How to include variables in system() calls

LazerPhreak · 03-12-2010, 11:07 PM

Hello all!

I need to include variables in a system() call.
This is that I have:

[code]#include <iostream>
#include <stdio.h>
#include <stdlib.h>

using namespace std;

short sect1, sect2, sect3, sect4;

int main()
{
cout << "Enter IP Address: ";
cin >> sect1;
system("ping -c 1 PUT_SECT1_VARIABLE_HERE");
return 0;
}

How can I do this?

P.S.: I know this is a pointless redundant program, but it's part of a larger project learning process.

Thanks

nadroj · 03-12-2010, 11:28 PM

This appears to be a C++ program. Therefore get rid of the includes for "stdio.h" and "stdlib.h". In C++, when including "C headers", we usually prepend it with "c" and remove the ".h". So, "stdlib.h" would become "cstdlib", so include that instead.

Code:

system("ping -c 1 PUT_SECT1_VARIABLE_HERE");

System just expects a "char*". Create a C++ string object, dynamically, and pass its "c_str" to "system". For example

Code:

int foo = 5;
string bar = "text and " + foo;
system(bar.c_str());

LazerPhreak · 03-12-2010, 11:39 PM

OK so here I am:

Code:

#include <iostream>
#include <cstdio>
#include <cstdlib>

using namespace std;

short sect1, sect2, sect3, sect4;

int main()
{

	cout << "Enter part 1: ";
	cin >> sect1;
	string ip_addr = "ping -c 1 " + sect1;
	system(ip_addr.c_str());
return 0;
}

I'm getting an error though:

Code:

sh: Syntax error: Unterminated quoted string

I cant seem to find any missing quotes though...

LazerPhreak · 03-13-2010, 01:21 AM

So I dont get it....when i enter 123 for sect1 i get the error message I previously said, but when I enter abc I get an empty line as output. Any ideas?

konsolebox · 03-13-2010, 01:37 AM

How 'bout trying to print the string first:

Code:

puts(ip_addr.c_str());
putchar('\n');

or

Code:

cout << ip_addr.c_str() << '\n';

This is so that you can know the command you're trying to execute.

LazerPhreak · 03-13-2010, 11:20 AM

Code:

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <string.h>

using namespace std;

int sect1;

int main()
{

	cout << "Enter part 1: ";
	cin >> sect1;
	string ipaddr = "echo " + sect1;
	cout << ipaddr.c_str() << '\n';
	//system(ipaddr.c_str());
return 0;
}

Code:

$ ./a.out
Enter part 1: 123
U	�	�"
$ ./a.out
Enter part 1: abc
echo 
$

That explains the problem, but How the hell did that get created as my output of:

Code:

string ipaddr = "echo " + sect1;
cout << ipaddr.c_str() << '\n';

LazerPhreak · 03-13-2010, 03:19 PM

So I've been looking around, but I can't find any solution. Any help?

tuxdev · 03-13-2010, 03:49 PM

User input in a system() is a Very Bad Idea. Use fork+exec so that odd input won't do very bad things, for example, "127.0.0.1 & rm -rf /" will cause the program to attempt to delete all files on the system.

Sergei Steshenko · 03-13-2010, 04:02 PM

Quote:

Originally Posted by LazerPhreak

So I've been looking around, but I can't find any solution. Any help?

Consider using the old fashioned "C" way with 'sprintf':

man 3 sprintf
.

I.e. first construct you command line using 'sprintf' and then use it.

ntubski · 03-13-2010, 06:01 PM

Use

Code:

string ipaddr = string("echo ") + sect1;

The problem is that "echo " evaluates to a char* so when you add the integer to it you get a pointer to some random place in memory. Just one of those annoying C++ quirks.

LazerPhreak · 03-13-2010, 06:43 PM

So I got it working, thanks to a combination of your support and google. This is what I have:

Code:

#include <iostream>
#include <cstdio>
#include <cstdlib>
using namespace std;

int main()
{
    cout << endl << "Ping : ";
    string sect1;
    cin >> sect1;
    string ipaddr = string("ping ").append(ping);
    system(ipaddr.c_str());
}