Set a variable from system("command") (C++)

Question

Before nothing: I searched similar problems but none of the solutions seemed to work for me. Also please, I'm noob in C so sorry for the stupid mistakes that I can do. Thanks.

I have a little problem with my C++ file. What I want, is set a variable from a system("command"). I don't know if I'm explaining myself well so I put my file as an example. This is my file:

#include <stdlib.h>
#include <iostream>

using namespace std;

int main()
{
    int kernel = system("uname -a");
    //Here It should print the value but instead of that prints a zero :/
    printf("%d \n", kernel);
    return 0;
}

I'm trying to define the variable "kernel" from the output of system("uname -a") command that should be something like:

$ uname -a
Linux 5.0.0-27-generic #28~18.04.1-Ubuntu SMP Thu Aug 22 03:00:32 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

However, when I try to execute the code the output is just:

0

I'm missing something or this cannot be done in C++? Sorry for the noob question.

Best regards.

`system()` returns the exit code, it doesn't return the output. — Vaughn Cato, Oct 06 '19 at 16:22
`system()` returns return status of the command (0 in this case). To capture its output, you need to redirect its output. As much as I hate to say it, `boost::process` is probably your friend in this case. — Super-intelligent Shade, Oct 06 '19 at 16:23
have a look at [`boost::process`](https://www.boost.org/doc/libs/1_71_0/doc/html/process.html) — Darklighter, Oct 06 '19 at 16:23
I see.. So what will be the best alternative? Save the output into a txt and then read the variable from it? Regards and thanks for you quickly response :). EDIT: Just saw the other comments. Going to take a look. Thanks to all. — , Oct 06 '19 at 16:24

273K · Answer 1 · 2019-10-06T16:46:30.750

2

system returns the error code, not a command output. 0 means no error. What you are looking for is popen.

#include <stdio.h>

char uname[1024];

FILE* fp = popen("uname -a", "r");
if (!fp)
    /* Handle error */;

if (fgets(uname, sizeof(uname), fp) != 0)
    std::cout << uname << std::endl;

int status = pclose(fp);
if (status == -1) {
    /* Error reported by pclose() */
}

Similar question demonstrates reading output data without length limit: popen() writes output of command executed to cout

edited Oct 06 '19 at 16:46

answered Oct 06 '19 at 16:31

273K

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This is pretty C for a C++ question and it also only works on Linux. – Darklighter Oct 06 '19 at 16:32
1

@Darklighter It works not only on Linux. Mac OSX, Windows have `popen`/`_popen` too. Where have you seen `uname -a` on Windows? – 273K Oct 06 '19 at 16:37
Thanks, this works in C but needs modifications. Regards. – Oct 06 '19 at 17:31
@273K what if my command output is an integer for example? It looks like the output of `popen` is always a `FILE *` and I am working on a problem where the output of `system` is an integer? I am also a c++ noob if you can't tell and I'm not sure where to look... – ydd Jul 27 '23 at 01:06
@ydd The output is a byte stream, it doesn't define what is a string or a number, the reader should convert bytes to desired types. – 273K Jul 27 '23 at 01:35

score 1 · Accepted Answer · edited Oct 07 '19 at 06:36

1

You need to use other function because system only return the code returned by the execution of the program. For example boost::process, this is a simple example

#include <boost/process.hpp>

using namespace boost::process;

int main()
{
    ipstream pipe_stream;
    child c("gcc --version", std_out > pipe_stream);

    std::string line;

    while (pipe_stream && std::getline(pipe_stream, line) && !line.empty())
        std::cerr << line << std::endl;

    c.wait();
}

Example taken from https://www.boost.org/doc/libs/1_71_0/doc/html/process.html

edited Oct 07 '19 at 06:36

Empty Space

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answered Oct 06 '19 at 16:32

Mauricio Ruiz

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Muchas Gracias :). – Oct 06 '19 at 16:37
trying to use boot/process I get errors. I added them in OP. – Oct 06 '19 at 16:43
Try to add `#include `. – Mauricio Ruiz Oct 06 '19 at 16:51
Now the error is different. I updated the OP. Thanks! – Oct 06 '19 at 16:54

Set a variable from system("command") (C++)

2 Answers2