Is there a way to redirect output of a command line which returns integer as an output to a variable in C?

for example, if the command is "cmd", then is there a way to redirect its output (an integer) and store it in variable in C? I tried using popen and fgets but it seems to be working only with characters. Any suggestions?

CodePudding user response：

It works perfectly fine with popen and fgets:

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

int
main(int argc, char *argv[])
{
    const char *cmd = argc > 1 ? argv[1] : "echo 42";
    char buf[32];
    FILE *fp = popen(cmd, "r");
    if( fp == NULL ){
        perror("popen");
        return 1;
    }
    if( fgets(buf, sizeof buf, fp) == buf ){
        int v = strtol(buf, NULL, 10);
        printf("read: %d\n", v);
    }
    return 0;
}

CodePudding user response：

If you want to convert a character string from the standard input, you could use fgets and then use atoi to convert the input to an integer.

If you want to convert the output of a command, let's say ls and store the output of the command to a variable, you could learn about fork, dup2, pipe, and exec function family.

More about this topic on this tutorial : Capture the output of a child in C. This tutorial also provide an example with popen if you want to keep things "high level".

CodePudding user response：

Here is an even simpler example using popen() and fscanf():

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

int main(int argc, char *argv[]) {
    FILE *fp = popen("date ' %s'", "r");
    long seconds;
    if (fp == NULL) {
        fprintf(stderr, "popen failed: %s\n", strerror(errno));
        return 1;
    }
    if (fscanf(fp, "%ld", &seconds) == 1) {
        printf("epoch seconds: %ld\n", seconds);
        pclose(fp);
        return 0;
    } else {
        fprintf(stderr, "invalid program output\n");
        pclose(fp);
        return 1;
    }
}