How to do what GPGTools's context menu does, but using GPG in the command line?

I use Mac and sometimes I use GPGTools to do the following:

open the TextEdit application and write, let's say, "Hello World" there.
select "Hello World" and control-click the selection, to open a context menu.
in the context menu, select "Services > OpenPGP: Encrypt Selection to New Window" and select the recipients in the dialog box that appears.

Then a new window pops up, showing an encrypted message that looks like this:

I would like to be able to do the same thing using the command line. And I would like to know how to do that in a way that is independent of the operating system. The solution should work in Windows and Linux too.

I know that GPGTools is based on GPG (GnuPG) and, therefore, I would assume that it should be possible to achieve what I want using the "gpg" command line tool. I know how to use "gpg" to encrypt, for instance, a "file.txt" file and obtain an encrypted "file.txt.gpg" file, but this encrypted file doesn't contain a "PGP MESSAGE" as shown above.

In summary, my question is: how can I encrypt a text string (not a file) and obtain a "PGP MESSAGE", using the command line?

Solution

To encrypt a string via standard input you would do this:

$ gpg --encrypt --armor --hidden-recipient "[email protected]" --output -

After executing this command it will appear to hang. Go ahead and type (copy/paste) your string.

Pasted string goes here.
Hitting enter creates a new line.
Backspace deletes characters.
You get the idea.

Now press [Enter] to create a new line. Then hit [CTRL+D] (this signals "end-of-file") to terminate the program.

gpg will then dump your ASCII armored encrypted text directly to the shell:

Side note:

If you are using OS X it is also possible to paste text directly from the pasteboard buffer using pbpaste. You can pipe its output into gpg by doing this instead:

$ pbpaste | gpg --encrypt --armor --hidden-recipient "[email protected]" --output -

Just be aware that the pasteboard buffer is not cleared automatically. If you copy a secret string there, be sure to copy something else (random irrelevant text) to clear it when you're done. Anyone (an attacker) with access to your account via ssh or other means could, for example, read your plain-text secret without any effort whatsoever.

To simply clear the pasteboard buffer via the shell, do: echo | pbcopy