use warnings;
use strict;

while(<DATA>){
 chomp;
 if(/^user(1|3):/){
    s{(?<=/bin)(.+?)$}{/nologin};
 }
 
  print $_,$/;
}


__DATA__
user1:x:9760822:3241:USER 1:/home/user1:/bin/bash

user2:x:9760822:3242:USER 2:/home/user2:/bin/ksh

user3:x:9760822:3243:USER 3:/home/user3:/bin/sh


##</code><code>##

user1:x:9760822:3241:USER 1:/home/user1:/bin/nologin

user2:x:9760822:3242:USER 2:/home/user2:/bin/ksh

user3:x:9760822:3243:USER 3:/home/user3:/bin/nologin