How do I get output of command in C++?

Question

How do I get the output of a sub process and assign it to a variable. EG: assign the output of wmic csproduct get uuid returns a long string of characters in CMD, how would I run that command in C++ and assign it to a variable?

Answer 1

There are two main options for capturing the output of a process:

• Redirect it to a file, then read the file later.
• Use a pipe.

Pipes are not supported by the standard library, other than hidden behind the common stream abstraction. Effectively that means that a process A that creates a process B, can't communicate with B via pipes, unless platform-specific functionality for that is used. In Windows the scripting support, Windows Script Host, has direct functionality for the pipe approach, based on polling for output from B. At a much lower level you can use the CreateProcess API function. But it's not trivial to do.

With a temporary file as communication channel, and using only standard C++ functionality, you simply invoke program B via a command interpreter that can redirect B's output. This command interpreter and its syntax then constitutes the platform specific part. So even this approach is platform specific, but the C++ code as such is portable.

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In Windows the standard command intepreter is cmd.exe, and it uses the ordinary Unix > syntax in order to redirect output of a program, so the command to be executed can be like

cmd /c "b.exe >tempfile"

You can execute such a command via C++ system. The system function already executes the specified command via a command intepreter, which in Windows is cmd.exe. And so the argument to system only needs to be

b.exe > tempfile

where tempfile is the name or full path to the file where you want the result.

You can create a fresh temporary filename via the tmpnam function.

Answer 2

I assume the winapi tag is referring to the answer you need and it also hints to the platform. So, I think the answer is _popen

Example:

#include <stdio.h>

int main()
{
    FILE*  CommandResult = _popen("wmic csproduct get uuid", "rt"); //send the command to the cmd and return a pointer to the command result
    char   line[256]; //a buffer to read from the file
    while(fgets(line, 256, CommandResult)) printf(line); //read and print all lines one by one
    _pclose(CommandResult);
    scanf(line); //prevent the console from closing immediately
}

Edit:

I just realized that I must attach the following disclaimer with any line of code before I can publish it.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.