How to redirect stdout with '\r' printed on the same line?

Question

Check below the code snippet of using the character \r:

#include <stdio.h>
#include <unistd.h>
int main() {
    for(int i=1; i<=100; i++) {
        printf("%d%%\r", i);
        fflush(stdout);
        usleep(1e4);
    }
    printf("\n");
    return 0;
}

The output of this program on the console has only 1 line:

$ ./main
100%

However if the output is redirected to a file ./main > main.log, the file has 100 lines:

1%
2%
3%
...
100%

Question:

How do I keep only the last line of output to the log file?

Answer 1

I am assuming this is part of a larger program where the "percentage" may or may not be 100% (otherwise, you could just print 100% right away). So you could check if the output goes to a terminal using isatty. If not, print the "last percentage" value:

int main(void)
{
    int b = isatty(STDOUT_FILENO);
    int i;
    for(i=1; i<=100; i++) {
        if (b) printf("%d%%\r", i);
        fflush(stdout);
        usleep(1e4);
    }
    if (!b) printf("%d%%\r", i-1);
    printf("\n");
    return 0;
}