Question

How can you know if popen() succeeded/done running the shell or failed?

Answer 1

popen return value:

Upon successful completion, popen() shall return a pointer to an open
stream that can be used to read or write to the pipe. Otherwise, it
shall return a null pointer and may set errno to indicate the error.

fp = popen("ls *", "r");
if (fp == NULL)
    /* Handle error */;