How to find out which user is logged into the linux system in si?

Question

linux

Hello. How to find out which user is logged into the linux system in si? That is, which one is active now? Is there any library for these things?

Answer 1

Option 1:

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

int main(int argc,char **argv)
{
    char *cmd = "whoami";
    char buf[BUFSIZ];
    FILE *ptr;
 
    if ((ptr = popen(cmd, "r")) != NULL)
        while (fgets(buf, BUFSIZ, ptr) != NULL)
            (void) printf("%s", buf);
            (void) pclose(ptr);
    return 0;
}

Option 2:

#include <stdlib.h>
 
int main(int argc,char **argv)
{
    system("whoami");
    return 0;
}

Both options will return the name of the current user.

Answer 2

You can start with man utmp.

Answer 3

If it works for postgresql, then it probably works for you too: https://github.com/postgres/postgres/blob/REL_13_S...
(geteuid + getpwuid under linux)

Answer 4

man getuid
man getpwuid
There are standard functions for such tasks. They are much more correct to use than parsing utmp (which the user may not get into, especially if it is a non-interactive login or su / sudo) and /etc/passwd (since users can be stored not only in passwd, but also in winbind/ldap/nis/ etc).

Answer 5

Would this not work: `system("who -a")`? Need 'stdlib.h'. Source: https://www.techonthenet.com/c_language/standard_l...