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How do I execute a command and get the output of the command within C++ using POSIX?

Submitted by: @import:stackoverflow-api·Mar 10, 2026·

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howwithinoutputandtheusingposixcommandexecuteget

Problem

I am looking for a way to get the output of a command when it is run from within a C++ program. I have looked at using the system() function, but that will just execute a command. Here's an example of what I'm looking for:

std::string result = system("./some_command");

I need to run an arbitrary command and get its output. I've looked at boost.org, but I have not found anything that will give me what I need.

Solution

#include 
#include 
#include 
#include 
#include 
#include 

std::string exec(const char* cmd) {
    std::array buffer;
    std::string result;
    std::unique_ptr pipe(popen(cmd, "r"), pclose);
    if (!pipe) {
        throw std::runtime_error("popen() failed!");
    }
    while (fgets(buffer.data(), static_cast(buffer.size()), pipe.get()) != nullptr) {
        result += buffer.data();
    }
    return result;
}

Pre-C++11 version:

#include 
#include 
#include 
#include 

std::string exec(const char* cmd) {
    char buffer[128];
    std::string result = "";
    FILE* pipe = popen(cmd, "r");
    if (!pipe) throw std::runtime_error("popen() failed!");
    try {
        while (fgets(buffer, sizeof buffer, pipe) != NULL) {
            result += buffer;
        }
    } catch (...) {
        pclose(pipe);
        throw;
    }
    pclose(pipe);
    return result;
}

Replace popen and pclose with _popen and _pclose for Windows.

2024 Edit:

With newer versions of gnu g++ such as the one in Ubuntu 24.04, the code above results in an error because the std::unique_ptr deleter is ignoring the return value from pclose().

error: ignoring attributes on template argument ‘int (*)(FILE*)’ [-Werror=ignored-attributes]

Had to modify the code to wrap pclose(). Used a lambda for the wrapper. Now looks like this:

std::unique_ptr pipe(popen(cmd.c_str(), "r"),
    [](FILE * f) -> void
    {
        // wrapper to ignore the return value from pclose() is needed with newer versions of gnu g++
        std::ignore = pclose(f);
    });

Code Snippets

#include <cstdio>
#include <iostream>
#include <memory>
#include <stdexcept>
#include <string>
#include <array>

std::string exec(const char* cmd) {
    std::array<char, 128> buffer;
    std::string result;
    std::unique_ptr<FILE, decltype(&pclose)> pipe(popen(cmd, "r"), pclose);
    if (!pipe) {
        throw std::runtime_error("popen() failed!");
    }
    while (fgets(buffer.data(), static_cast<int>(buffer.size()), pipe.get()) != nullptr) {
        result += buffer.data();
    }
    return result;
}

#include <iostream>
#include <stdexcept>
#include <stdio.h>
#include <string>

std::string exec(const char* cmd) {
    char buffer[128];
    std::string result = "";
    FILE* pipe = popen(cmd, "r");
    if (!pipe) throw std::runtime_error("popen() failed!");
    try {
        while (fgets(buffer, sizeof buffer, pipe) != NULL) {
            result += buffer;
        }
    } catch (...) {
        pclose(pipe);
        throw;
    }
    pclose(pipe);
    return result;
}

error: ignoring attributes on template argument ‘int (*)(FILE*)’ [-Werror=ignored-attributes]

std::unique_ptr<FILE, void(*)(FILE*)> pipe(popen(cmd.c_str(), "r"),
    [](FILE * f) -> void
    {
        // wrapper to ignore the return value from pclose() is needed with newer versions of gnu g++
        std::ignore = pclose(f);
    });

Context

Stack Overflow Q#478898, score: 809

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