Access c variable in system command

Question

I want to access variable of c in system command but i do not know how to do it i tried like below but this does not works

#include<stdlib.h>
int main(){
int a=12;
system("echo $a");
}

Answer 1

You can't do this via any kind of string interpolation such as you've tried. What you need to do is build the command string before passing it to system().

#include <stdio.h>
#include <stdlib.h>

int main() {
    int a = 12;
    char command[100];
    sprintf(command, "echo %d", a);
    system(command);
}

Answer 2

The system function takes a const char* argument and returns an integer value depending on the system; usually its the status code of the command after being executed.

int system (const char* command);

So to embed variables from your c program you'd have to build a command string and then pass it to system(); Apart from using sprintf() as suggested above you can use string functions such as strcat() as well to build complex commands from your C variables. For example

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
    int a = 12;
    char command[] = "ls";
    char opt[] = " -l";
    char cmd[50];
    strcat(command, opt);

    sprintf(cmd, " | head -n %d", a);

    strcat(command, cmd);
    printf("%s\n", command );
    int rv = system(command);
    printf("Return value : %d\n", rv);

    return 0;
}