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.
You should look at the chpasswd command (if available in your linux flavor):
echo 'userid:newpasswd' | chpasswd
Or, you can cat a file listing userid:passwd for each account on a separate line.
That's it.
echo -e -n "newpasswd\nnewpasswd" | passwd user
$> 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.
passwd program reads from /dev/tty rather than standard input, so redirecting the standard input like that doesn't achieve anything. To get around that, you normally resort to the expect program, which arranges to run the program with its /dev/tty connected to a pseudo-tty or pty that expect can drive programmatically.
-e enable interpretation of backslash escapes