Question

I want to change the password of the Unix user through a shell script, but without being prompted to enter the old password.

That means I want to provide the old and new password in the shell script itself, such that during the prompt it will read it from the script file.

Answer 1

$> echo -e -n "oldpasswd\nnewpasswd\nnewpasswd" | passwd

Or to make things a cleaner way, write a file with your passwords

oldpassword
newpassword
newpassword

and use the following command: $> passwd < file

The pipe and redirection operators are replacing the standard input with either the content of the file redirected, either the output of the command piped.