GPG : "No secret key found" error

[Robert J. Hansen]

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> Step 2. Instead, I have thought of storing the passphrase in a file 
> (passphrase.dat.pgp), encrypted that file without password and passing 
> the password to do the work using below command.

How exactly do you "encrypt that file without password"?

At any rate, this is probably a bad idea.  Often the best way to proceed 
for scripting GnuPG tasks is to remove the passphrase from the certificate.

> Step 3. To my wonder, when I execute Step 1 first and then Step 2 
> (within a short span), it works, but if I directly run Step 2 ( which 
> actually will be happening as a part of solution), then it doesn't and 
> fails for "No secret key" error.

This tells me that GnuPG is caching your passphrase with gpg-agent. 
When you run it the second time GnuPG sees the passphrase is in the 
cache and uses that, without ever needing to ask you for the passphrase.