Re: [PATCH v4 2/7] Documentation: add shell guidelines

From: Junio C Hamano <gitster@pobox.com>
To: Matthew DeVore <matvore@google.com>
Cc: git@vger.kernel.org, peff@peff.net, jonathantanmy@google.com,
	jrn@google.com, sunshine@sunshineco.com, szeder.dev@gmail.com
Subject: Re: [PATCH v4 2/7] Documentation: add shell guidelines
Date: Fri, 05 Oct 2018 09:48:23 -0700	[thread overview]
Message-ID: <xmqq5zygwd9k.fsf@gitster-ct.c.googlers.com> (raw)
In-Reply-To: a45b646fced228e5a7fab4db7a35c1c6084e58d0.1538581868.git.matvore@google.com

Matthew DeVore <matvore@google.com> writes:

> Add the following guideline to Documentation/CodingGuidelines:
>
> 	&&, ||, and | should appear at the end of lines, not the
> 	beginning, and the \ line continuation character should be
> 	omitted

"should be omitted" sounds as if it is the norm to have such a
character, but it is not.  The text in the actual patch body does a
much better job than this.

Perhaps

	Break overlong lines after "&&", "||", and "|", not before
	them; that way the command can continue to subsequent lines
	without backslash at the end.

> And the following to t/README (since it is specific to writing tests):
>
> 	pipes and $(git ...) should be avoided when they swallow exit
> 	codes of Git processes

Good.

> Signed-off-by: Matthew DeVore <matvore@google.com>
> ---
>  Documentation/CodingGuidelines | 18 ++++++++++++++++++
>  t/README                       | 28 ++++++++++++++++++++++++++++
>  2 files changed, 46 insertions(+)
>
> diff --git a/Documentation/CodingGuidelines b/Documentation/CodingGuidelines
> index 48aa4edfb..72967deb7 100644
> --- a/Documentation/CodingGuidelines
> +++ b/Documentation/CodingGuidelines
> @@ -118,6 +118,24 @@ For shell scripts specifically (not exhaustive):
>  		do this
>  	fi
>  
> + - If a command sequence joined with && or || or | spans multiple
> +   lines, put each command on a separate line and put && and || and |
> +   operators at the end of each line, rather than the start. This
> +   means you don't need to use \ to join lines, since the above
> +   operators imply the sequence isn't finished.

Correct.  Even though I wonder if we need to say the last sentence,
which is rather obvious, patch is already written and I do not see
much point editing this further.

> +	(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 &&
> +	...
> +
>   - We prefer "test" over "[ ... ]".
>  
>   - We do not write the noiseword "function" in front of shell
> diff --git a/t/README b/t/README
> index 85024aba6..9a71d5732 100644
> --- a/t/README
> +++ b/t/README
> @@ -466,6 +466,34 @@ And here are the "don'ts:"
>     platform commands; just use '! cmd'.  We are not in the business
>     of verifying that the world given to us sanely works.
>  
> + - Don't use Git upstream in the non-final position in a piped chain, as
> +   in:

"upstream in the non-final position" is a bit redundant, isn't it?

  - Don't feed the output of 'git' to a pipe, as in:

> +
> +     git -C repo ls-files |
> +     xargs -n 1 basename |
> +     grep foo
> +
> +   which will discard git's exit code and may mask a crash. In the
> +   above example, all exit codes are ignored except grep's.

Good.

> +   Instead, write the output of that command to a temporary
> +   file with ">" or assign it to a variable with "x=$(git ...)" rather
> +   than pipe it.
> +
> + - Don't use command substitution in a way that discards git's exit
> +   code. When assigning to a variable, the exit code is not discarded,
> +   e.g.:
> +
> +     x=$(git cat-file -p $sha) &&
> +     ...
> +
> +   is OK because a crash in "git cat-file" will cause the "&&" chain
> +   to fail, but:
> +
> +     test "refs/heads/foo" = "$(git symbolic-ref HEAD)"
> +
> +   is not OK and a crash in git could go undetected.

Good.

>   - Don't use perl without spelling it as "$PERL_PATH". This is to help
>     our friends on Windows where the platform Perl often adds CR before
>     the end of line, and they bundle Git with a version of Perl that