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Use grep Output as if-else Conditional
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This was my notation of how to use ``grep``'s standard output as an ``if-else`` conditional in a shell script.

Basically, just about all programs have at least two outputs: the textual output that it sends to ``STDOUT`` - the stuff that you read - and a basic ``0``, ``1`` or higher, which you use in shell scripts, ``if-else`` conditionals,  loops, and other tests.

The ``if-else`` conditionals or ``while`` loops like to use simple numbers: ``0`` or ``1`` and higher for testing for the success or failure of a once-running program. ``0`` meaning successful run, or ``1`` and above meaning failed. If we are testing for ``openvpn`` being installed, and it returns ``0`` for yes, ``1`` for error, you can make a conditional that says ``if 0, yes, then do not attempt install. If NOT 0, then attempt install.``

Now, when I say "return", I mean it presents the output, by default, essentially in the background where you might not see it, but the computer certainly knows is there. And, depending on the program/programs running, they can see the result, too.

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Basic Use of grep in if-else
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This would be a simple layout of how you could potentially use this in a shell script.

.. code-block:: bash

  #!/bin/bash

  dpkg -l | grep openvpn &> /dev/null
  if [[ $? == 0 ]]; then
    echo "matched"
  else
    echo "Not Matched"
  fi

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So, lets break that down
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1. ``dpkg -l`` - this, by itself, will output all installed programs that dpkg manages.
2. ``|`` - this is the ``pipe`` command.

  - What this says is:
    take the outputted text from the last thing and use it as input for the next thing
  - So, the entire list of installed apps then is searched using ``grep`` for ``openvpn``

3. ``&> /dev/null`` - this is a redirection of the output, negating any textual output, leaving only the ``1`` or ``0`` or more outputs to use
4. ``$?`` - this basically says ``take the numbered output, whatever it is, and stick it here``
5. So, when you do ``$? == 0`` you're saying ``if the output was 0, then do this command``

AND be careful, 0 might not always be yes or confirm. Make sure to check the program's MAN - manual - pages for clarification, especially if it uses different error codes for different responses.