How do I change user password using a shell script under FreeBSD operating systems?

You need to use the -h fd option for pw command. It provides a special interface by which interactive scripts can set an account password using pw. Because the command line and environment are fundamentally insecure mechanisms by which programs can accept information, pw will only allow setting of account and group passwords via a file descriptor (usually a pipe between an interactive script and the program). sh, bash, ksh and perl all possess mechanisms by which this can be done. Linux / UNIX should able to use passwd command or perl script to do the same.

Task: Add User And Set A Password

Add a new user called vivek with password called topSecrete:
# echo PASSWORD | pw add user USERNAME -h 0
# echo topSecrete | pw add user vivek -h 0

Task: Modify Existing User Password

Change the password for a user called chitra
# echo newPassword | pw mod user chitra -h 0

A Sample Shell Script To Change User Password

#!/bin/sh
PW=/usr/sbin/pw
SHELL=/bin/csh
echo "*** ADD NEW USER SHELL SCRIPT ***"
echo -n "Username : " 
read user
echo "Password : " 
read -n passwd
echo ${passwd} | ${PW} add user ${user} -m -s ${SHELL} -h 0 >/dev/null 2>&1
if [ $? -eq 0 ]
then
	echo "User added to the system."
else
	echo "Failed to add user to the system."
fi