Hi,

/etc/passwd file has write permission only for the root user.

Now when a normal user changes the its own password using passwd command, how this information has been written to the /etc/passwd file when the user is not having write permission to this file.

~santosh

Google for "setuid". If this is homework, your teacher probably already told you about this mechanism.

Does this really smell like homework?
I would have taken it for a legitimate newbie question,
especially since it was posted in the Dummies section.

Have a look at the user interface's binary to change one's password
which is the command passwd
As you can see from the owner's (i.e. root) perm bits instead of the usual x
for execute there appears an s, the so called suid bit (that's what era referred to).
This means that although most of the times executed by ordinary users,
the command actually runs under the effective uid of root who is the only one on the system (normally) who is allowed to write to the file /etc/passwd.

If you look at the perms of the ping command you will find another suid binary
because ordinary users are not allowed to emit ICMP echo request packets.

Because of the security implications those elevated privileges impose
you generally will find only very few suid enabled executables on most Unix systems.
And their number should be observed in e.g. security audits or when hardening a system.