$ echo "cleartext" | gpg --passphrase "some-passphrase" -c --no-use-agent > text.encrypted
$ cat text.encrypted | gpg --passphrase "passphrase" --no-use-agent 2>/dev/null
$ cat text.encrypted | gpg --passphrase "some-passphrase" --no-use-agent 2>/dev/null
cleartext


Without “–no-use-agent”, you might very well be prompted by some system keyring/agent every time.