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


Question

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.

1
412
1/5/2019 3:50:19 PM

Accepted Answer

#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(), buffer.size(), pipe.get()) != nullptr) {
        result += buffer.data();
    }
    return result;
}

Pre-C++11 version:

#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;
}

Replace popen and pclose with _popen and _pclose for Windows.

545
12/5/2018 2:16:04 AM

Getting both stdout and stderr (and also writing to stdin, not shown here) is easy peasy with my pstreams header, which defines iostream classes that work like popen:

#include <pstream.h>
#include <string>
#include <iostream>

int main()
{
  // run a process and create a streambuf that reads its stdout and stderr
  redi::ipstream proc("./some_command", redi::pstreams::pstdout | redi::pstreams::pstderr);
  std::string line;
  // read child's stdout
  while (std::getline(proc.out(), line))
    std::cout << "stdout: " << line << '\n';
  // read child's stderr
  while (std::getline(proc.err(), line))
    std::cout << "stderr: " << line << '\n';
} 

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