From: Jeff King <peff@peff.net>
Date: Sun, 06 Nov 2016 14:29:58 GMT
Subject: Re: [PATCH 1/4] t0021: use write_script to create rot13 shell script
Message-ID: <20161106142957.52wx2llanw5bis4i@sigill.intra.peff.net>
In-Reply-To: <22DAA280-1857-4E22-914A-1208D784DA98@gmail.com>

On Sun, Nov 06, 2016 at 03:25:33PM +0100, Lars Schneider wrote:

> This looks good to me (and it works on my machine).
> However, I took a look at the "write_script" function and found this,
> added by Junio in 840c519d:
> 
> echo "#!${2-"$SHELL_PATH"}" &&
> 
> There is some kind of variable expansion happening with the "2-" but
> I can't quite figure out what is going on. Plus, I can't find anything 
> about this in the sh docs.
> 
> Can anyone help me to understand it?

See the section on parameter expansion in "man bash". Basically:

 ${foo:-bar}

expands to $foo, or "bar" if it is unset or empty. Without the colon:

  ${foo-bar}

expands to $foo, "bar" if it unset (but not if it is empty). I don't
think we really care about the distinction here, and either is fine (you
would not ever pass an empty argument).

So in this context you may pass in the interpreter:

  write_script "$PERL_PATH" <<\EOF
  ... some perl code ...
  EOF

or it defaults to shell, which is what most of the callers want:

  write_script <<\EOF
  ... some shell code ...
  EOF

-Peff