Making a shell in c++, trying to create shell variables - c++

Question

So I have a relatively simple shell, it handles pipes, chdir, redirects and running programs. But I need a way of implementing shell variables like you would have in a normal shell(E.g HELLO=world).
int main()
{
while(true)
{
string result;
char * left[128];
char * right[128];
cout << "$$ ";
char command[128];
cin.getline(command,128);
if(strlen(command) != 0)
{
vector<char*>args;
char* prog = strtok(command, " ");
char* tmp = prog;
while ( tmp != NULL )
{
args.push_back( tmp );
tmp = strtok( NULL, " " );
}
char** argv = new char*[args.size()+1];
for ( int k = 0; k < args.size(); k++ )
{
argv[k] = args[k];
}
argv[args.size()] = NULL;
if ( strcmp( command, "exit" ) == 0 )
{
return 0;
}
if(!strcmp(prog,"cd"))
{
chdir(argv);
}
if(prog[0] == '.')
{
std::system(args[0]);
}
else
{
pid_t kidpid = fork();
if(kidpid < 0)
{
perror("Could not fork");
return -1;
}
else if (kidpid == 0)
{
execvp(prog,argv);
}
else
{
if(waitpid(kidpid,0,0) <0 )
{
return -1;
}
}
}
}
}
return 0;
}
Here's the shell from the simplest form, the function calls do pretty much what they say.

Answer 1

You need 3 things:
Parse FOO=foo variable assignments in the input lines
Parse $FOO variable references in the input lines, replace with value
Storage of the variable names and values
There are endless possibilities how to do the latter.
Single dynamic char array, all variables stored in a single string, with a magic character of your choice separating the entries: FOO=foo#BAR=baz#SPAM=eggs. Scales O(n) with the number of entries.
Dynamic array containing pairs of char pointers for variable names and values. Scales O(n).
Linked list, where you insert above pairs in a sorted way. Scales O(log n) if you do insertions and lookups with a binary search.
Binary tree for above pairs, if unbalanced, scales between O(log n) and O(n).
Hash table. Scales O(1).
Etc, etc, etc.
With a dynamic array, I mean that you always realloc the whole thing upon insertions.

Answer 2

If you want to store the variables in your shell, you should look into getenv() and setenv() found in stdlib.
http://pubs.opengroup.org/onlinepubs/009695399/functions/setenv.html
http://pubs.opengroup.org/onlinepubs/009695399/functions/getenv.html
This avoids having to use storage in your C/C++ program with the STL. For example, you can set variables by
setenv("variablename", "value", 1);
Where the 1 turns on overwrite for the current variable if it exists. So in your example, we would use
setenv("HELLO", "world", 1);
You can also retrieve the value of the variable by using
char *value = getenv("variablename");
You do not need to dynamically allocate value.
Note: These values persist for the life of the program that simulates the shell, after which they no longer exist.