C++ popen() 将执行的命令的输出写入 cout
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popen() writes output of command executed to cout
提问by John Smith
I am writing an application that needs to open another process and get it's output. Online everywhere I read I have to use popenand read from the file.
我正在编写一个需要打开另一个进程并获取它的输出的应用程序。在线阅读我必须使用popen并从文件中读取的任何地方。
But I can't read from it. The output of the command gets output into the console window of the calling application. Below is the code I am using. I added some prints to debug.
但我无法从中读取。命令的输出将输出到调用应用程序的控制台窗口中。下面是我正在使用的代码。我添加了一些打印来调试。
#include <string>
#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <array>
int main()
{
// some command that fails to execute properly.
std::string command("ls afskfksakfafkas");
std::array<char, 128> buffer;
std::string result;
std::cout << "Opening reading pipe" << std::endl;
FILE* pipe = popen(command.c_str(), "r");
if (!pipe)
{
std::cerr << "Couldn't start command." << std::endl;
return 0;
}
while (fgets(buffer.data(), 128, pipe) != NULL) {
std::cout << "Reading..." << std::endl;
result += buffer.data();
}
auto returnCode = pclose(pipe);
std::cout << result << std::endl;
std::cout << returnCode << std::endl;
return 0;
}
Reading is never actually printed to the my coutand result is an empty string. I clearly see the output of the command in my terminal. If the command exits gracefully the behaviour is as expected. But I only capture the output for error cases.
Reading 从未真正打印到 mycout并且结果是一个空字符串。我在终端中清楚地看到了命令的输出。如果命令正常退出，则行为符合预期。但我只捕获错误情况的输出。
回答by John Smith
Popen doesn't capture stderr only stdout. Redirecting stderr to stdout fixes the issue.
Popen 不只捕获 stderr 的 stdout。将 stderr 重定向到 stdout 可以解决此问题。
#include <string>
#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <array>
int main()
{
std::string command("ls afskfksakfafkas 2>&1");
std::array<char, 128> buffer;
std::string result;
std::cout << "Opening reading pipe" << std::endl;
FILE* pipe = popen(command.c_str(), "r");
if (!pipe)
{
std::cerr << "Couldn't start command." << std::endl;
return 0;
}
while (fgets(buffer.data(), 128, pipe) != NULL) {
std::cout << "Reading..." << std::endl;
result += buffer.data();
}
auto returnCode = pclose(pipe);
std::cout << result << std::endl;
std::cout << returnCode << std::endl;
return 0;
}
回答by Антон М
You have to add "2>&1" at the end of command string
您必须在命令字符串的末尾添加“2>&1”
command.append(" 2>&1");
there is a full example https://www.jeremymorgan.com/tutorials/c-programming/how-to-capture-the-output-of-a-linux-command-in-c/
