How to store the result of a system command call?

Question

linux

Hello. Please help me understand the following question:
In a C / C ++ program, using a command in bash, I need to count the number of lines in a file containing the letter a . I am writing a program.

#include<stdio.h>
int main() {
      
    system("cat 1.txt | grep -c \"a\" ");
       return 0;
}

But I need to capture the output of system("cat 1.txt | grep -c \"a\" "); to a variable within the program itself. Something like this int i = system("cat 1.txt | grep -c \"a\" "); . How can this be organized?

Answer 1

man popen

#include <stdio.h>

int main()
{
    int n;
    FILE *f = popen("grep -c \"a\" < 1.txt", "r");
    fscanf(f, "%d", &n);
    pclose(f);
    ...
}

Answer 2

perversion, but,
redefine console out with a string stream, run a command, change out back, output a line.
Run a process that will have an out in your pipe, etc.

Answer 3

The most optimal would be to add output redirection to a file to the command passed to system (), and then read this file through fopen (). After all, remove the file with remove().