Re: [PATCH v2 1/6] CodingGuidelines: add shell piping guidelines

From: Matthew DeVore <matvore@google.com>
Date: Sep 19, 2018, 02:11 UTC
Message-ID: <CAMfpvhKejvbgzwtTv93iqLG8fMxqZW_MRTAU0q9bDArqJU2zUg@mail.gmail.com>
In-Reply-To: <CAPig+cSzddcS+8mx=GMbJ5BP+=fPtza+7UdA5ugN+83NuOHyiw@mail.gmail.com>

On Mon, Sep 17, 2018 at 5:16 PM Eric Sunshine <sunshine@sunshineco.com> wrote:

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>
> On Mon, Sep 17, 2018 at 6:24 PM Matthew DeVore <matvore@google.com> wrote:
> > diff --git a/Documentation/CodingGuidelines b/Documentation/CodingGuidelines
> > @@ -163,6 +163,35 @@ For shell scripts specifically (not exhaustive):
> > + - In a piped sequence which spans multiple lines, put each statement
> > +   on a separate line and put pipes on the end of each line, rather
> > +   than the start. This means you don't need to use \ to join lines,
> > +   since | implies a join already. Also, do not indent subsequent
> > +   lines; if you need a sequence to visually stand apart from the
> > +   surrounding code, use a blank line before and/or after the piped
> > +   sequence.
> > +
> > +       (incorrect)
> > +       [...]
> > +       (correct)
> > +       echo '...' > expected
>
> Existing tests seem to favor the name "expect" over "expected", so
> perhaps use that instead.
>
>     $ git grep '>expect\b' -- t | wc -l
>     2674
>     $ git grep '>expected\b' -- t | wc -l
>     1406

Thank you for clarifying that out for me, but I'm not longer using that example, so it's moot.

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>
> > +       git ls-files -s file.1 file.2 file.3 file.4 file.5 |
> > +       awk '{print $1}' |
> > +       sort >observed
>
> This is not a great example since it flatly contradicts the very next
> bit of advice added by this patch about not placing a Git command
> upstream in a pipe. Perhaps come up with an example which doesn't
> suffer this shortcoming.

Done.

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>
> I've seen the advice earlier in the thread of not indenting the
> sub-commands in a pipe, but I find that the result makes it far more
> difficult to see which commands are part of the pipe sequence than
> with them indented, so I'm not convinced that this advice should be in
> the guidelines. (But that just my opinion.)

I'm not totally sure either way, nor do I have a strong opinion. I agree it's probably better to not codify this in the documentation until there's a great reason to.

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>
> > + - In a pipe, any non-zero exit codes returned by processes besides
> > +   the last will be ignored. If there is any possibility some
> > +   non-final command in the pipe will raise an error, prefer writing
> > +   the output of that command to a temporary file with '>' rather than
> > +   pipe it.
>
> It's not so much that we care about losing a non-zero exit code (which
> might be perfectly acceptable depending upon the context) but that we
> care about missing a Git command which outright crashes. So, it might
> make sense to make this text more specific by saying that ("exit code
> indicating a crash" and "Git command") rather than being generic in
> saying only "exit code" and "command".

Fixed.

>
> Also, what about expression like $(git foo) by which a crash of a Git
> command can also be lost? Do we want to talk about that, as well?

Yes, it's probably better to add a point about that. Here is the new documentation after applying your suggestions:

 - If a piped sequence which spans multiple lines, put each statement
   on a separate line and put pipes on the end of each line, rather
   than the start. This means you don't need to use \ to join lines,
   since | implies a join already.

        (incorrect)
        grep blob verify_pack_result \
        | awk -f print_1.awk \
        | sort >actual &&
        ...

        (correct)
        grep blob verify_pack_result |
        awk -f print_1.awk |
        sort >actual &&
        ...

 - In a pipe, any exit codes returned by processes besides the last
   are ignored. This means that if git crashes at the beginning or
   middle of a pipe, it may go undetected. Prefer writing the output
   of that command to a temporary file with '>' rather than pipe it.

 - The $(git ...) construct also discards git's exit code, so if the
   goal is to test that particular command, redirect its output to a
   temporary file rather than wrap it with $( ).