Question

I’m trying to do this in C, I have some code in Python but I can’t figure how to make it work in C, I’m used to high level languages, and C is really new to me, maybe you can help me to translate it, I have nothing so far as I don’t know where to begin …

Python code:

archivo = open('passwd.txt')
for linea in archivo:
        campos = linea.strip().split(':')
        print 'User',campos[0],'has shell',campos[6]
archivo.close()

passwd.txt is the following:

root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh

The output should be:

User root has shell /bin/bash
User daemon has shell /bin/sh
User bin has shell /bin/sh

Can anyone help me?

Answer 1

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

int main(void) {
   FILE *fp = fopen("/etc/passwd", "r");
   char line[999];
   while (fgets(line, sizeof line, fp)) {
      char *user = strtok(line, ":");
      char *shell;
      int i;
      for (i = 0; i < 4; i++)
         strtok(0, ":");
      shell = strtok(0, ":");
      printf("User %s has shell %s\n", user, shell);
   }
   fclose(fp);
   return 0;
}

However, if /etc/passwd has empty fields, indicated by ::, then strtok will not work. You could use strsep instead, or just strchr, as in:

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

int main(void) {
   FILE *fp = fopen("/etc/passwd", "r");
   char line[999];
   while (fgets(line, sizeof line, fp)) {
      char *user = line;
      char *p = line;
      char *shell;
      int i;
      for (i = 0; i < 6; i++)
         p = strchr(p + 1, ':');
      shell = p + 1;
      strchr(user, ':')[0] = 0;
      printf("User %s has shell %s", user, shell);
   }
   fclose(fp);
   return 0;
}

On my machine, some of the so-called GECOS fields are empty, and user news has no shell.