
{
    int ret;

    for (int i = 0; i < paths.size(); ++i)
    {
       std::string temp = paths[i];

       temp = temp + "/" + pgm;

       // let execv determine if it is executable
       // or you can do that here if required
       if ((ret = access(temp.c_str(), F_OK)) == 0) 
           return temp;
    }

    return("");
}

int main(int argc, char *argv[])
{
    pathArray pa;

    pa.push_back("/bin");
    pa.push_back("/usr/bin"); //and the rest of PATH contents

    std::string pgm;
    std::string fullpath;

    pgm = "ls";

    if ((fullpath = lookupPath(pgm, pa)) != "")
    {
        //execv(fullpath.c_str(), ....);
        //etc...
    }
    else
    {  //could not find pgm
    }

    return(0);
}