What an useradd command in Linux does?

When you execute an useradd command  (# useradd <username>”), the following happens.

1.       Add the new user entry in the following files: /etc/passwd and /etc/shadow

2.       A group will be created with the same username and it will be updated in the following files: /etc/group and /etc/gshadow.

3.       Home folder for the user will be created (/home/<username>) and the default profile settings will be copied from /etc/skel to it.

# ls -la /etc/skel/
total 56
drwxr-xr-x   3 root root  4096 Aug 16 13:03 .
drwxr-xr-x 111 root root 12288 Oct  6 15:10 ..
-rw-r--r--   1 root root    33 Jan 21  2009 .bash_logout
-rw-r--r--   1 root root   176 Jan 21  2009 .bash_profile
-rw-r--r--   1 root root   124 Jan 21  2009 .bashrc
-rw-r--r--   1 root root   515 May 24  2008 .emacs
drwxr-xr-x   4 root root  4096 Jul 26 18:19 .mozilla
-rw-r--r--   1 root root   658 Sep 21  2009 .zshrc
 #

Please note in the case, if you happen to accidently delete either /etc/passwd or /etc/shadow files, you can restore it from its corresponding backup files (i.e /etc/passwd- ,  /etc/shadow- ) respectively. These files are updated upon the system reboot.  So you can’t expect these files to be having the entries for user accounts which are added after the system reboot.