gpg-agent forwarding: inappropriate ioctl for device

When running gpg from a script, the --batch argument must be provided.

It happens when GPG is confused where to read input from. Simply configuring it to look for input from tty (the terminal connected to standard input) fixes it:

export GPG_TTY=$(tty)

This method does not work when you are inside an LXC container. Instead, add this to ~/.gnupg/gpg.conf:

use-agent 
pinentry-mode loopback

Then add this to ~/.gnupg/gpg-agent.conf

allow-loopback-pinentry

Then restart the agent with echo RELOADAGENT | gpg-connect-agent.

(source)